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{{short description|Chemical compound with formula H₂O}}
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{{redirect|H2O}}
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'''Water''' is an [[inorganic compound]] with the [[chemical formula]] {{chem2|H2O}}. It is a transparent, tasteless, odorless,{{efn|see the [[water#Taste and odor|taste and odor]] section}} and [[Color of water|nearly colorless]] [[chemical substance]], and it is the main constituent of [[Earth]]'s [[hydrosphere]] and the [[fluid]]s of all known living organisms (in which it acts as a [[solvent]]<ref>{{Cite web |url=https://www.usgs.gov/special-topic/water-science-school/science/water-qa-why-water-universal-solvent?qt-science_center_objects=0#qt-science_center_objects |title=Water Q&A: Why is water the "universal solvent"? |date=20 June 2019 |website=Water Science School |publisher=[[United States Geological Survey]], [[U.S. Department of the Interior]] |access-date=15 January 2021 |archive-date=6 February 2021 |archive-url=https://web.archive.org/web/20210206061114/https://www.usgs.gov/special-topic/water-science-school/science/water-qa-why-water-universal-solvent?qt-science_center_objects=0#qt-science_center_objects |url-status=live }}</ref>). It is vital for all known forms of [[life]], despite not providing [[food energy]] or organic [[micronutrient]]s. Its chemical formula, {{chem2|H2O}}, indicates that each of its [[molecule]]s contains one [[oxygen]] and two [[hydrogen]] [[atom]]s, connected by [[covalent bond]]s. The hydrogen atoms are attached to the oxygen atom at an angle of 104.45°.<ref>{{Cite web |url=https://chem.libretexts.org/Courses/Pacific_Union_College/Quantum_Chemistry/10%3A_Bonding_in_Polyatomic_Molecules/10.02%3A_Hybrid_Orbitals_in_Water |title=10.2: Hybrid Orbitals in Water |date=18 March 2020 |website=Chemistry LibreTexts |access-date=11 April 2021 |language=English |archive-date=30 July 2022 |archive-url=https://web.archive.org/web/20220730092130/https://chem.libretexts.org/Courses/Pacific_Union_College/Quantum_Chemistry/10%3A_Bonding_in_Polyatomic_Molecules/10.02%3A_Hybrid_Orbitals_in_Water |url-status=live }}</ref> In liquid form, {{chem2|H2O}} is also called "water" at [[standard temperature and pressure]].

Because Earth's environment is relatively close to water's [[triple point]], water exists on Earth as a [[solid]], a [[liquid]], and a [[gas]].<ref>{{cite web |last1=Butler |first1=John |title=The Earth – Introduction – Weathering |url=https://uh.edu/~jbutler/physical/chapter6notes.html |publisher=University of Houston |access-date=30 January 2023 |quote=Note that the Earth environment is close to the triple point and that water, steam and ice can all exist at the surface. |archive-date=30 January 2023 |archive-url=https://web.archive.org/web/20230130051934/https://uh.edu/~jbutler/physical/chapter6notes.html |url-status=live }}</ref> It forms [[precipitation]] in the form of [[rain]] and [[aerosol]]s in the form of [[fog]]. [[Cloud]]s consist of suspended droplets of water and [[ice]], its solid state. When finely divided, [[crystal]]line ice may precipitate in the form of [[snow]]. The gaseous state of water is [[steam]] or [[water vapor]].

Water covers about 71% of the Earth's surface, with seas and oceans making up most of the water volume (about 96.5%).<ref name="WSS">{{cite web |url=https://www.usgs.gov/special-topics/water-science-school/science/how-much-water-there-earth |title=How Much Water is There on Earth? |date=13 November 2019 |website=Water Science School |publisher=[[United States Geological Survey]], [[U.S. Department of the Interior]] |access-date=8 June 2022 |archive-date=9 June 2022 |archive-url=https://web.archive.org/web/20220609050627/https://www.usgs.gov/special-topics/water-science-school/science/how-much-water-there-earth |url-status=live }}</ref> Small portions of water occur as [[groundwater]] (1.7%), in the [[glaciers]] and the [[ice caps]] of [[Antarctica]] and [[Greenland]] (1.7%), and in the air as [[vapor]], clouds (consisting of ice and liquid water suspended in air), and precipitation (0.001%).<ref name="b1">{{cite book |title=Water in Crisis: A Guide to the World's Freshwater Resources |editor=Gleick, P.H. |publisher=Oxford University Press |year=1993 |page=13, Table 2.1 "Water reserves on the earth" |url=http://www.oup.com/us/catalog/general/subject/EarthSciences/Oceanography/?view=usa&ci=9780195076288 |url-status=dead |archive-url=https://web.archive.org/web/20130408091921/http://www.oup.com/us/catalog/general/subject/EarthSciences/Oceanography/?view=usa&ci=9780195076288 |archive-date=8 April 2013 }}</ref><ref>[http://www.agu.org/sci_soc/mockler.html Water Vapor in the Climate System] {{Webarchive|url=https://web.archive.org/web/20070320034158/http://www.agu.org/sci_soc/mockler.html |date=20 March 2007 }}, Special Report, [AGU], December 1995 (linked 4/2007). [http://www.unep.org/dewa/assessments/ecosystems/water/vitalwater/ Vital Water] {{Webarchive|url=https://web.archive.org/web/20080220070111/http://www.unep.org/dewa/assessments/ecosystems/water/vitalwater/ |date=20 February 2008 }} [[UNEP]].</ref> Water moves continually through the [[water cycle]] of [[evaporation]], [[transpiration]] ([[evapotranspiration]]), [[condensation]], precipitation, and [[Surface runoff|runoff]], usually reaching the sea.

Water plays an important role in the [[world economy]]. Approximately 70% of the [[freshwater|fresh water]] used by humans [[Irrigation|goes to agriculture]].<ref name=Baroni2007>{{cite journal |author=Baroni, L. |author2=Cenci, L. |author3=Tettamanti, M. |author4=Berati, M. |year=2007 |title=Evaluating the environmental impact of various dietary patterns combined with different food production systems |journal=European Journal of Clinical Nutrition |volume=61 |pages=279–286 |doi=10.1038/sj.ejcn.1602522 |pmid=17035955 |issue=2|doi-access=free }}</ref> Fishing in [[Saline water|salt]] and [[fresh water]] bodies has been, and continues to be, a major source of food for many parts of the world, providing 6.5% of global protein.<ref>{{Cite journal |last1=Troell |first1=Max |last2=Naylor |first2=Rosamond L. |last3=Metian |first3=Marc |last4=Beveridge |first4=Malcolm |last5=Tyedmers |first5=Peter H. |last6=Folke |first6=Carl |last7=Arrow |first7=Kenneth J. |last8=Barrett |first8=Scott |last9=Crépin |first9=Anne-Sophie |last10=Ehrlich |first10=Paul R. |last11=Gren |first11=Åsa |date=2014-09-16 |title=Does aquaculture add resilience to the global food system? |journal=Proceedings of the National Academy of Sciences |language=en |volume=111 |issue=37 |pages=13257–13263 |doi=10.1073/pnas.1404067111 |issn=0027-8424 |pmc=4169979 |pmid=25136111|bibcode=2014PNAS..11113257T |doi-access=free }}</ref> Much of the long-distance trade of commodities (such as oil, natural gas, and manufactured products) is [[Maritime transport|transported by boats]] through seas, rivers, lakes, and canals. Large quantities of water, ice, and steam are used for cooling and heating in industry and homes. Water is an excellent solvent for a wide variety of substances, both mineral and organic; as such, it is widely used in industrial processes and in cooking and washing. Water, ice, and snow are also central to many sports and other forms of entertainment, such as [[swimming]], pleasure boating, [[boat racing]], [[surfing]], [[Recreational fishing|sport fishing]], [[Diving (sport)|diving]], [[ice skating]], [[snowboarding]], and [[skiing]].

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==Etymology==
The word ''water'' comes from [[Old English]] '''{{Lang|ang|wæter}}''', from [[Proto-Germanic language|Proto-Germanic]] {{lang|gem-x-proto|*watar}} (source also of [[Old Saxon]] {{Lang|osx|watar}}, [[Old Frisian]] {{Lang|ofs|wetir}}, [[Dutch language|Dutch]] {{Lang|nl|water}}, [[Old High German]] {{Lang|goh|wazzar}}, [[German language|German]] {{Lang|de|Wasser}}, {{Lang|non|vatn}}, [[Gothic language|Gothic]] {{Lang|got|𐍅𐌰𐍄𐍉}} ({{transliteration|got|wato}})), from [[Proto-Indo-European language|Proto-Indo-European]] {{lang|ine-x-proto|*wod-or}}, suffixed form of root {{lang|ine-x-proto|*wed-}} ({{gloss|water}}; {{gloss|wet}}).<ref>{{cite web |url=http://www.etymonline.com/index.php?allowed_in_frame=0&search=water |title=Water (v.) |author=<!--Not stated--> |website=www.etymonline.com |publisher=Online Etymology Dictionary |access-date=20 May 2017 |archive-url=https://web.archive.org/web/20170802204905/http://www.etymonline.com/index.php?allowed_in_frame=0&search=water |archive-date=2 August 2017 |url-status=live }}</ref> Also [[cognate]], through the Indo-European root, with [[Greek language|Greek]] {{Lang|el|ύδωρ}} ({{transliteration|el|ýdor}}; from Ancient Greek {{Lang|grc|ὕδωρ}} ({{transl|grc|hýdōr}}), whence English {{gloss|hydro-}}), [[Russian language|Russian]] {{Lang|ru|вода́}} ({{transliteration|ru|vodá}}), [[Irish language|Irish]] {{Lang|ga|uisce}}, and [[Albanian language|Albanian]] {{Lang|sq|ujë}}.

==History==
{{main|Origin of water on Earth#History of water on Earth|Properties of water#History}}
===On Earth===
{{excerpt|Origin of water on Earth|section =History of water on Earth}}

==Properties==
{{Main|Properties of water}}
{{see also||Water (data page)|Water model}}
[[File:Water molecule (1).svg|thumb|right|A water molecule consists of two hydrogen atoms and one oxygen atom.]]
Water ({{chem2|H2O|auto=1}}) is a [[Chemical polarity|polar]] [[inorganic compound]]. At [[room temperature]] it is a [[taste]]less and [[odor]]less [[liquid]], nearly [[Transparency and translucency|colorless]] with a [[Color of water|hint of blue]].<!--please read the article before considering removing it.--> This simplest [[hydrogen chalcogenide]] is by far the most studied chemical compound and is described as the "universal solvent" for its ability to dissolve many substances.<ref>{{Greenwood&Earnshaw2nd|page=620}}</ref><ref>{{cite web |title=Water, the Universal Solvent |url=http://water.usgs.gov/edu/solvent.html |website=[[USGS]] |access-date=27 June 2017 |archive-url=https://web.archive.org/web/20170709141251/https://water.usgs.gov/edu/solvent.html |archive-date=9 July 2017 |url-status=live }}</ref> This allows it to be the "[[solvent]] of life":<ref>{{Cite book |title=Campbell Biology |last=Reece |first=Jane B. |date=2013 |publisher=[[Pearson Education|Pearson]] |isbn=978-0-321-77565-8 |edition=10th |page=48 }}</ref> indeed, water as found in nature almost always includes various dissolved substances, and special steps are required to obtain chemically [[pure water]]. Water is the only common substance to exist as a [[solid]], liquid, and [[gas]] in normal terrestrial conditions.<ref>{{Cite book |title=Campbell Biology |last=Reece |first=Jane B. |year=2013 |publisher=[[Pearson Education|Pearson]] |isbn=978-0-321-77565-8 |edition=10th |page=44 }}</ref>

===States===
[[File: States of Matter.svg|thumb|The three common states of matter]]
Along with ''oxidane'', ''water'' is one of the two official names for the chemical compound {{chem|H|2|O}};<ref>{{Cite book|url=http://old.iupac.org/publications/books/principles/principles_of_nomenclature.pdf |title=Principles of chemical nomenclature: a guide to IUPAC recommendations |last1=Leigh |first1=G. J. |last2 = Favre| first2 = H. A|last3 = Metanomski|first3 = W. V.|date=1998 |publisher=Blackwell Science|location=Oxford|oclc=37341352|isbn=978-0-86542-685-6|url-status=dead |archive-url=https://web.archive.org/web/20110726171925/http://old.iupac.org/publications/books/principles/principles_of_nomenclature.pdf |archive-date=2011-07-26}}</ref> it is also the liquid phase of {{chem|H|2|O}}.<ref name=pubchem>{{cite web |last1=PubChem |title=Water |url=https://pubchem.ncbi.nlm.nih.gov/compound/Water |publisher=National Center for Biotechnology Information |access-date=25 March 2020 |language=en |archive-date=3 August 2018 |archive-url=https://web.archive.org/web/20180803194841/https:m//pubchem.ncbi.nlm.nih.gov/compound/water |url-status=live }}</ref> The other two common [[states of matter]] of water are the solid phase, [[ice]], and the gaseous phase, [[water vapor]] or [[steam]]. The addition or removal of heat can cause [[phase transition]]s: [[freezing]] (water to ice), [[melting]] (ice to water), [[vaporization]] (water to vapor), [[condensation]] (vapor to water), [[sublimation (phase transition)|sublimation]] (ice to vapor) and [[Deposition (phase transition)|deposition]] (vapor to ice).<ref name=Belnay>{{cite web |last1=Belnay |first1=Louise |title=The water cycle |url=https://www.esrl.noaa.gov/gmd/education/info_activities/pdfs/Teacher_CTA_the_water_cycle.pdf |website=Critical thinking activities |publisher=Earth System Research Laboratory |access-date=25 March 2020 |archive-date=20 September 2020 |archive-url=https://web.archive.org/web/20200920152817/https://www.esrl.noaa.gov/gmd/education/info_activities/pdfs/Teacher_CTA_the_water_cycle.pdf |url-status=live }}</ref>

==== Density ====
{{See also|Frost weathering}}
Water differs from most liquids in that it becomes less [[density|dense]] as it freezes.{{efn|Other substances with this property include [[bismuth]], [[silicon]], [[germanium]] and [[gallium]].<ref name=Oliveira/>}} In 1&nbsp;atm pressure, it reaches its maximum density of {{convert|999.972|kg/m3|lb/cuft|sigfig=6|abbr=on}} at {{convert|3.98|°C}}, or almost {{convert|1000|kg/m3|lb/cuft|sigfig=4|abbr=on}} at almost {{convert|4|°C}}.<ref>{{cite web |title=What is Density? |url=https://www.mt.com/sg/en/home/applications/Application_Browse_Laboratory_Analytics/Density/density-measurement.html |website=Mettler Toledo |access-date=11 November 2022 |archive-date=11 November 2022 |archive-url=https://web.archive.org/web/20221111064630/https://www.mt.com/sg/en/home/applications/Application_Browse_Laboratory_Analytics/Density/density-measurement.html |url-status=live }}</ref><ref name="NatureWaterStructure">{{cite journal |url=https://www.academia.edu/2230441 |title=Water: Water—an enduring mystery |access-date=15 November 2016 |journal=Nature |volume=452 |issue=7185 |pages=291–2 |archive-url=https://web.archive.org/web/20161117211552/http://www.academia.edu/2230441/Water_Water_an_enduring_mystery |archive-date=17 November 2016 |url-status=live |bibcode=2008Natur.452..291B |last1=Ball |first1=Philip |year=2008 |doi=10.1038/452291a |pmid=18354466 |s2cid=4365814 |doi-access=free }}</ref> The density of ice is {{convert|917|kg/m3|lb/cuft|sigfig=4|abbr=on}}, an expansion of 9%.<ref>{{cite book |last1=Kotz |first1=J. C. |last2=Treichel |first2=P. |last3=Weaver |first3=G. C. |year=2005 |title=Chemistry & Chemical Reactivity |publisher=Thomson Brooks/Cole |isbn=978-0-534-39597-1}}</ref><ref>{{cite book |last1=Ben-Naim |first1=Ariel |display-authors=etal |title=Alice's Adventures in Water-land |year=2011 |doi=10.1142/8068 |last2=Ben-Naim |first2=Roberta |isbn=978-981-4338-96-7}}</ref> This expansion can exert enormous pressure, bursting pipes and cracking rocks.<ref name="MM">{{cite journal |last1=Matsuoka |first1=N. |last2=Murton |first2=J. |title=Frost weathering: recent advances and future directions |journal=Permafrost and Periglacial Processes |volume=19 |issue= 2|pages=195–210 |year=2008 |doi=10.1002/ppp.620 |bibcode=2008PPPr...19..195M |s2cid=131395533 }}</ref>

In a lake or ocean, water at {{cvt|4|C|F}} sinks to the bottom, and ice forms on the surface, floating on the liquid water. This ice insulates the water below, preventing it from freezing solid. Without this protection, most aquatic organisms residing in lakes would perish during the winter.<ref>{{cite web |last1=Wiltse |first1=Brendan |title=A Look Under The Ice: Winter Lake Ecology |url=https://www.ausableriver.org/blog/look-under-ice-winter-lake-ecology |website=Ausable River Association |access-date=23 April 2020 |archive-date=19 June 2020 |archive-url=https://web.archive.org/web/20200619081813/https://www.ausableriver.org/blog/look-under-ice-winter-lake-ecology |url-status=live }}</ref>

==== Magnetism ====
Water is a [[Diamagnetism|diamagnetic]] material.<ref name="Chen-2010">{{Cite web|last=Chen|first=Zijun|date=2010-04-21|title=Measurement of Diamagnetism in Water|url=http://conservancy.umn.edu/handle/11299/90865|language=en-US|journal=|hdl=11299/90865 |access-date=8 January 2022|archive-date=8 January 2022|archive-url=https://web.archive.org/web/20220108015508/https://conservancy.umn.edu/handle/11299/90865|url-status=live}}</ref> Though interaction is weak, with superconducting magnets it can attain a notable interaction.<ref name="Chen-2010" />

==== Phase transitions ====
At a pressure of one [[Standard atmosphere (unit)|atmosphere]] (atm), ice melts or water freezes (solidifies) at {{cvt|0|C|}} and water boils or vapor condenses at {{cvt|100|C|F}}. However, even below the boiling point, water can change to vapor at its surface by [[evaporation]] (vaporization throughout the liquid is known as [[boiling]]). Sublimation and deposition also occur on surfaces.<ref name=Belnay/> For example, [[frost]] is deposited on cold surfaces while [[snowflake]]s form by deposition on an aerosol particle or ice nucleus.<ref>{{cite web |last1=Wells |first1=Sarah |title=The Beauty and Science of Snowflakes |url=https://ssec.si.edu/stemvisions-blog/beauty-and-science-snowflakes |website=Smithsonian Science Education Center |access-date=25 March 2020 |language=en |date=21 January 2017 |archive-date=25 March 2020 |archive-url=https://web.archive.org/web/20200325185513/https://ssec.si.edu/stemvisions-blog/beauty-and-science-snowflakes |url-status=live }}</ref> In the process of [[freeze-drying]], a food is frozen and then stored at low pressure so the ice on its surface sublimates.<ref name=FreezeDrying>{{Cite book|title=Food processing technology: principles and practice|last=Fellows|first=Peter|date=2017|publisher=Woodhead Publishing/Elsevier Science|isbn=978-0-08-100523-1|edition=4th|location=Kent|pages=929–940|chapter=Freeze drying and freeze concentration|oclc=960758611}}</ref>

The melting and boiling points depend on pressure. A good approximation for the rate of change of the melting temperature with pressure is given by the [[Clausius–Clapeyron relation]]:

<math display="block"> \frac{d T}{d P} = \frac{T \left(v_\text{L}-v_\text{S}\right) }{L_\text{f}} </math>

where <math>v_\text{L}</math> and <math>v_\text{S}</math> are the [[molar volume]]s of the liquid and solid phases, and <math>L_\text{f}</math> is the molar [[latent heat]] of melting. In most substances, the volume increases when melting occurs, so the melting temperature increases with pressure. However, because ice is less dense than water, the melting temperature decreases.<ref name=Oliveira>{{cite book |last1=Oliveira |first1=Mário J. de |title=Equilibrium Thermodynamics |date=2017 |publisher=Springer |isbn=978-3-662-53207-2 |pages=120–124 |url=https://books.google.com/books?id=F8GRDgAAQBAJ&dq=denser+liquid+than+solid+phase+water+silicon+bismuth&pg=PA122 |access-date=26 March 2020 |language=en |archive-date=8 March 2021 |archive-url=https://web.archive.org/web/20210308003011/https://www.google.com/books/edition/Equilibrium_Thermodynamics/F8GRDgAAQBAJ?hl=en&gbpv=1&dq=denser+liquid+than+solid+phase+water+silicon+bismuth&pg=PA122&printsec=frontcover |url-status=live }}</ref> In glaciers, [[pressure melting point|pressure melting]] can occur under sufficiently thick volumes of ice, resulting in [[subglacial lake]]s.<ref>{{cite journal |last1=Siegert |first1=Martin J. |last2=Ellis-Evans |first2=J. Cynan |last3=Tranter |first3=Martyn |last4=Mayer |first4=Christoph |last5=Petit |first5=Jean-Robert |last6=Salamatin |first6=Andrey |last7=Priscu |first7=John C. |title=Physical, chemical and biological processes in Lake Vostok and other Antarctic subglacial lakes |journal=Nature |date=December 2001 |volume=414 |issue=6864 |pages=603–609 |doi=10.1038/414603a|pmid=11740551 |bibcode=2001Natur.414..603S |s2cid=4423510 }}</ref><ref>{{cite web |last1=Davies |first1=Bethan |title=Antarctic subglacial lakes |url=http://www.antarcticglaciers.org/glacier-processes/glacial-lakes/subglacial-lakes/ |website=AntarcticGlaciers |access-date=25 March 2020 |archive-date=3 October 2020 |archive-url=https://web.archive.org/web/20201003171536/http://www.antarcticglaciers.org/glacier-processes/glacial-lakes/subglacial-lakes/ |url-status=live }}</ref>

The Clausius-Clapeyron relation also applies to the boiling point, but with the liquid/gas transition the vapor phase has a much lower density than the liquid phase, so the boiling point increases with pressure.<ref>{{cite book |last1=Masterton |first1=William L. |last2=Hurley |first2=Cecile N. |title=Chemistry: principles and reactions |date=2008 |publisher=Cengage Learning |isbn=978-0-495-12671-3 |page=230 |edition=6th |url=https://books.google.com/books?id=teubNK-b2bsC&q=clapeyron%20equation%20boiling |access-date=3 April 2020 |archive-date=8 March 2021 |archive-url=https://web.archive.org/web/20210308080844/https://www.google.com/books/edition/Chemistry_Principles_and_Reactions/teubNK-b2bsC?hl=en&gbpv=1&bsq=clapeyron%20equation%20boiling |url-status=live }}</ref> Water can remain in a liquid state at high temperatures in the deep ocean or underground. For example, temperatures exceed {{convert|205|C}} in [[Old Faithful]], a geyser in [[Yellowstone National Park]].<ref>{{cite web |last1=Peaco |first1=Jim |title=Yellowstone Lesson Plan: How Yellowstone Geysers Erupt |location=Yellowstone National Park |publisher=U.S. National Park Service |url=https://www.nps.gov/yell/learn/education/classrooms/how-yellowstone-geysers-erupt.htm |access-date=5 April 2020 |language=en |archive-date=2 March 2020 |archive-url=https://web.archive.org/web/20200302093350/https://www.nps.gov/yell/learn/education/classrooms/how-yellowstone-geysers-erupt.htm |url-status=live }}</ref> In [[hydrothermal vent]]s, the temperature can exceed {{convert|400|C}}.<ref>{{cite news |last1=Brahic |first1=Catherine |title=Found: The hottest water on Earth |url=https://www.newscientist.com/article/dn14456-found-the-hottest-water-on-earth/ |access-date=5 April 2020 |work=New Scientist |archive-date=9 May 2020 |archive-url=https://web.archive.org/web/20200509103747/https://www.newscientist.com/article/dn14456-found-the-hottest-water-on-earth/ |url-status=live }}</ref>

At [[sea level]], the boiling point of water is {{convert|100|C}}. As atmospheric pressure decreases with altitude, the boiling point decreases by 1&nbsp;°C every 274&nbsp;meters. [[High-altitude cooking]] takes longer than sea-level cooking. For example, at {{convert|1524|m}}, cooking time must be increased by a fourth to achieve the desired result.<ref>{{cite web |last1=USDA Food Safety and Inspection Service |title=High Altitude Cooking and Food Safety |url=https://www.fsis.usda.gov/shared/PDF/High_Altitude_Cooking_and_Food_Safety.pdf |access-date=5 April 2020 |archive-date=20 January 2021 |archive-url=https://web.archive.org/web/20210120010850/https://www.fsis.usda.gov/shared/PDF/High_Altitude_Cooking_and_Food_Safety.pdf |url-status=dead }}</ref> Conversely, a [[pressure cooker]] can be used to decrease cooking times by raising the boiling temperature.<ref>{{cite web |title=Pressure Cooking – Food Science |url=https://www.exploratorium.edu/food/pressure-cooking |website=Exploratorium |language=en |date=26 September 2019 |access-date=21 April 2020 |archive-date=19 June 2020 |archive-url=https://web.archive.org/web/20200619044746/https://www.exploratorium.edu/food/pressure-cooking |url-status=live }}</ref> In a vacuum, water will boil at room temperature.<ref>{{cite news |last1=Allain |first1=Rhett |title=Yes, You Can Boil Water at Room Temperature. Here's How |url=https://www.wired.com/story/yes-you-can-boil-water-at-room-temperature-heres-how/ |access-date=5 April 2020 |magazine=Wired |date=12 September 2018 |language=en |archive-date=28 September 2020 |archive-url=https://web.archive.org/web/20200928044101/https://www.wired.com/story/yes-you-can-boil-water-at-room-temperature-heres-how/ |url-status=live }}</ref>

==== Triple and critical points ====
[[File:Phase diagram of water simplified.svg|thumb|Phase diagram of water (simplified)]]
On a pressure/temperature [[phase diagram]] (see figure), there are curves separating solid from vapor, vapor from liquid, and liquid from solid. These meet at a single point called the [[triple point]], where all three phases can coexist. The triple point is at a temperature of {{convert|273.16|K|C F}} and a pressure of {{convert|611.657|Pa|atm psi|sigfig=3}};<ref>{{cite journal |last1=Murphy |first1=D. M. |last2=Koop |first2=T. |title=Review of the vapour pressures of ice and supercooled water for atmospheric applications |journal=Quarterly Journal of the Royal Meteorological Society |date=1 April 2005 |volume=131 |issue=608 |pages=1540 |doi=10.1256/qj.04.94 |bibcode=2005QJRMS.131.1539M |s2cid=122365938 |url=https://zenodo.org/record/1236243 |access-date=31 August 2020 |archive-date=18 August 2020 |archive-url=https://web.archive.org/web/20200818105335/https://zenodo.org/record/1236243 |url-status=live |doi-access=free }}</ref> it is the lowest pressure at which liquid water can exist. [[2019 redefinition of the SI base units|Until 2019]], the triple point was used to define the [[Kelvin|Kelvin temperature scale]].<ref>{{cite book |author=International Bureau of Weights and Measures |author-link=International Bureau of Weights and Measures |date=2006 |url=http://www.bipm.org/utils/common/pdf/si_brochure_8_en.pdf |url-status=live |archive-url=https://web.archive.org/web/20170814094625/http://www.bipm.org/utils/common/pdf/si_brochure_8_en.pdf |archive-date=2017-08-14 |title=The International System of Units (SI) |edition=8th |isbn=92-822-2213-6 |page=114|publisher=Bureau International des Poids et Mesures }}</ref><ref name=Brochure9_2019>{{cite web |title = 9th edition of the SI Brochure |publisher = BIPM |url = https://www.bipm.org/en/publications/si-brochure/ |date = 2019 |access-date = 20 May 2019 |df = dmy-all |archive-date = 19 April 2021 |archive-url = https://web.archive.org/web/20210419211921/https://www.bipm.org/en/publications/si-brochure |url-status = live }}</ref>

The water/vapor phase curve terminates at {{convert|647.096|K|C F}} and {{convert|22.064|MPa|psi atm}}.<ref name=IAPWS95>{{cite journal |last1=Wagner |first1=W. |last2=Pruß |first2=A. |title=The IAPWS Formulation 1995 for the Thermodynamic Properties of Ordinary Water Substance for General and Scientific Use |journal=Journal of Physical and Chemical Reference Data |date=June 2002 |volume=31 |issue=2 |page=398 |doi=10.1063/1.1461829}}</ref> This is known as the [[critical point (thermodynamics)|critical point]]. At higher temperatures and pressures the liquid and vapor phases form a continuous phase called a [[supercritical fluid]]. It can be gradually compressed or expanded between gas-like and liquid-like densities; its properties (which are quite different from those of ambient water) are sensitive to density. For example, for suitable pressures and temperatures it can [[miscibility|mix freely]] with [[Nonpolar molecule|nonpolar compounds]], including most [[organic compound]]s. This makes it useful in a variety of applications including high-temperature [[electrochemistry]] and as an ecologically benign solvent or [[catalysis|catalyst]] in chemical reactions involving organic compounds. In Earth's mantle, it acts as a solvent during mineral formation, dissolution and deposition.<ref>{{cite journal |last1=Weingärtner |first1=Hermann |last2=Franck |first2=Ernst Ulrich |title=Supercritical Water as a Solvent |journal=Angewandte Chemie International Edition |date=29 April 2005 |volume=44 |issue=18 |pages=2672–2692 |doi=10.1002/anie.200462468|pmid=15827975 }}</ref><ref>{{cite journal |last1=Adschiri |first1=Tadafumi |last2=Lee |first2=Youn-Woo |last3=Goto |first3=Motonobu |last4=Takami |first4=Seiichi |title=Green materials synthesis with supercritical water |journal=Green Chemistry |date=2011 |volume=13 |issue=6 |pages=1380 |doi=10.1039/c1gc15158d}}</ref>

==== Phases of ice and water ====
{{main|Ice}}
The normal form of ice on the surface of Earth is [[Ice Ih|ice I<sub>h</sub>]], a phase that forms crystals with [[Hexagonal crystal family|hexagonal symmetry]]. Another with [[Cubic crystal system|cubic crystalline symmetry]], [[Ice Ic|ice I<sub>c</sub>]], can occur in the upper atmosphere.<ref>{{cite journal |last1=Murray |first1=Benjamin J.|last2=Knopf |first2=Daniel A. |last3=Bertram |first3=Allan K. |year=2005|title=The formation of cubic ice under conditions relevant to Earth's atmosphere|journal=Nature|volume=434|pages=202–205|doi=10.1038/nature03403|pmid=15758996|issue=7030|bibcode=2005Natur.434..202M|s2cid=4427815}}</ref> As the pressure increases, ice forms other [[crystal structure]]s. As of 2024, twenty have been experimentally confirmed and several more are predicted theoretically.<ref>{{cite journal |last1=Salzmann |first1=Christoph G. |title=Advances in the experimental exploration of water's phase diagram |journal=The Journal of Chemical Physics |date=14 February 2019 |volume=150 |issue=6 |pages=060901 |doi=10.1063/1.5085163|pmid=30770019 |arxiv=1812.04333 |bibcode=2019JChPh.150f0901S |doi-access=free }}</ref> The eighteenth form of ice, [[ice XVIII]], a face-centred-cubic, superionic ice phase, was discovered when a droplet of water was subject to a shock wave that raised the water's pressure to millions of atmospheres and its temperature to thousands of degrees, resulting in a structure of rigid oxygen atoms in which hydrogen atoms flowed freely.<ref name="Sokol2021">{{cite magazine |url=https://www.wired.com/story/a-bizarre-form-of-water-may-exist-all-over-the-universe/ |title=A Bizarre Form of Water May Exist All Over the Universe |last=Sokol |first=Joshua |magazine=Wired |date=12 May 2019 |access-date=1 September 2021 |archive-url=https://web.archive.org/web/20190512130533/https://www.wired.com/story/a-bizarre-form-of-water-may-exist-all-over-the-universe/|archive-date=12 May 2019|url-status=live}}</ref><ref name="Millotetal2019">{{cite journal |last1=Millot |first1=M. |last2=Coppari |first2=F. |last3=Rygg |first3=J. R. |last4=Barrios |first4=Antonio Correa |last5=Hamel |first5=Sebastien |last6=Swift |first6=Damian C. |last7=Eggert |first7=Jon H. |year=2019 |title=Nanosecond X-ray diffraction of shock-compressed superionic water ice |journal=Nature |publisher=Springer |volume=569 |issue=7755 |pages=251–255 |doi=10.1038/s41586-019-1114-6 |pmid=31068720 |bibcode=2019Natur.569..251M |osti=1568026 |s2cid=148571419 |url=https://www.osti.gov/biblio/1568026 |access-date=5 March 2024 |archive-date=9 July 2023 |archive-url=https://web.archive.org/web/20230709172600/https://www.osti.gov/biblio/1568026 |url-status=live }}</ref> When sandwiched between layers of [[graphene]], ice forms a square lattice.<ref>{{cite journal |last1=Peplow |first1=Mark |title=Graphene sandwich makes new form of ice |journal=Nature |date=25 March 2015 |doi=10.1038/nature.2015.17175|s2cid=138877465 }}</ref>

The details of the chemical nature of liquid water are not well understood; some theories suggest that its unusual behaviour is due to the existence of two liquid states.<ref name="NatureWaterStructure" /><ref>{{Cite journal |last1=Maestro |first1=L. M. |last2=Marqués |first2=M. I. |last3=Camarillo |first3=E. |last4=Jaque |first4=D. |last5=Solé |first5=J. García |last6=Gonzalo |first6=J. A. |last7=Jaque |first7=F. |last8=Valle |first8=Juan C. Del |last9=Mallamace |first9=F. |date=1 January 2016 |title=On the existence of two states in liquid water: impact on biological and nanoscopic systems |journal=International Journal of Nanotechnology |volume=13 |issue=8–9 |pages=667–677 |doi=10.1504/IJNT.2016.079670 |bibcode=2016IJNT...13..667M |s2cid=5995302 |archive-url=https://web.archive.org/web/20231115003311/http://pdfs.semanticscholar.org/fc61/afe755fe34c5e163daa3c402bb8f03c40d7f.pdf |archive-date=15 November 2023 |url-status=live |url=http://pdfs.semanticscholar.org/fc61/afe755fe34c5e163daa3c402bb8f03c40d7f.pdf |access-date=5 March 2024 }}</ref><ref>{{cite journal |first1=Francesco |last1=Mallamace |first2=Carmelo |last2=Corsaro |first3=H. Eugene |last3=Stanley |title=A singular thermodynamically consistent temperature at the origin of the anomalous behavior of liquid water|journal=Scientific Reports |date=18 December 2012 |volume=2 |issue=1 |page=993 |doi=10.1038/srep00993 |pmid=23251779 |pmc=3524791 |bibcode=2012NatSR...2E.993M}}</ref><ref>{{Cite journal |last1=Perakis |first1=Fivos |last2=Amann-Winkel |first2=Katrin |last3=Lehmkühler |first3=Felix |last4=Sprung |first4=Michael |last5=Mariedahl |first5=Daniel |last6=Sellberg |first6=Jonas A. |last7=Pathak |first7=Harshad |last8=Späh |first8=Alexander |last9=Cavalca |first9=Filippo|last10=Ricci|first10=Alessandro |last11=Jain |first11=Avni |last12=Massani |first12=Bernhard |last13=Aubree |first13=Flora |last14=Benmore |first14=Chris J. |last15=Loerting|author15-link=Thomas Loerting |first15=Thomas |last16=Grübel |first16=Gerhard |last17=Pettersson |first17=Lars G. M. |last18=Nilsson |first18=Anders |date=26 June 2017 |title=Diffusive dynamics during the high-to-low density transition in amorphous ice |journal=Proceedings of the National Academy of Sciences of the United States of America |volume=13 |issue=8–9 |pages=667–677 |doi=10.1073/pnas.1705303114|pmc=5547632 |pmid=28652327|bibcode=2017PNAS..114.8193P |doi-access=free }}</ref>

===Taste and odor===
Pure water is usually described as tasteless and odorless, although [[humans]] have specific sensors that can feel the presence of water in their mouths,<ref name="pmid28553944">{{cite journal | vauthors = Zocchi D, Wennemuth G, Oka Y | title = The cellular mechanism for water detection in the mammalian taste system | journal = Nature Neuroscience | volume = 20 | issue = 7 | pages = 927–933 | date = July 2017 | pmid = 28553944 | doi = 10.1038/nn.4575 | s2cid = 13263401 | url = https://authors.library.caltech.edu/77104/6/nn.4575-S2.pdf | access-date = 27 January 2024 | archive-date = 5 March 2024 | archive-url = https://web.archive.org/web/20240305154837/https://s3.us-west-2.amazonaws.com/caltechauthors/99/15/d0ca-f08f-4315-b32e-c758f8dd1cc8/data?response-content-type=application%2Foctet-stream&response-content-disposition=attachment%3B%20filename%3Dnn.4575-S2.pdf&X-Amz-Algorithm=AWS4-HMAC-SHA256&X-Amz-Credential=AKIARCVIVNNAKP37N3MU%2F20240305%2Fus-west-2%2Fs3%2Faws4_request&X-Amz-Date=20240305T154835Z&X-Amz-Expires=60&X-Amz-SignedHeaders=host&X-Amz-Signature=c12110c390e86eaaada9c08cfa75fbc87beb2c703250bafb9358fda4dfc2acf4 | url-status = live }}</ref><ref name=emo>Edmund T. Rolls (2005). ''Emotion Explained''. Oxford University Press, Medical. {{ISBN|978-0198570035}}.</ref> and frogs are known to be able to smell it.<ref name=frog>R. Llinas, W. Precht (2012), ''Frog Neurobiology: A Handbook''. Springer Science & Business Media. {{ISBN|978-3642663161}}</ref> However, water from ordinary sources (including [[mineral water]]) usually has many dissolved substances that may give it varying tastes and odors. Humans and other animals have developed senses that enable them to evaluate the [[potability]] of water in order to avoid water that is too salty or [[putrid]].<ref name=candau>{{cite journal |last1=Candau |first1=Joël |year=2004 |title=The Olfactory Experience: constants and cultural variables |url=https://halshs.archives-ouvertes.fr/halshs-00130924 |journal=Water Science and Technology |volume=49 |issue=9 |pages=11–17 |access-date=28 September 2016 |archive-url=https://web.archive.org/web/20161002152229/https://halshs.archives-ouvertes.fr/halshs-00130924 |archive-date=2 October 2016 |url-status=live |doi=10.2166/wst.2004.0522 |pmid=15237601 }}</ref>

===Color and appearance===
{{Main|Color of water}}
{{See also|Electromagnetic absorption by water}}
Pure water is [[Visual perception|visibly]] blue due to [[electromagnetic absorption by water|absorption]] of light in the region c. 600–800&nbsp;nm.<ref>{{cite journal |last=Braun |first=Charles L. |author2=Sergei N. Smirnov |title=Why is water blue? |journal=Journal of Chemical Education |volume=70 |issue=8 |page=612 |year=1993 |url=http://www.dartmouth.edu/~etrnsfer/water.htm |doi=10.1021/ed070p612 |bibcode=1993JChEd..70..612B |access-date=21 April 2007 |archive-url=https://web.archive.org/web/20120320060654/http://www.dartmouth.edu/~etrnsfer/water.htm |archive-date=20 March 2012 |url-status=live |url-access=subscription }}</ref> The color can be easily observed in a glass of tap-water placed against a pure white background, in daylight. The principal absorption bands responsible for the color are [[Overtone band|overtone]]s of the O–H stretching [[Molecular vibration|vibrations]]. The apparent intensity of the color increases with the depth of the water column, following [[Beer's law]]. This also applies, for example, with a swimming pool when the light source is sunlight reflected from the pool's white tiles.

In nature, the color may also be modified from blue to green due to the presence of suspended solids or algae.

In industry, [[near-infrared spectroscopy]] is used with aqueous solutions as the greater intensity of the lower overtones of water means that glass [[cuvette]]s with short path-length may be employed. To observe the fundamental stretching absorption spectrum of water or of an aqueous solution in the region around 3,500&nbsp;cm{{sup|−1}} (2.85&nbsp;μm)<ref>{{cite book |last1=Nakamoto |first1=Kazuo |title=Infrared and Raman Spectra of Inorganic and Coordination Compounds, Part A: Theory and Applications in Inorganic Chemistry |date=1997 |publisher=Wiley |location=New York |isbn=0-471-16394-5 |page=170 |edition=5th}}</ref> a path length of about 25&nbsp;μm is needed. Also, the cuvette must be both transparent around 3500&nbsp;cm{{sup|−1}} and insoluble in water; [[calcium fluoride]] is one material that is in common use for the cuvette windows with aqueous solutions.

The [[Raman spectroscopy|Raman-active]] fundamental vibrations may be observed with, for example, a 1&nbsp;cm sample cell.

[[Aquatic plant]]s, [[algae]], and other [[Photosynthesis|photosynthetic]] organisms can live in water up to hundreds of meters deep, because [[sunlight]] can reach them.
Practically no sunlight reaches the parts of the oceans below {{convert|1000|m|sp=us}} of depth.

The [[refractive index]] of liquid water (1.333 at {{convert|20|C}}) is much higher than that of air (1.0), similar to those of [[alkane]]s and [[ethanol]], but lower than those of [[glycerol]] (1.473), [[benzene]] (1.501), [[carbon disulfide]] (1.627), and common types of glass (1.4 to 1.6). The refraction index of ice (1.31) is lower than that of liquid water.

=== Molecular polarity ===
[[File:Tetrahedral Structure of Water.png|thumb|Tetrahedral structure of water]]
In a water molecule, the hydrogen atoms form a 104.5° angle with the oxygen atom. The hydrogen atoms are close to two corners of a tetrahedron centered on the oxygen. At the other two corners are ''[[lone pairs]]'' of valence electrons that do not participate in the bonding. In a perfect tetrahedron, the atoms would form a 109.5° angle, but the repulsion between the lone pairs is greater than the repulsion between the hydrogen atoms.<ref>{{harvnb|Ball|2001|p=168}}</ref><ref>{{harvnb|Franks|2007|p=10}}</ref> The O–H bond length is about 0.096&nbsp;nm.<ref>{{cite web |title=Physical Chemistry of Water |url=https://msu.edu/course/css/850/snapshot.afs/teppen/physical_chemistry_of_water.htm |publisher=Michigan State University |access-date=11 September 2020 |archive-date=20 October 2020 |archive-url=https://web.archive.org/web/20201020055601/https://msu.edu/course/css/850/snapshot.afs/teppen/physical_chemistry_of_water.htm |url-status=live }}</ref>

Other substances have a tetrahedral molecular structure, for example [[methane]] ({{chem|C|H|4}}) and [[hydrogen sulfide]] ({{chem|H|2|S}}). However, oxygen is more [[electronegativity|electronegative]] than most other elements, so the oxygen atom has a negative partial charge while the hydrogen atoms are partially positively charged. Along with the bent structure, this gives the molecule an [[electrical dipole moment]] and it is classified as a [[polar molecule]].<ref>{{harvnb|Ball|2001|p=169}}</ref>

Water is a good polar [[solvent]], dissolving many [[salt (chemistry)|salts]] and [[hydrophilic]] organic molecules such as sugars and simple alcohols such as [[ethanol]]. Water also dissolves many gases, such as oxygen and [[carbon dioxide]]—the latter giving the fizz of [[carbonation|carbonated]] beverages, [[sparkling wine]]s and beers. In addition, many substances in living organisms, such as [[protein]]s, [[DNA]] and [[polysaccharide]]s, are dissolved in water. The interactions between water and the subunits of these biomacromolecules shape [[protein folding]], [[Base pairing|DNA base pairing]], and other phenomena crucial to life ([[hydrophobic effect]]).

Many organic substances (such as [[lipids|fats and oils]] and [[alkanes]]) are [[hydrophobic]], that is, insoluble in water. Many inorganic substances are insoluble too, including most metal [[oxide]]s, [[sulfide]]s, and [[silicate]]s.

===Hydrogen bonding===
{{See also|Chemical bonding of water}}
[[File:3D model hydrogen bonds in water.svg|thumb|Model of [[hydrogen bond]]s (1) between molecules of water]]

Because of its polarity, a molecule of water in the liquid or solid state can form up to four [[hydrogen bonds]] with neighboring molecules. Hydrogen bonds are about ten times as strong as the [[Van der Waals force]] that attracts molecules to each other in most liquids. This is the reason why the melting and boiling points of water are much higher than those of [[Hydrogen chalcogenide|other analogous compounds]] like hydrogen sulfide. They also explain its exceptionally high [[specific heat capacity]] (about 4.2 [[Joule|J]]/(g·K)), [[heat of fusion]] (about 333 J/g), [[heat of vaporization]] ({{nowrap|2257 J/g}}), and [[thermal conductivity]] (between 0.561 and 0.679 W/(m·K)). These properties make water more effective at moderating Earth's [[climate]], by storing heat and transporting it between the oceans and the atmosphere. The hydrogen bonds of water are around 23 kJ/mol (compared to a covalent O-H bond at 492 kJ/mol). Of this, it is estimated that 90% is attributable to electrostatics, while the remaining 10% is partially covalent.<ref>{{Cite journal |date=1 March 2000 |title=Compton scattering evidence for covalency of the hydrogen bond in ice|journal=Journal of Physics and Chemistry of Solids |volume=61 |issue=3 |pages=403–406 |doi=10.1016/S0022-3697(99)00325-X |last1=Isaacs |first1=E. D. |last2=Shukla |first2=A |last3=Platzman |first3=P. M. |last4=Hamann |first4=D. R. |last5=Barbiellini |first5=B. |last6=Tulk |first6=C. A. |bibcode=2000JPCS...61..403I}}</ref>

These bonds are the cause of water's high [[surface tension]]<ref>{{cite book |last1=Campbell |first1=Neil A. |first2=Brad |last2=Williamson |first3=Robin J. |last3=Heyden |title=Biology: Exploring Life |publisher=Pearson Prentice Hall |year=2006 |location=Boston, Massachusetts |url=http://www.phschool.com/el_marketing.html |isbn=978-0-13-250882-7 |access-date=11 November 2008 |archive-url=https://web.archive.org/web/20141102041816/http://www.phschool.com/el_marketing.html |archive-date=2 November 2014 |url-status=live }}</ref> and capillary forces. The [[capillary action]] refers to the tendency of water to move up a narrow tube against the force of [[gravity]]. This property is relied upon by all [[vascular plant]]s, such as trees.{{Citation needed|date=August 2022}}

[[File:Heat capacity of water 2.jpg|thumb|upright=1.4|Specific heat capacity of water<ref>{{Cite web |title=Heat capacity water online |url=https://www.desmos.com/calculator/wicmrvrznj?lang=ru |access-date=2022-06-03 |website=Desmos |language=ru |archive-date=6 June 2022 |archive-url=https://web.archive.org/web/20220606020344/https://www.desmos.com/calculator/wicmrvrznj?lang=ru |url-status=live }}</ref>]]

===Self-ionization===
{{main|Self-ionization of water}}

Water is a weak solution of hydronium hydroxide—there is an equilibrium {{Nowrap|{{chem|2H|2|O}} ⇌ {{chem|H|3|O|+}} + {{chem|OH|-}}}}, in combination with solvation of the resulting [[hydronium]] and [[hydroxide]] ions.

===Electrical conductivity and electrolysis===
Pure water has a low [[electrical conductivity]], which increases with the [[dissolution (chemistry)|dissolution]] of a small amount of ionic material such as [[sodium chloride|common salt]].

Liquid water can be split into the [[Chemical element|elements]] hydrogen and oxygen by passing an electric current through it—a process called [[Electrolysis of water|electrolysis]]. The decomposition requires more energy input than the [[standard enthalpy of formation|heat released by the inverse process]] (285.8 kJ/[[mole (unit)|mol]], or 15.9 MJ/kg).<ref>{{cite journal |last=Ball |first=Philip |author-link=Philip Ball |title=Burning water and other myths |url=http://www.nature.com/news/2007/070910/full/070910-13.html |journal=News@nature |date=14 September 2007 |access-date=14 September 2007 |archive-url=https://web.archive.org/web/20090228054247/http://www.nature.com/news/2007/070910/full/070910-13.html |archive-date=28 February 2009 |url-status=live |doi=10.1038/news070910-13 |s2cid=129704116 |doi-access=free }}</ref>

===Mechanical properties===
Liquid water can be assumed to be incompressible for most purposes: its compressibility ranges from 4.4 to {{val|5.1|e=-10|u=Pa<sup>−1</sup>}} in ordinary conditions.<ref>{{cite journal |last1=Fine |first1=R. A. |last2=Millero |first2=F. J.|date=1973 |title=Compressibility of water as a function of temperature and pressure |volume=59 |issue=10 |page=5529 |journal=Journal of Chemical Physics |doi=10.1063/1.1679903 |bibcode=1973JChPh..59.5529F}}</ref> Even in oceans at 4&nbsp;km depth, where the pressure is 400 atm, water suffers only a 1.8% decrease in volume.<ref name=nave>{{cite web |title=Bulk Elastic Properties |last=Nave |first=R. |website=HyperPhysics |publisher=[[Georgia State University]] |url=http://hyperphysics.phy-astr.gsu.edu/hbase/hph.html |access-date=26 October 2007 |archive-url=https://web.archive.org/web/20071028155517/http://hyperphysics.phy-astr.gsu.edu/hbase/hph.html |archive-date=28 October 2007 |url-status=live }}</ref>

The [[viscosity]] of water is about 10{{sup|−3}} Pa·[[second|s]] or 0.01 [[Poise (unit)|poise]] at {{convert|20|C}}, and the [[speed of sound]] in liquid water ranges between {{convert|1400|and|1540|m/s|sp=us}} depending on temperature. Sound travels long distances in water with little [[attenuation]], especially at low frequencies (roughly 0.03 [[decibel|dB]]/km for 1 k[[hertz|Hz]]), a property that is exploited by [[cetaceans]] and humans for communication and environment sensing ([[sonar]]).<ref name=NPLcalc>UK National Physical Laboratory, [http://resource.npl.co.uk/acoustics/techguides/seaabsorption/ Calculation of absorption of sound in seawater] {{Webarchive|url=https://web.archive.org/web/20161003014920/http://resource.npl.co.uk/acoustics/techguides/seaabsorption/ |date=3 October 2016 }}. Online site, last accessed on 28 September 2016.</ref>

===Reactivity===
Metallic elements which are more [[Electronegativity|electropositive]] than hydrogen, particularly the [[alkali metals]] and [[alkaline earth metals]] such as [[lithium]], [[sodium]], [[calcium]], [[potassium]] and [[Caesium|cesium]] displace hydrogen from water, forming [[hydroxide]]s and releasing hydrogen. At high temperatures, carbon reacts with steam to form [[carbon monoxide]] and hydrogen.{{citation needed|date=November 2023}}

==On Earth==
{{Main|Hydrology|Water distribution on Earth}}
<!-- Commented out because image was deleted: [[File:Water Distribution.jpg|thumb|Graphical illustration of the Earth's relative water distribution at various locations on or near its surface<ref name="Garrison">{{cite book |author=Tom Garrison |title=Oceanography: An Invitation to Marine Science |edition=7th |publisher=Yolanda Cossio |year=2009 |isbn=978-0-495-39193-7}}</ref>]] -->

Hydrology is the study of the movement, distribution, and quality of water throughout the Earth. The study of the distribution of water is [[hydrography]]. The study of the distribution and movement of [[groundwater]] is [[hydrogeology]], of glaciers is [[glaciology]], of inland waters is [[limnology]] and distribution of oceans is [[oceanography]]. Ecological processes with hydrology are in the focus of [[ecohydrology]].

The collective mass of water found on, under, and over the surface of a planet is called the [[hydrosphere]]. Earth's approximate water volume (the total water supply of the world) is {{convert|1.386|e9km3|e6mi3|abbr=off}}.<ref name=b1 />

Liquid water is found in [[body of water|bodies of water]], such as an ocean, sea, lake, river, stream, [[canal]], pond, or [[puddle]]. The majority of water on Earth is [[seawater]]. Water is also present in the atmosphere in solid, liquid, and vapor states. It also exists as groundwater in [[aquifer]]s.

Water is important in many geological processes. Groundwater is present in most [[rock (geology)|rocks]], and the pressure of this groundwater affects patterns of [[fault (geology)|faulting]]. Water in the [[Mantle (geology)|mantle]] is responsible for the melt that produces [[volcano]]es at [[subduction zone]]s. On the surface of the Earth, water is important in both chemical and physical [[weathering]] processes. Water, and to a lesser but still significant extent, ice, are also responsible for a large amount of [[sediment transport]] that occurs on the surface of the earth. [[Deposition (geology)|Deposition]] of transported sediment forms many types of [[sedimentary rock]]s, which make up the [[geologic record]] of [[History of the Earth|Earth history]].

===Water cycle===
{{Main|Water cycle}}
[[File:Water cycle.png|thumb|Water cycle]]

The water cycle (known scientifically as the hydrologic cycle) is the continuous exchange of water within the [[hydrosphere]], between the [[Earth atmosphere|atmosphere]], [[soil]] water, [[surface water]], groundwater, and plants.

Water moves perpetually through each of these regions in the ''water cycle'' consisting of the following transfer processes:
* [[evaporation]] from oceans and other water bodies into the air and [[transpiration]] from land plants and animals into the air.
* [[precipitation (meteorology)|precipitation]], from water vapor condensing from the air and falling to the earth or ocean.
* [[runoff (water)|runoff]] from the land usually reaching the sea.
Most water vapors found mostly in the ocean returns to it, but winds carry water vapor over land at the same rate as runoff into the sea, about 47&nbsp;[[Metric tonne unit|Tt]] per year whilst evaporation and transpiration happening in land masses also contribute another 72&nbsp;Tt per year. Precipitation, at a rate of 119 Tt per year over land, has several forms: most commonly rain, snow, and [[hail]], with some contribution from [[fog]] and [[dew]].<ref>{{cite book |title=Water in Crisis: A Guide to the World's Freshwater Resources |editor-last=Gleick |editor-first=P. H. |publisher=Oxford University Press |year=1993 |page=15, Table 2.3 |url=http://www.oup.com/us/catalog/general/subject/EarthSciences/Oceanography/?view=usa&ci=9780195076288 |url-status=dead |archive-url=https://web.archive.org/web/20130408091921/http://www.oup.com/us/catalog/general/subject/EarthSciences/Oceanography/?view=usa&ci=9780195076288 |archive-date=8 April 2013}}</ref> Dew is small drops of water that are condensed when a high density of water vapor meets a cool surface. Dew usually forms in the morning when the temperature is the lowest, just before sunrise and when the temperature of the earth's surface starts to increase.<ref>{{cite book |title=Alice's Adventures in Water-land |last1=Ben-Naim |first1=A. |last2=Ben-Naim |first2=R. |publisher=World Scientific Publishing |year=2011 |page=31 |doi=10.1142/8068 |isbn=978-981-4338-96-7}}</ref> Condensed water in the air may also [[refract]] [[sunlight]] to produce [[rainbow]]s.

Water runoff often collects over [[Drainage basin|watersheds]] flowing into rivers. Through [[erosion]], runoff shapes the environment creating river [[valley]]s and [[river delta|deltas]] which provide rich soil and level ground for the establishment of population centers. A flood occurs when an area of land, usually low-lying, is covered with water which occurs when a river overflows its banks or a storm surge happens. On the other hand, drought is an extended period of months or years when a region notes a deficiency in its water supply. This occurs when a region receives consistently below average precipitation either due to its topography or due to its location in terms of [[latitude]].

===Water resources===
{{Main|Water resources}}
Water resources are [[natural resource]]s of water that are potentially useful for humans,<ref>{{Cite encyclopedia |title=water resource |encyclopedia=Encyclopædia Britannica |url=https://www.britannica.com/science/water-resource |access-date=2022-05-17 |language=en |archive-date=2 October 2022 |archive-url=https://web.archive.org/web/20221002130105/https://www.britannica.com/science/water-resource |url-status=live }}</ref> for example as a source of drinking [[water supply]] or [[irrigation]] water. Water occurs as both "stocks" and "flows". Water can be stored as lakes, water vapor, groundwater or aquifers, and ice and snow. Of the total volume of global freshwater, an estimated 69 percent is stored in glaciers and permanent snow cover; 30 percent is in groundwater; and the remaining 1 percent in lakes, rivers, the atmosphere, and biota.<ref>{{Cite book |title=Water in Crisis |last=Gleick |first=Peter H. |date=1993 |publisher=[[Oxford University Press]] |isbn=0-19-507627-3 |edition= |location=New York |publication-date=1993 |page=[https://archive.org/details/waterincrisisgui00glei/page/13 13] |url=https://archive.org/details/waterincrisisgui00glei/page/13 }}</ref> The length of time water remains in storage is highly variable: some aquifers consist of water stored over thousands of years but lake volumes may fluctuate on a seasonal basis, decreasing during dry periods and increasing during wet ones. A substantial fraction of the water supply for some regions consists of water extracted from water stored in stocks, and when withdrawals exceed recharge, stocks decrease. By some estimates, as much as 30 percent of total water used for irrigation comes from unsustainable withdrawals of groundwater, causing [[overdrafting|groundwater depletion]].<ref>{{cite journal |first1=Yoshihide|last1=Wada |first2=L. P. H.|last2=Van Beek|first3=Marc F. P. |last3=Bierkens|title=Nonsustainable groundwater sustaining irrigation: A global assessment|journal=Water Resources Research |date= 2012 |volume=48 |issue=6 |pages=W00L06 |doi=10.1029/2011WR010562|bibcode=2012WRR....48.0L06W |doi-access=free }}</ref>

===Seawater and tides===
{{Main|Seawater|Tides}}
Seawater contains about 3.5% [[sodium chloride]] on average, plus smaller amounts of other substances. The physical properties of seawater differ from [[fresh water]] in some important respects. It freezes at a lower temperature (about {{convert|-1.9|C}}) and its density increases with decreasing temperature to the freezing point, instead of reaching maximum density at a temperature above freezing. The salinity of water in major seas varies from about 0.7% in the [[Baltic Sea]] to 4.0% in the [[Red Sea]]. (The [[Dead Sea]], known for its ultra-high salinity levels of between 30 and 40%, is really a [[salt lake]].)

[[Tide]]s are the cyclic rising and falling of local sea levels caused by the [[tidal force]]s of the Moon and the Sun acting on the oceans. Tides cause changes in the depth of the marine and [[estuary|estuarine]] water bodies and produce oscillating currents known as tidal streams. The changing tide produced at a given location is the result of the changing positions of the Moon and Sun relative to the Earth coupled with the [[Coriolis effect|effects of Earth rotation]] and the local [[bathymetry]]. The strip of seashore that is submerged at high tide and exposed at low tide, the [[intertidal zone]], is an important ecological product of ocean tides.
{{gallery
| title = The [[Bay of Fundy]] at high tide and low tide
| width = 150
| align = center
|File:Bay of Fundy High Tide.jpg
 |High tide
|File:Bay of Fundy Low Tide.jpg
 |Low tide
}}

==Effects on life==
[[File:Auto-and heterotrophs.svg|thumb|upright|Overview of [[photosynthesis]] <span style="color:green;">(green)</span> and [[cellular respiration|respiration]] <span style="color:red;">(red)</span>]]

From a [[biology|biological]] standpoint, water has many distinct properties that are critical for the proliferation of life. It carries out this role by allowing [[organic compound]]s to react in ways that ultimately allow [[Self-replication|replication]]. All known forms of life depend on water. Water is vital both as a [[solvent]] in which many of the body's solutes dissolve and as an essential part of many [[metabolism|metabolic]] processes within the body. Metabolism is the sum total of [[anabolism]] and [[catabolism]]. In anabolism, water is removed from molecules (through energy requiring enzymatic chemical reactions) in order to grow larger molecules (e.g., starches, triglycerides, and proteins for storage of fuels and information). In catabolism, water is used to break bonds in order to generate smaller molecules (e.g., glucose, fatty acids, and amino acids to be used for fuels for energy use or other purposes). Without water, these particular metabolic processes could not exist.

Water is fundamental to both photosynthesis and respiration. Photosynthetic cells use the sun's energy to split off water's hydrogen from oxygen.<ref>{{Cite web|title=Catalyst helps split water: Plants|url=https://asknature.org/strategy/catalyst-helps-split-water/|access-date=2020-09-10|website=AskNature|language=en-US|archive-date=28 October 2020|archive-url=https://web.archive.org/web/20201028194047/https://asknature.org/strategy/catalyst-helps-split-water/|url-status=live}}</ref> In the presence of sunlight, hydrogen is combined with {{chem|C|O|2}} (absorbed from air or water) to form glucose and release oxygen.<ref>{{cite book | last=Hall | first=D.O. | date=2001 | title=Photosynthesis, Sixth edition | url=https://books.google.com/books?id=6F7yuf1Sj30C&dq=process+of+photosynthesis&pg=PR7 | publisher=University of Cambridge | isbn=0-521-64497-6 | access-date=26 August 2023 | archive-date=5 October 2023 | archive-url=https://web.archive.org/web/20231005012445/https://books.google.com/books?id=6F7yuf1Sj30C&dq=process+of+photosynthesis&pg=PR7 | url-status=live }}</ref> All living cells use such fuels and oxidize the hydrogen and carbon to capture the sun's energy and reform water and {{chem|C|O|2}} in the process (cellular respiration).

Water is also central to acid-base neutrality and enzyme function. An acid, a hydrogen ion ({{chem|H|+}}, that is, a proton) donor, can be neutralized by a base, a proton acceptor such as a hydroxide ion ({{chem|O|H|−}}) to form water. Water is considered to be neutral, with a [[pH]] (the negative log of the hydrogen ion concentration) of 7. [[Acids]] have pH values less than 7 while [[Base (chemistry)|bases]] have values greater than 7.

===Aquatic life forms===
{{Further|Hydrobiology|Marine life|Aquatic plant}}
Earth's surface waters are filled with life. The earliest life forms appeared in water; nearly all fish live exclusively in water, and there are many types of marine mammals, such as dolphins and whales. Some kinds of animals, such as [[amphibian]]s, spend portions of their lives in water and portions on land. Plants such as [[kelp]] and [[algae]] grow in the water and are the basis for some underwater ecosystems. [[Plankton]] is generally the foundation of the ocean [[food chain]].

Aquatic vertebrates must obtain oxygen to survive, and they do so in various ways. Fish have [[gills]] instead of [[lungs]], although some species of fish, such as the [[lungfish]], have both. [[Marine mammal]]s, such as dolphins, whales, [[otter]]s, and [[pinniped|seals]] need to surface periodically to breathe air. Some amphibians are able to absorb oxygen through their skin. Invertebrates exhibit a wide range of modifications to survive in poorly oxygenated waters including breathing tubes (see [[Siphon (insect)|insect]] and [[Siphon (mollusc)|mollusc siphons]]) and [[gills]] (''[[Carcinus]]''). However, as invertebrate life evolved in an aquatic habitat most have little or no specialization for respiration in water.

{{gallery
|align=center
|File:Blue Linckia Starfish.JPG|Some of the [[biodiversity]] of a [[coral reef]]
|File:Diatoms through the microscope.jpg|Some marine [[diatom]]s – a key [[phytoplankton]] group
|File:VonDamm Crustaceans.jpg|[[Squat lobster]] and [[Alvinocarididae]] shrimp at the Von Damm [[Hydrothermal vent|hydrothermal field]] survive by altered water chemistry.}}

==Effects on human civilization==
{{More citations needed section|date=May 2018}}
[[File:Longwood Gardens-Italian Garden.jpg|thumb|right|Water [[fountain]]]]

Civilization has historically flourished around rivers and major waterways; [[Mesopotamia]], one of the so-called [[cradles of civilization]], was situated between the major rivers [[Tigris]] and [[Euphrates]]; the ancient society of the [[Egyptians]] depended entirely upon the [[Nile]]. The early [[Indus Valley civilization]] ({{Circa|3300 BCE|1300 BCE}}) developed along the Indus River and tributaries that flowed out of the [[Himalayas]]. [[Rome]] was also founded on the banks of the Italian river [[Tiber]]. Large [[metropolis]]es like [[Rotterdam]], [[London]], [[Montreal]], [[Paris]], [[New York City]], [[Buenos Aires]], [[Shanghai]], [[Tokyo]], [[Chicago]], and [[Hong Kong]] owe their success in part to their easy accessibility via water and the resultant expansion of trade. Islands with safe water ports, like [[Singapore]], have flourished for the same reason. In places such as North Africa and the Middle East, where water is more scarce, access to clean drinking water was and is a major factor in human development.

===Health and pollution===
[[File: Field Trip- water sampling.jpg|thumb|An environmental science program – a student from [[Iowa State University]] sampling water]]
Water fit for human consumption is called [[drinking water]] or potable water. Water that is not potable may be made potable by filtration or [[distillation]], or by a range of [[Water treatment|other methods]]. More than 660 million people do not have access to safe drinking water.<ref>{{Cite web|title=On Water|url=https://www.eib.org/en/essays/on-water|access-date=2020-10-13|website=European Investment Bank|language=en|archive-date=14 October 2020|archive-url=https://web.archive.org/web/20201014022119/https://www.eib.org/en/essays/on-water|url-status=live}}</ref><ref>{{Cite web|title=2.4 billion Without Adequate Sanitation. 600 million Without Safe Water. Can We Fix it by 2030?|url=https://ieg.worldbankgroup.org/blog/over-24-billion-without-adequate-sanitation-600-million-without-safe-water-how-do-we-bridge|access-date=2020-10-13|publisher=World Bank Group|first=Ramachandra |last=Jammi|date=13 March 2018 |language=en|archive-date=14 October 2020|archive-url=https://web.archive.org/web/20201014022128/https://ieg.worldbankgroup.org/blog/over-24-billion-without-adequate-sanitation-600-million-without-safe-water-how-do-we-bridge|url-status=live}}</ref>

Water that is not fit for drinking but is not harmful to humans when used for swimming or bathing is called by various names other than potable or drinking water, and is sometimes called [[safe water]], or "safe for bathing". Chlorine is a skin and mucous membrane irritant that is used to make water safe for bathing or drinking. Its use is highly technical and is usually monitored by government regulations (typically 1 part per million (ppm) for drinking water, and 1–2 ppm of chlorine not yet reacted with impurities for bathing water). Water for bathing may be maintained in satisfactory microbiological condition using chemical disinfectants such as [[chlorine]] or [[ozone]] or by the use of [[ultraviolet]] light.

[[Reclaimed water|Water reclamation]] is the process of converting wastewater (most commonly [[sewage]], also called municipal wastewater) into water that can be [[reuse]]d for other purposes. There are 2.3 billion people who reside in nations with water scarcities, which means that each individual receives less than {{convert|1,700|m3}} of water annually. {{convert|380|e9m3}} of municipal wastewater are produced globally each year.<ref name="EIB-2022">{{Cite web |title=Wastewater resource recovery can fix water insecurity and cut carbon emissions |url=https://www.eib.org/en/essays/wastewater-resource-recovery |access-date=2022-08-29 |website=European Investment Bank |language=en |archive-date=29 August 2022 |archive-url=https://web.archive.org/web/20220829150040/https://www.eib.org/en/essays/wastewater-resource-recovery |url-status=live }}</ref><ref>{{Cite web |title=International Decade for Action 'Water for Life' 2005–2015. Focus Areas: Water scarcity |url=https://www.un.org/waterforlifedecade/scarcity.shtml |access-date=2022-08-29 |publisher=United Nations |archive-date=23 May 2020 |archive-url=https://web.archive.org/web/20200523125706/https://www.un.org/waterforlifedecade/scarcity.shtml |url-status=live }}</ref><ref>{{Cite web |title=The State of the World's Land and Water Resources for Food and Agriculture |url=https://www.fao.org/3/i1688e/i1688e.pdf |access-date=30 August 2022 |archive-date=31 August 2022 |archive-url=https://web.archive.org/web/20220831234648/http://www.fao.org/3/i1688e/i1688e.pdf |url-status=live }}</ref>

Freshwater is a renewable resource, recirculated by the natural [[hydrologic cycle]], but pressures over access to it result from the naturally uneven distribution in space and time, growing economic demands by agriculture and industry, and rising populations. Currently, nearly a billion people around the world lack access to safe, affordable water. In 2000, the [[United Nations]] established the [[Millennium Development Goals]] for water to halve by 2015 the proportion of people worldwide without access to safe water and [[sanitation]]. Progress toward that goal was uneven, and in 2015 the UN committed to the [[Sustainable Development Goals]] of achieving universal access to safe and affordable water and sanitation by 2030. Poor [[water quality]] and bad sanitation are deadly; some five million deaths a year are caused by water-related diseases. The [[World Health Organization]] estimates that [[safe water]] could prevent 1.4 million child deaths from [[diarrhoea]] each year.<ref>{{cite web |url=https://www.who.int/features/QA/70/en/ |title=World Health Organization. Safe Water and Global Health |publisher=World Health Organization |date=25 June 2008 |access-date=25 July 2010 |archive-url=https://web.archive.org/web/20101224174349/http://www.who.int/features/qa/70/en/ |archive-date=24 December 2010 |url-status=live }}</ref>

In developing countries, 90% of all [[Sewage|municipal wastewater]] still goes untreated into local rivers and streams.<ref>{{cite book |title=Environmentally Sound Technology for Wastewater and Stormwater Management: An International Source Book |author=UNEP International Environment |year=2002 |publisher=IWA |isbn=978-1-84339-008-4 |oclc=49204666}}</ref> Some 50 countries, with roughly a third of the world's population, also suffer from medium or high [[water scarcity]] and 17 of these extract more water annually than is recharged through their natural water cycles.<ref>{{cite book |title=Climate Change and Developing Countries |last1=Ravindranath |first1=Nijavalli H. |first2=Jayant A. |last2=Sathaye |year=2002 |publisher=Springer |isbn=978-1-4020-0104-8 |oclc=231965991}}</ref> The strain not only affects surface freshwater bodies like rivers and lakes, but it also degrades groundwater resources.

===Human uses===
{{Further|Water supply}}
[[File:Water withdrawals per capita, OWID.svg|thumb|upright=1.6|Total water withdrawals for agricultural, industrial and municipal purposes per capita, measured in cubic metres (m{{sup|3}}) per year in 2010<ref>{{cite web |title=Water withdrawals per capita |url=https://ourworldindata.org/grapher/water-withdrawals-per-capita |website=Our World in Data |access-date=6 March 2020 |archive-date=12 March 2020 |archive-url=https://web.archive.org/web/20200312112519/https://ourworldindata.org/grapher/water-withdrawals-per-capita |url-status=live }}</ref>]]

====Agriculture====
The most substantial human use of water is for agriculture, including irrigated agriculture, which accounts for as much as 80 to 90 percent of total human water consumption.<ref>{{cite web |url=http://www.wbcsd.org/includes/getTarget.asp?type=d&id=MTYyNTA|archive-url=https://web.archive.org/web/20120301011840/http://www.wbcsd.org/includes/getTarget.asp?type=d&id=MTYyNTA|url-status=dead|archive-date=1 March 2012 |title=WBCSD Water Facts & Trends |access-date=25 July 2010}}</ref> In the United States, 42% of freshwater withdrawn for use is for irrigation, but the vast majority of water "consumed" (used and not returned to the environment) goes to agriculture.<ref name="Estimated use of water in the United States in 2015">{{cite book |chapter-url=https://pubs.er.usgs.gov/publication/cir1441 |chapter=Estimated use of water in the United States in 2015 |publisher=U.S. Geological Survey |doi=10.3133/cir1441 |title=Circular |year=2018 |last1=Dieter |first1=Cheryl A. |last2=Maupin |first2=Molly A. |last3=Caldwell |first3=Rodney R. |last4=Harris |first4=Melissa A. |last5=Ivahnenko |first5=Tamara I. |last6=Lovelace |first6=John K. |last7=Barber |first7=Nancy L. |last8=Linsey |first8=Kristin S. |page=76 |access-date=21 May 2019 |archive-date=28 April 2019 |archive-url=https://web.archive.org/web/20190428190636/https://pubs.er.usgs.gov/publication/cir1441 |url-status=live }}</ref>

Access to fresh water is often taken for granted, especially in developed countries that have built sophisticated water systems for collecting, purifying, and delivering water, and removing wastewater. But growing economic, demographic, and climatic pressures are increasing concerns about water issues, leading to increasing competition for fixed water resources, giving rise to the concept of [[peak water]].<ref>{{Cite journal |last1=Gleick |first1=P. H. |title=Peak Water |url=http://www.pacinst.org/press_center/press_releases/peak_water_pnas.pdf |access-date=11 October 2011 |year=2010 |doi=10.1073/pnas.1004812107 |pmid=20498082 |journal=Proceedings of the National Academy of Sciences |volume=107 |issue=125 |pages=11155–11162 |last2=Palaniappan |first2=M. |bibcode=2010PNAS..10711155G |pmc=2895062 |archive-url=https://web.archive.org/web/20111108224340/http://www.pacinst.org/press_center/press_releases/peak_water_pnas.pdf |archive-date=8 November 2011 |url-status=live |doi-access=free }}</ref> As populations and economies continue to grow, consumption of water-thirsty meat expands, and new demands rise for biofuels or new water-intensive industries, new water challenges are likely.<ref>United Nations Press Release POP/952 (13 March 2007). [https://www.un.org/News/Press/docs/2007/pop952.doc.htm "World population will increase by 2.5 billion by 2050"]. {{Webarchive|url=https://web.archive.org/web/20140727030018/http://www.un.org/News/Press/docs/2007/pop952.doc.htm |date=27 July 2014 }}</ref>

An assessment of water management in agriculture was conducted in 2007 by the [[International Water Management Institute]] in Sri Lanka to see if the world had sufficient water to provide food for its growing population.<ref>, Molden, D. (Ed). ''Water for food, Water for life: [[A Comprehensive Assessment of Water Management in Agriculture]].'' Earthscan/IWMI, 2007.</ref> It assessed the current availability of water for agriculture on a global scale and mapped out locations suffering from water scarcity. It found that a fifth of the world's people, more than 1.2 billion, live in areas of [[physical water scarcity]], where there is not enough water to meet all demands. A further 1.6 billion people live in areas experiencing [[economic water scarcity]], where the lack of investment in water or insufficient human capacity make it impossible for authorities to satisfy the demand for water. The report found that it would be possible to produce the food required in the future, but that continuation of today's food production and environmental trends would lead to crises in many parts of the world. To avoid a global water crisis, farmers will have to strive to increase productivity to meet growing demands for food, while industries and cities find ways to use water more efficiently.<ref>Chartres, C. and Varma, S. (2010) ''Out of water. From Abundance to Scarcity and How to Solve the World's Water Problems''. FT Press (US).</ref>

Water scarcity is also caused by production of water intensive products. For example, [[cotton]]: 1&nbsp;kg of cotton—equivalent of a pair of jeans—requires {{convert|10.9|m3|sp=us}} water to produce. While cotton accounts for 2.4% of world water use, the water is consumed in regions that are already at a risk of water shortage. Significant environmental damage has been caused: for example, the diversion of water by the former [[Soviet Union]] from the [[Amu Darya]] and [[Syr Darya]] rivers to produce cotton was largely responsible for the disappearance of the [[Aral Sea]].<ref>{{cite web |first1=A. K. |last1=Chapagain |first2=A. Y. |last2=Hoekstra |first3=H. H. G. |last3=Savenije |first4=R. |last4=Guatam |title=The Water Footprint of Cotton Consumption |url=http://waterfootprint.org/media/downloads/Report18.pdf |publisher=[[IHE Delft Institute for Water Education]] |date=September 2005 |access-date=24 October 2019 |archive-url=https://web.archive.org/web/20190326141524/https://waterfootprint.org/media/downloads/Report18.pdf |archive-date=26 March 2019 |url-status=live }}</ref>

<gallery width="280px" height="200px">
File:Water requirement per tonne of food product, OWID.svg|Water requirement per tonne of food product
File:Subsurface drip emission on loamy soil.ogv|Water distribution in subsurface [[drip irrigation]]
File:SiphonTubes.JPG|[[Irrigation]] of field crops
</gallery>

====As a scientific standard====
On 7 April 1795, the gram was defined in France to be equal to "the absolute weight of a volume of pure water equal to a cube of one-hundredth of a meter, and at the temperature of melting ice".<ref>[http://smdsi.quartier-rural.org/histoire/18germ_3.htm "Décret relatif aux poids et aux mesures"] [Decree relating to weights and measures] (in French). 18 [[French revolutionary calendar|germinal]] an 3 (7 April 1795). {{Webarchive|url=https://web.archive.org/web/20130225163152/http://smdsi.quartier-rural.org/histoire/18germ_3.htm |date=25 February 2013 }}. quartier-rural.org</ref> For practical purposes though, a metallic reference standard was required, one thousand times more massive, the kilogram. Work was therefore commissioned to determine precisely the mass of one liter of water. In spite of the fact that the decreed definition of the gram specified water at {{convert|0|C}}—a highly reproducible ''temperature''—the scientists chose to redefine the standard and to perform their measurements at the temperature of highest water ''density'', which was measured at the time as {{convert|4|C}}.<ref>[http://histoire.du.metre.free.fr/fr/index.htm here "L'Histoire Du Mètre, La Détermination De L'Unité De Poids"] {{Webarchive|url=https://web.archive.org/web/20130725163108/http://histoire.du.metre.free.fr/fr/index.htm |date=25 July 2013 }}. histoire.du.metre.free.fr</ref>

The [[Kelvin temperature scale]] of the [[International System of Units|SI]] system was based on the [[triple point]] of water, defined as exactly {{convert|273.16|K|C F}}, but as of May 2019 is based on the [[Boltzmann constant]] instead. The scale is an [[absolute temperature]] scale with the same increment as the Celsius temperature scale, which was originally defined according to the [[boiling point]] (set to {{convert|100|C}}) and [[melting point]] (set to {{convert|0|C}}) of water.

Natural water consists mainly of the isotopes hydrogen-1 and oxygen-16, but there is also a small quantity of heavier isotopes oxygen-18, oxygen-17, and hydrogen-2 ([[deuterium]]). The percentage of the heavier isotopes is very small, but it still affects the properties of water. Water from rivers and lakes tends to contain less heavy isotopes than seawater. Therefore, standard water is defined in the [[Vienna Standard Mean Ocean Water]] specification.

====For drinking====
{{Main|Drinking water}}
[[File:Humanitarian aid OCPA-2005-10-28-090517a.jpg|thumb|A young girl drinking [[bottled water]]]]
[[File:2006 Global Water Availability.svg|thumb|right|Water availability: the fraction of the population using improved water sources by country]]
[[File:Roadside fresh water outlet from glacier, Nubra, Ladakh.jpg|thumb|Roadside fresh water outlet from glacier, [[Nubra]]]]
The [[human body]] contains from 55% to 78% water, depending on body size.<ref>[http://www.madsci.org/posts/archives/2000-05/958588306.An.r.html "Re: What percentage of the human body is composed of water?"] {{Webarchive|url=https://web.archive.org/web/20071125073713/http://madsci.org/posts/archives/2000-05/958588306.An.r.html |date=25 November 2007 }} Jeffrey Utz, M.D., The MadSci Network</ref>{{ugc|date=November 2022}} To function properly, the body requires between {{convert|1|and|7|L|sp=us|spell=in}}{{citation needed|date=April 2019}} of water per day to avoid [[dehydration]]; the precise amount depends on the level of activity, temperature, humidity, and other factors. Most of this is ingested through foods or beverages other than drinking straight water. It is not clear how much water intake is needed by healthy people, though the British Dietetic Association advises that 2.5 liters of total water daily is the minimum to maintain proper hydration, including 1.8 liters (6 to 7 glasses) obtained directly from beverages.<ref>{{cite web |url=https://www.bbc.co.uk/health/healthy_living/nutrition/drinks_water.shtml |title=Healthy Water Living |work=BBC Health |access-date=1 February 2007 |url-status=dead |archive-url=https://web.archive.org/web/20070101100025/http://www.bbc.co.uk/health/healthy_living/nutrition/drinks_water.shtml |archive-date=1 January 2007}}</ref> Medical literature favors a lower consumption, typically 1 liter of water for an average male, excluding extra requirements due to fluid loss from exercise or warm weather.<ref name=Rhoades_2003>{{cite book |vauthors=Rhoades RA, Tanner GA |title=Medical Physiology |publisher=Lippincott Williams & Wilkins |edition=2nd |location=Baltimore |year=2003 |isbn=978-0-7817-1936-0 |oclc=50554808 |url=https://archive.org/details/medicalphysiolog0000unse }}</ref>

Healthy kidneys can excrete 0.8 to 1 liter of water per hour, but stress such as exercise can reduce this amount. People can drink far more water than necessary while exercising, putting them at risk of [[water intoxication]] (hyperhydration), which can be fatal.<ref>{{cite journal |author=Noakes TD |author2=Goodwin N |author3=Rayner BL |display-authors=etal |title=Water intoxication: a possible complication during endurance exercise |journal=Medicine and Science in Sports and Exercise |year=1985 |volume=17 |issue=3 |pages=370–375 |pmid=4021781 |doi=10.1249/00005768-198506000-00012|doi-access=free }}</ref><ref>{{cite journal |vauthors=Noakes TD, Goodwin N, Rayner BL, Branken T, Taylor RK |title=Water intoxication: a possible complication during endurance exercise, 1985 |journal=Wilderness and Environmental Medicine |year=2005 |volume=16 |issue=4 |pages=221–227 |pmid=16366205 |doi=10.1580/1080-6032(2005)16[221:WIAPCD]2.0.CO;2|s2cid=28370290 |doi-access= }}</ref> The popular claim that "a person should consume eight glasses of water per day" seems to have no real basis in science.<ref>{{cite journal |title='Drink at least eight glasses of water a day.' Really? Is there scientific evidence for '8 × 8'? |journal=American Journal of Physiology. Regulatory, Integrative and Comparative Physiology |volume=283 |issue=5 |pages=R993–R1004 |doi=10.1152/ajpregu.00365.2002 |pmid=12376390 |year=2002 |last1=Valtin |first1=Heinz |s2cid=2256436 |url=http://pdfs.semanticscholar.org/3595/81eb8fa614a2f8c765dc1d4fed3c0e39ee7e.pdf |archive-url=https://web.archive.org/web/20190222112803/http://pdfs.semanticscholar.org/3595/81eb8fa614a2f8c765dc1d4fed3c0e39ee7e.pdf |url-status=dead |archive-date=2019-02-22 }}</ref> Studies have shown that extra water intake, especially up to {{convert|500|mL|sp=us}} at mealtime, was associated with weight loss.<ref>{{cite journal |vauthors=Stookey JD, Constant F, Popkin BM, Gardner CD |title=Drinking water is associated with weight loss in overweight dieting women independent of diet and activity |journal=Obesity |volume=16 |issue=11 |pages=2481–2488 |date=November 2008 |pmid=18787524 |doi=10.1038/oby.2008.409|s2cid=24899383 }}</ref><ref>{{cite web |url=https://www.sciencedaily.com/releases/2010/08/100823142929.htm |title=Drink water to curb weight gain? Clinical trial confirms effectiveness of simple appetite control method |date=23 August 2010 |website=Science Daily |access-date=14 May 2017 |archive-url=https://web.archive.org/web/20170707071448/https://www.sciencedaily.com/releases/2010/08/100823142929.htm |archive-date=7 July 2017 |url-status=live}}</ref><ref>{{cite journal |vauthors=Dubnov-Raz G, Constantini NW, Yariv H, Nice S, Shapira N |title=Influence of water drinking on resting energy expenditure in overweight children |journal=International Journal of Obesity |volume=35 |issue=10 |pages=1295–1300 |date=October 2011 |pmid=21750519 |doi=10.1038/ijo.2011.130|s2cid=27561994 |doi-access= }}</ref><ref>{{cite journal |author=Dennis EA |author2=Dengo AL |author3=Comber DL |display-authors=etal |title=Water consumption increases weight loss during a hypocaloric diet intervention in middle-aged and older adults |journal=Obesity |volume=18 |issue=2 |pages=300–307 |date=February 2010 |pmid=19661958 |pmc=2859815 |doi=10.1038/oby.2009.235}}</ref><ref>{{cite journal |vauthors=Vij VA, Joshi AS |title=Effect of 'water induced thermogenesis' on body weight, body mass index and body composition of overweight subjects |journal=Journal of Clinical and Diagnostic Research |volume=7 |issue=9 |pages=1894–1896 |date=September 2013 |pmid=24179891 |pmc=3809630 |doi=10.7860/JCDR/2013/5862.3344}}</ref><ref>{{cite journal |vauthors=Muckelbauer R, Sarganas G, Grüneis A, Müller-Nordhorn J |title=Association between water consumption and body weight outcomes: a systematic review |journal=The American Journal of Clinical Nutrition |volume=98 |issue=2 |pages=282–299 |date=August 2013 |pmid=23803882 |doi=10.3945/ajcn.112.055061|s2cid=12265434 |doi-access=free }}</ref> Adequate fluid intake is helpful in preventing constipation.<ref>[http://www.webmd.com/digestive-disorders/water-a-fluid-way-to-manage-constipation "Water, Constipation, Dehydration, and Other Fluids"]. {{Webarchive|url=https://web.archive.org/web/20150304043454/http://www.webmd.com/digestive-disorders/water-a-fluid-way-to-manage-constipation |date=4 March 2015 }}. ''Science Daily''. Retrieved on 28 September 2015.</ref>

[[File:DIN 4844-2 D-P005.svg|thumb|right|[[Hazard symbol]] for non-potable water]]
An original recommendation for water intake in 1945 by the Food and Nutrition Board of the [[U.S. National Research Council]] read: "An ordinary standard for diverse persons is 1 milliliter for each calorie of food. Most of this quantity is contained in prepared foods."<ref>{{cite book |title=Food and Nutrition Board, National Academy of Sciences. Recommended Dietary Allowances |publisher=National Research Council, Reprint and Circular Series, No. 122 |year=1945 |pages=3–18}}</ref> The latest dietary reference intake report by the U.S. National Research Council in general recommended, based on the median total water intake from US survey data (including food sources): {{convert|3.7|L|sp=us}} for men and {{convert|2.7|L|sp=us}} of water total for women, noting that water contained in food provided approximately 19% of total water intake in the survey.<ref>{{Cite book|url=https://www.nap.edu/read/10925/chapter/6|title=4 Water {{!}} Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate |publisher=The National Academies Press|doi=10.17226/10925|year=2005|isbn=978-0-309-09169-5|author1=Institute of Medicine|author2=Food Nutrition Board|author3=Standing Committee on the Scientific Evaluation of Dietary Reference Intakes|author4=Panel on Dietary Reference Intakes for Electrolytes and Water|access-date=11 January 2017|archive-url=https://web.archive.org/web/20170113063638/https://www.nap.edu/read/10925/chapter/6|archive-date=13 January 2017|url-status=live}}</ref>

Specifically, pregnant and breastfeeding women need additional fluids to stay hydrated. The US [[Institute of Medicine]] recommends that, on average, men consume {{convert|3|L|sp=us}} and women {{convert|2.2|L|sp=us}}; pregnant women should increase intake to {{convert|2.4|L|sp=us}} and breastfeeding women should get 3 liters (12 cups), since an especially large amount of fluid is lost during nursing.<ref>{{cite web |url=http://www.mayoclinic.com/health/water/NU00283 |title=Water: How much should you drink every day? |publisher=Mayo Clinic |access-date=25 July 2010 |archive-url=https://web.archive.org/web/20101204012725/http://www.mayoclinic.com/health/water/NU00283 |archive-date=4 December 2010 |url-status=live}}</ref> Also noted is that normally, about 20% of water intake comes from food, while the rest comes from drinking water and beverages ([[Caffeine|caffeinated]] included). Water is excreted from the body in multiple forms; through [[urine]] and [[feces]], through [[sweat]]ing, and by exhalation of water vapor in the breath. With physical exertion and heat exposure, water loss will increase and daily fluid needs may increase as well.

Humans require water with few impurities. Common impurities include metal salts and oxides, including copper, iron, calcium and lead,<ref>''Conquering Chemistry'' (4th ed.), 2008</ref>{{full citation needed|date=November 2022}} and harmful bacteria, such as ''[[Vibrio]]''. Some [[solutes]] are acceptable and even desirable for taste enhancement and to provide needed [[electrolyte]]s.<ref>{{cite book |last1=Maton |first1=Anthea |first2=Jean |last2=Hopkins |first3=Charles William |last3=McLaughlin |first4=Susan |last4=Johnson |first5=Maryanna Quon |last5=Warner |first6=David |last6=LaHart |first7=Jill D. |last7=Wright |title=Human Biology and Health |publisher=Prentice Hall |year=1993 |location=Englewood Cliffs, New Jersey |isbn=978-0-13-981176-0 |oclc=32308337 |url=https://archive.org/details/humanbiologyheal00scho }}</ref>

The single largest (by volume) freshwater resource suitable for drinking is [[Lake Baikal]] in Siberia.<ref>{{cite book |url=https://archive.org/details/bub_gb_ujf0kkNF2H8C |page=[https://archive.org/details/bub_gb_ujf0kkNF2H8C/page/n140 125] |title=Water: a shared responsibility |author=Unesco |publisher=Berghahn Books |year=2006 |isbn=978-1-84545-177-6}}</ref>

====Washing====
{{excerpt|washing}}

====Transportation====
{{excerpt|maritime transport|only=paragraphs}}

====Chemical uses====
Water is widely used in chemical reactions as a [[solvent]] or [[reactant]] and less commonly as a [[solute]] or catalyst. In inorganic reactions, water is a common solvent, dissolving many ionic compounds, as well as other polar compounds such as [[ammonia]] and [[Hydrogen chalcogenide|compounds closely related to water]]. In organic reactions, it is not usually used as a reaction solvent, because it does not dissolve the reactants well and is [[amphoteric]] (acidic ''and'' basic) and [[nucleophilic]]. Nevertheless, these properties are sometimes desirable. Also, acceleration of [[Diels-Alder reaction]]s by water has been observed. [[Supercritical water]] has recently been a topic of research. Oxygen-saturated supercritical water combusts organic pollutants efficiently.

====Heat exchange====
Water and steam are a common fluid used for [[heat exchanger|heat exchange]], due to its availability and high [[Heat capacity of water|heat capacity]], both for cooling and heating. Cool water may even be naturally available from a lake or the sea. It is especially effective to transport heat through [[vaporization]] and [[condensation]] of water because of its large [[latent heat of vaporization]]. A disadvantage is that metals commonly found in industries such as steel and copper are [[oxidation|oxidized]] faster by untreated water and steam. In almost all [[thermal power station]]s, water is used as the working fluid (used in a closed-loop between boiler, steam turbine, and condenser), and the coolant (used to exchange the waste heat to a water body or carry it away by [[evaporation]] in a [[cooling tower]]). In the United States, cooling power plants is the largest use of water.<ref name="Water Use in the United States">[http://nationalatlas.gov/articles/water/a_wateruse.html "Water Use in the United States"], ''National Atlas''. {{webarchive|url=https://web.archive.org/web/20090814045418/http://nationalatlas.gov/articles/water/a_wateruse.html |date=14 August 2009 }}</ref>

In the [[nuclear power]] industry, water can also be used as a [[neutron moderator]]. In most [[nuclear reactor]]s, water is both a coolant and a moderator. This provides something of a passive safety measure, as removing the water from the reactor also [[void coefficient|slows the nuclear reaction down]]. However other methods are favored for stopping a reaction and it is preferred to keep the nuclear core covered with water so as to ensure adequate cooling.

====Fire considerations====
[[File:MH-60S Helicopter dumps water onto Fire.jpg|right|thumb|Water is used for [[fire fighting|fighting]] [[wildfire]]s.]]
Water has a high heat of vaporization and is relatively inert, which makes it a good [[Fire fighting#Use of water|fire extinguishing]] fluid. The evaporation of water carries heat away from the fire. It is dangerous to use water on fires involving oils and organic solvents because many organic materials float on water and the water tends to spread the burning liquid.

Use of water in fire fighting should also take into account the hazards of a [[steam explosion]], which may occur when water is used on very hot fires in confined spaces, and of a hydrogen explosion, when substances which react with water, such as certain metals or hot carbon such as coal, [[charcoal]], or [[coke (fuel)|coke]] graphite, decompose the water, producing [[water gas]].

The power of such explosions was seen in the [[Chernobyl disaster]], although the water involved in this case did not come from fire-fighting but from the reactor's own water cooling system. A steam explosion occurred when the extreme overheating of the core caused water to flash into steam. A hydrogen explosion may have occurred as a result of a reaction between steam and hot [[zirconium]].

Some metallic oxides, most notably those of [[alkali metals]] and [[alkaline earth metals]], produce so much heat in reaction with water that a fire hazard can develop. The alkaline earth oxide [[Calcium oxide|quicklime]], also known as calcium oxide, is a mass-produced substance that is often transported in paper bags. If these are soaked through, they may ignite as their contents react with water.<ref>{{cite web |title=Material Safety Data Sheet: Quicklime |url=https://www.lhoist.com/sites/lhoist/files/lna_msds_quicklime_2012-3.pdf |publisher=Lhoist North America |date=6 August 2012 |access-date=24 October 2019 |archive-url=https://web.archive.org/web/20160705030051/http://www.lhoist.com/sites/lhoist/files/lna_msds_quicklime_2012-3.pdf |archive-date=5 July 2016 |url-status=live }}</ref>

====Recreation====
{{Main|Water sport (recreation)}}
[[File:Johny Cay.jpg|thumb|right|[[San Andrés (island)|San Andrés island]], [[Colombia]]]]

Humans use water for many recreational purposes, as well as for exercising and for sports. Some of these include swimming, [[waterskiing]], [[boating]], [[surfing]] and [[Underwater diving|diving]]. In addition, some sports, like [[ice hockey]] and [[ice skating]], are played on ice. Lakesides, beaches and [[water park]]s are popular places for people to go to relax and enjoy recreation. Many find the sound and appearance of flowing water to be calming, and fountains and other flowing water structures are popular decorations. Some keep fish and other flora and fauna inside [[aquarium]]s or ponds for show, fun, and companionship. Humans also use water for snow sports such as [[skiing]], [[sledding]], [[snowmobiling]] or [[snowboarding]], which require the water to be at a low temperature either as ice or crystallized into [[snow]].

====Water industry====
The [[water industry]] provides drinking water and [[wastewater]] services (including [[sewage treatment]]) to households and industry. [[Water supply]] facilities include [[water well]]s, [[cistern]]s for [[rainwater harvesting]], [[water supply network]]s, and [[water purification]] facilities, [[water tank]]s, [[water tower]]s, [[water pipe]]s including old [[Aqueduct (watercourse)|aqueducts]]. [[Atmospheric water generator]]s are in development.

Drinking water is often collected at [[spring (hydrosphere)|springs]], extracted from artificial [[Boring (earth)|borings]] (wells) in the ground, or pumped from lakes and rivers. Building more wells in adequate places is thus a possible way to produce more water, assuming the aquifers can supply an adequate flow. Other water sources include rainwater collection. Water may require purification for human consumption. This may involve the removal of undissolved substances, dissolved substances and harmful [[microbe]]s. Popular methods are [[filter (water)|filtering]] with sand which only removes undissolved material, while [[Water chlorination|chlorination]] and [[boiling]] kill harmful microbes. [[Distillation]] does all three functions. More advanced techniques exist, such as [[reverse osmosis]]. [[Desalination]] of abundant [[seawater]] is a more expensive solution used in coastal [[arid]] [[climate]]s.

The distribution of drinking water is done through [[municipal water system]]s, tanker delivery or as [[bottled water]]. Governments in many countries have programs to distribute water to the needy at no charge.

Reducing usage by using drinking (potable) water only for human consumption is another option. In some cities such as Hong Kong, seawater is extensively used for flushing toilets citywide in order to [[Water conservation|conserve freshwater resources]].

[[Water pollution|Polluting water]] may be the biggest single misuse of water; to the extent that a pollutant limits other uses of the water, it becomes a waste of the resource, regardless of benefits to the polluter. Like other types of pollution, this does not enter standard accounting of market costs, being conceived as [[externality|externalities]] for which the market cannot account. Thus other people pay the price of water pollution, while the private firms' profits are not redistributed to the local population, victims of this pollution. [[Pharmaceuticals]] consumed by humans often end up in the waterways and can have detrimental effects on [[marine biology|aquatic]] life if they [[bioaccumulation|bioaccumulate]] and if they are not [[biodegradable]].

Municipal and [[industrial wastewater treatment|industrial wastewater]] are typically treated at [[wastewater treatment plant]]s. Mitigation of polluted [[surface runoff]] is addressed through a variety of [[Surface runoff#Mitigation and treatment|prevention and treatment techniques]].
{{gallery
|align=center
|File:Water carrier in India.jpg|A water-carrier in India, 1882. In many places where running water is not available, water has to be transported by people.
|File:TapWater-china.JPG|A manual water [[pump]] in China
|File:Usine Bret MG 1648.jpg|[[Water purification]] facility
|File:Reverse osmosis desalination plant.JPG|[[Reverse osmosis]] (RO) [[desalination]] plant in [[Barcelona]], Spain
}}

====Industrial applications====
Many industrial processes rely on reactions using chemicals dissolved in water, suspension of solids in water [[slurry|slurries]] or using water to dissolve and extract substances, or to wash products or process equipment. Processes such as [[mining]], [[chemical pulping]], [[pulp bleaching]], [[paper manufacturing]], textile production, dyeing, printing, and cooling of power plants use large amounts of water, requiring a dedicated water source, and often cause significant water pollution.

Water is used in [[power generation]]. [[Hydroelectricity]] is electricity obtained from [[hydropower]]. Hydroelectric power comes from water driving a water turbine connected to a generator. Hydroelectricity is a low-cost, non-polluting, renewable energy source. The energy is supplied by the motion of water. Typically a dam is constructed on a river, creating an artificial lake behind it. Water flowing out of the lake is forced through turbines that turn generators.

{{wide image|200407-sandouping-sanxiadaba-4.med.jpg|800px|[[Three Gorges Dam]] is the [[List of the largest hydroelectric power stations|largest hydro-electric power station]] in the world.}}

Pressurized water is used in [[Hydrodemolition|water blasting]] and [[water jet cutter]]s. High pressure water guns are used for precise cutting. It works very well, is relatively safe, and is not harmful to the environment. It is also used in the cooling of machinery to prevent overheating, or prevent saw blades from overheating.

Water is also used in many industrial processes and machines, such as the [[steam turbine]] and [[heat exchanger]], in addition to its use as a chemical [[solvent]]. Discharge of untreated water from industrial uses is [[water pollution|pollution]]. Pollution includes discharged solutes (chemical pollution) and discharged coolant water ([[thermal pollution]]). Industry requires pure water for many applications and uses a variety of purification techniques both in water supply and discharge.

====Food processing====
[[File:Cuisson des pates.jpg|thumb|Water can be used to cook foods such as [[noodles]].]]
[[File:Sterilewater.jpg|thumb|upright|Sterile water for injection]]
[[Boiling]], [[steaming]], and [[simmering]] are popular cooking methods that often require immersing food in water or its gaseous state, steam.<ref>{{Cite book|url=https://books.google.com/books?id=xZHUAAAAMAAJ&pg=PA54|title=A Course in Household Arts: Part I|last=Duff|first=Loretto Basil|date=1916|publisher=Whitcomb & Barrows|access-date=3 December 2017|archive-date=14 April 2021|archive-url=https://web.archive.org/web/20210414164100/https://books.google.com/books?id=xZHUAAAAMAAJ&pg=PA54|url-status=live}}</ref> Water is also used for [[dishwashing]]. Water also plays many critical roles within the field of [[food science]].

[[Solutes]] such as salts and sugars found in water affect the physical properties of water. The boiling and freezing points of water are affected by solutes, as well as [[air pressure]], which is in turn affected by altitude. Water boils at lower temperatures with the lower air pressure that occurs at higher elevations. One [[mole (unit)|mole]] of sucrose (sugar) per kilogram of water raises the boiling point of water by {{convert|0.51|C-change|3}}, and one mole of salt per kg raises the boiling point by {{convert|1.02|C-change|3}}; similarly, increasing the number of dissolved particles lowers water's freezing point.<ref name="vaclacik">{{cite book |title=Essentials of Food Science |url=https://books.google.com/books?id=iCCsvwZrguUC |year=2007 |last1=Vaclavik |first1=Vickie A. |last2=Christian |first2=Elizabeth W. |publisher=Springer |isbn=978-0-387-69939-4 |access-date=31 August 2020 |archive-date=14 April 2021 |archive-url=https://web.archive.org/web/20210414164352/https://books.google.com/books?id=iCCsvwZrguUC |url-status=live }}</ref>

Solutes in water also affect water activity that affects many chemical reactions and the growth of microbes in food.<ref name="deman">{{cite book |url=https://books.google.com/books?id=kDYJ7a1HbD0C&pg=PA434 |title=Principles of Food Chemistry |year=1999 |last=DeMan |first=John M. |publisher=Springer |isbn=978-0-8342-1234-3 |access-date=31 August 2020 |archive-date=14 April 2021 |archive-url=https://web.archive.org/web/20210414185952/https://books.google.com/books?id=kDYJ7a1HbD0C&pg=PA434 |url-status=live }}</ref> Water activity can be described as a ratio of the vapor pressure of water in a solution to the vapor pressure of pure water.<ref name="vaclacik" /> Solutes in water lower water activity—this is important to know because most bacterial growth ceases at low levels of water activity.<ref name="deman" /> Not only does microbial growth affect the safety of food, but also the preservation and shelf life of food.

[[Water hardness]] is also a critical factor in food processing and may be altered or treated by using a chemical ion exchange system. It can dramatically affect the quality of a product, as well as playing a role in sanitation. Water hardness is classified based on concentration of calcium carbonate the water contains. Water is classified as soft if it contains less than 100&nbsp;mg/L (UK)<ref name="DEFRA">{{cite web |url=http://dwi.defra.gov.uk/consumers/advice-leaflets/hardness_map.pdf |title=Map showing the rate of hardness in mg/L as Calcium carbonate in England and Wales |publisher=[[Department for Environment, Food and Rural Affairs|DEFRA]] Drinking Water Inspectorate |date=2009 |access-date=18 May 2015 |archive-url=https://web.archive.org/web/20150529054911/http://dwi.defra.gov.uk/consumers/advice-leaflets/hardness_map.pdf |archive-date=29 May 2015 |url-status=live }}</ref> or less than 60&nbsp;mg/L (US).<ref name="USGS">{{cite web |url=https://water.usgs.gov/edu/hardness.html |publisher=US Geological Service |title=Water hardness |date=8 April 2014 |access-date=18 May 2015 |archive-url=https://web.archive.org/web/20150518204909/https://water.usgs.gov/edu/hardness.html |archive-date=18 May 2015 |url-status=live}}</ref>

According to a report published by the Water Footprint organization in 2010, a single kilogram of beef requires {{convert|15|e3L|e3impgal+e3usgal|sp=us}} of water; however, the authors also make clear that this is a global average and circumstantial factors determine the amount of water used in beef production.<ref>{{cite report |title=The green, blue and grey water footprint of farm animals and animal products, Value of Water |series=Research Report Series |volume=1|issue=48 |url=http://www.waterfootprint.org/Reports/Report-48-WaterFootprint-AnimalProducts-Vol1.pdf |publisher=UNESCO – IHE Institute for Water Education |access-date=30 January 2014 |first1=M. M. |last1=Mekonnen |first2=A. Y. |last2=Hoekstra |date=December 2010 |archive-url=https://web.archive.org/web/20140527104135/http://www.waterfootprint.org/Reports/Report-48-WaterFootprint-AnimalProducts-Vol1.pdf |archive-date=27 May 2014 |url-status=live}}</ref>

====Medical use====
[[Water for injection]] is on the [[World Health Organization]]'s [[World Health Organization's list of essential medicines|list of essential medicines]].<ref>{{cite web |url=http://apps.who.int/iris/bitstream/10665/93142/1/EML_18_eng.pdf?ua=1 |title=WHO Model List of EssentialMedicines |date=October 2013 |website=World Health Organization |access-date=22 April 2014 |archive-url=https://web.archive.org/web/20140423005004/http://apps.who.int/iris/bitstream/10665/93142/1/EML_18_eng.pdf?ua=1 |archive-date=23 April 2014 |url-status=live}}</ref>

==Distribution in nature==

===In the universe===
[[File:Band 5 ALMA receiver.jpg|thumb|Band 5 [[Atacama Large Millimeter Array|ALMA]] receiver is an instrument specifically designed to detect water in the universe.<ref>{{cite web |title=ALMA Greatly Improves Capacity to Search for Water in Universe |url=http://www.eso.org/public/announcements/ann15059/ |access-date=20 July 2015 |archive-url=https://web.archive.org/web/20150723070436/http://www.eso.org/public/announcements/ann15059/ |archive-date=23 July 2015 |url-status=live }}</ref>]]

Much of the universe's water is produced as a byproduct of [[star formation]]. The formation of stars is accompanied by a strong outward wind of gas and dust. When this outflow of material eventually impacts the surrounding gas, the shock waves that are created compress and heat the gas. The water observed is quickly produced in this warm dense gas.<ref>Melnick, Gary, [[Harvard-Smithsonian Center for Astrophysics]] and Neufeld, David, [[Johns Hopkins University]] quoted in:
{{cite web |url=http://www.news.harvard.edu/gazette/1998/04.23/DiscoverofWater.html |title=Discover of Water Vapor Near Orion Nebula Suggests Possible Origin of H20 in Solar System (sic) |date=23 April 1998 |website=The Harvard University Gazette |url-status=dead |archive-url=https://web.archive.org/web/20000116054013/http://www.news.harvard.edu/gazette/1998/04.23/DiscoverofWater.html |archive-date=16 January 2000 }}
{{cite news |url=http://www.jhu.edu/news_info/news/home98/apr98/clouds.html |title=Space Cloud Holds Enough Water to Fill Earth's Oceans 1 Million Times |date=9 April 1998 |publisher=Headlines@Hopkins, JHU |access-date=21 April 2007 |archive-url=https://web.archive.org/web/20071109171410/http://www.jhu.edu/news_info/news/home98/apr98/clouds.html |archive-date=9 November 2007 |url-status=live }}
{{cite web |url=http://news.harvard.edu/gazette/1999/02.25/telescope.html |title=Water, Water Everywhere: Radio telescope finds water is common in universe |date=25 February 1999 |website=The Harvard University Gazette |access-date=19 September 2010 |archive-url=https://web.archive.org/web/20110519141432/http://news.harvard.edu/gazette/1999/02.25/telescope.html |archive-date=19 May 2011 |url-status=live }} ([https://web.archive.org/web/20160715053715/http://news.harvard.edu/gazette/1998/04.23/DiscoverofWater.html archive link])</ref>

On 22 July 2011, a report described the discovery of a gigantic cloud of water vapor containing "140 trillion times more water than all of Earth's oceans combined" around a [[quasar]] located 12 billion light years from Earth. According to the researchers, the "discovery shows that water has been prevalent in the universe for nearly its entire existence".<ref name="Clavin">{{cite web |last1=Clavin |first1=Whitney |last2=Buis |first2=Alan |title=Astronomers Find Largest, Most Distant Reservoir of Water |url=http://www.nasa.gov/topics/universe/features/universe20110722.html |date=22 July 2011 |publisher=[[NASA]] |access-date=25 July 2011 |archive-url=https://web.archive.org/web/20110724063244/http://www.nasa.gov/topics/universe/features/universe20110722.html |archive-date=24 July 2011 |url-status=live }}</ref><ref name="water vapor cloud">{{cite web |author=Staff |title=Astronomers Find Largest, Oldest Mass of Water in Universe |url=http://www.space.com/12400-universe-biggest-oldest-cloud-water.html |date=22 July 2011 |publisher=[[Space.com]] |access-date=23 July 2011 |archive-url=https://web.archive.org/web/20111029230319/http://www.space.com/12400-universe-biggest-oldest-cloud-water.html |archive-date=29 October 2011 |url-status=live }}</ref>

Water has been detected in [[interstellar cloud]]s within the [[Milky Way]].<ref>{{Cite book |url=https://books.google.com/books?id=m1gfe459yygC&pg=PA90 |title=Faint Echoes, Distant Stars: The Science and Politics of Finding Life Beyond Earth |last=Bova |first=Ben |year=2009 |publisher=Zondervan |isbn=978-0-06-185448-4 |access-date=31 August 2020 |archive-date=14 April 2021 |archive-url=https://web.archive.org/web/20210414164517/https://books.google.com/books?id=m1gfe459yygC&pg=PA90 |url-status=live }}</ref> Water probably exists in abundance in other galaxies, too, because its components, hydrogen, and oxygen, are among the most abundant elements in the universe. Based on models of the [[formation and evolution of the Solar System]] and that of other star systems, most other [[planetary system]]s are likely to have similar ingredients.

====Water vapor====
Water is present as vapor in:
* [[Solar atmosphere|Atmosphere of the Sun]]: in detectable trace amounts<ref name=Solanki1994>{{cite journal |last1=Solanki |first1=S.K. |last2=Livingston |first2=W. |last3=Ayres |first3=T. |year=1994 |title=New Light on the Heart of Darkness of the Solar Chromosphere |journal=[[Science (journal)|Science]] |pmid=17748350 |volume=263 |issue=5143 |pages=64–66 |bibcode=1994Sci...263...64S |doi=10.1126/science.263.5143.64 |s2cid=27696504 |url=http://pdfs.semanticscholar.org/f20e/89b9c386ff2dea7d990f8ff6a09d550e5e43.pdf |archive-url=https://web.archive.org/web/20190307030222/http://pdfs.semanticscholar.org/f20e/89b9c386ff2dea7d990f8ff6a09d550e5e43.pdf |url-status=dead |archive-date=2019-03-07 }}</ref>
* [[Atmosphere of Mercury]]: 3.4%, and large amounts of water in [[Mercury (planet)|Mercury's]] [[exosphere]]<ref name="planetary society">{{cite web |url=http://www.planetary.org/news/2008/0703_MESSENGER_Scientists_Astonished_to.html |title=MESSENGER Scientists 'Astonished' to Find Water in Mercury's Thin Atmosphere |access-date=5 July 2008 |publisher=Planetary Society |date=3 July 2008 |archive-url=https://web.archive.org/web/20100406034624/http://www.planetary.org/news/2008/0703_MESSENGER_Scientists_Astonished_to.html |url-status=dead |archive-date=6 April 2010}}</ref>
* [[Atmosphere of Venus]]: 0.002%<ref name=Bertaux2007>{{cite journal |last=Bertaux |first=Jean-Loup |title=A warm layer in Venus' cryosphere and high-altitude measurements of HF, HCl, H2O and HDO |journal=Nature |year=2007 |volume=450 |pages=646–649 |doi=10.1038/nature05974 |bibcode=2007Natur.450..646B |pmid=18046397 |issue=7170 |author2=Vandaele, Ann-Carine |last3=Korablev |first3=Oleg |last4=Villard |first4=E. |last5=Fedorova |first5=A. |last6=Fussen |first6=D. |last7=Quémerais |first7=E. |last8=Belyaev |first8=D. |last9=Mahieux |first9=A. |hdl=2268/29200 |s2cid=4421875 |url=https://orbi.uliege.be/bitstream/2268/29200/1/Bertaux-2007-a%20warm.pdf |access-date=8 October 2022 |archive-date=7 September 2022 |archive-url=https://web.archive.org/web/20220907122145/https://orbi.uliege.be/bitstream/2268/29200/1/Bertaux-2007-a%20warm.pdf |url-status=live }}</ref>
* [[Earth's atmosphere]]: ≈0.40% over full atmosphere, typically 1–4% at surface; as well as [[Atmosphere of the Moon|that of the Moon]] in trace amounts<ref name="Sridharan2010">{{cite journal |last1=Sridharan |first1=R. |first2=S.M. |last2=Ahmed |first3=Tirtha Pratim |last3=Dasa |first4=P. |last4=Sreelathaa |first5=P. |last5=Pradeepkumara |first6=Neha |last6=Naika |first7=Gogulapati |last7=Supriya |year=2010 |page=947 |issue=6 |volume=58 |title='Direct' evidence for water (H2O) in the sunlit lunar ambience from CHACE on MIP of Chandrayaan I |journal=Planetary and Space Science |doi=10.1016/j.pss.2010.02.013 |bibcode=2010P&SS...58..947S}}</ref>
* [[Atmosphere of Mars]]: 0.03%<ref name="Rapp2012">{{cite book |author=Rapp, Donald |title=Use of Extraterrestrial Resources for Human Space Missions to Moon or Mars |url=https://books.google.com/books?id=2xzxhnBRHCMC&pg=PA78 |year=2012 |publisher=Springer |isbn=978-3-642-32762-9 |page=78 |access-date=9 February 2016 |archive-url=https://web.archive.org/web/20160715154349/https://books.google.com/books?id=2xzxhnBRHCMC&pg=PA78 |archive-date=15 July 2016 |url-status=live }}</ref>
* [[Atmosphere of Ceres]]<ref name="Kuppers2014">{{cite journal |last1=Küppers |first1=M. |last2=O'Rourke |first2=L. |last3=Bockelée-Morvan |first3=D.|author3-link=Dominique Bockelée-Morvan |last4=Zakharov |first4=V. |last5=Lee |first5=S. |last6=Von Allmen |first6=P. |last7=Carry |first7=B. |last8=Teyssier |first8=D. |last9=Marston |first9=A. |last10=Müller |first10=T. |last11=Crovisier |first11=J. |last12=Barucci |first12=M.A. |last13=Moreno |first13=R. |title=Localized sources of water vapour on the dwarf planet (1) Ceres |journal=Nature |volume=505 |issue=7484 |date=23 January 2014 |pages=525–527|doi=10.1038/nature12918 |pmid=24451541 |bibcode=2014Natur.505..525K|s2cid=4448395 }}</ref>
* [[Atmosphere of Jupiter]]: 0.0004%<ref>{{cite journal |doi=10.1007/s11214-005-1951-5 |last1=Atreya |first1=Sushil K. |last2=Wong |first2=Ah-San |year=2005 |title=Coupled Clouds and Chemistry of the Giant Planets&nbsp;– A Case for Multiprobes |journal=Space Science Reviews |volume=116 |issue=1–2 |pages=121–136 |url=http://www-personal.umich.edu/~atreya/Chapters/2005_JovianCloud_Multiprobes.pdf |bibcode=2005SSRv..116..121A |access-date=1 April 2014 |archive-url=https://web.archive.org/web/20110722074717/http://www-personal.umich.edu/~atreya/Chapters/2005_JovianCloud_Multiprobes.pdf |archive-date=22 July 2011 |url-status=live |hdl=2027.42/43766 |s2cid=31037195 |hdl-access=free }}</ref> – in [[Volatile (astrogeology)|ices]] only; and that of its moon [[Europa (moon)|Europa]]<ref name="NASA-20131212-EU">{{cite web |last1=Cook |first1=Jia-Rui C. |last2=Gutro |first2=Rob |last3=Brown |first3=Dwayne |last4=Harrington |first4=J.D. |last5=Fohn |first5=Joe |title=Hubble Sees Evidence of Water Vapor at Jupiter Moon |url=http://www.jpl.nasa.gov/news/news.php?release=2013-363 |date=12 December 2013 |website=[[NASA]] |access-date=12 December 2013 |archive-url=https://web.archive.org/web/20131215053143/http://www.jpl.nasa.gov/news/news.php?release=2013-363 |archive-date=15 December 2013 |url-status=live}}</ref>
* [[Atmosphere of Saturn]] – in [[Volatile (astrogeology)|ices]] only; [[Enceladus (moon)|Enceladus]]: 91%<ref name="Hansen">{{cite journal |doi=10.1126/science.1121254 |title=Enceladus' Water Vapor Plume |year=2006 |author=Hansen |journal=Science |volume=311 |pages=1422–1425 |pmid=16527971 |issue=5766 |bibcode=2006Sci...311.1422H |author2=C.J.|last3=Stewart |first3=AI |last4=Colwell |first4=J |last5=Hendrix |first5=A |last6=Pryor |first6=W |last7=Shemansky |first7=D |last8=West |first8=R|s2cid=2954801 |url=https://pdfs.semanticscholar.org/89b1/1f34539a1b9b8a9dcb5a1d835e693bea1940.pdf |archive-url=https://web.archive.org/web/20200218132849/https://pdfs.semanticscholar.org/89b1/1f34539a1b9b8a9dcb5a1d835e693bea1940.pdf |url-status=dead |archive-date=2020-02-18 }}</ref> and [[Dione (moon)|Dione]] (exosphere){{Citation needed|date=May 2018}}
* [[Atmosphere of Uranus]] – in trace amounts below 50 bar
* [[Atmosphere of Neptune]] – found in the deeper layers<ref name=hubbard>{{cite journal |last=Hubbard |first=W.B. |title=Neptune's Deep Chemistry |journal=Science |year=1997 |volume=275 |issue=5304 |pages=1279–1280 |doi=10.1126/science.275.5304.1279 |pmid=9064785|s2cid=36248590 }}</ref>
* [[Extrasolar planet]] atmospheres: including those of [[HD 189733 b]]<ref>[http://www.time.com/time/health/article/0,8599,1642811,00.html Water Found on Distant Planet] {{Webarchive|url=https://web.archive.org/web/20070716081124/http://www.time.com/time/health/article/0,8599,1642811,00.html |date=16 July 2007 }} 12 July 2007 By Laura Blue, ''[[Time (magazine)|Time]]''</ref> and [[HD 209458 b]],<ref name="Space.com water">[http://www.space.com/scienceastronomy/070410_water_exoplanet.html Water Found in Extrasolar Planet's Atmosphere] {{Webarchive|url=https://web.archive.org/web/20101230065702/http://www.space.com/scienceastronomy/070410_water_exoplanet.html |date=30 December 2010 }} – Space.com</ref> [[Tau Boötis b]],<ref>{{Cite journal |arxiv = 1402.0846|last1 = Lockwood|first1 = Alexandra C|title = Near-IR Direct Detection of Water Vapor in Tau Boo B|journal = The Astrophysical Journal|volume = 783|issue = 2|pages = L29|last2 = Johnson|first2 = John A|last3 = Bender|first3 = Chad F|last4 = Carr|first4 = John S|last5 = Barman|first5 = Travis|last6 = Richert|first6 = Alexander J.W.|last7 = Blake|first7 = Geoffrey A|year = 2014|doi = 10.1088/2041-8205/783/2/L29|bibcode = 2014ApJ...783L..29L|s2cid = 8463125}}</ref> [[HAT-P-11b]],<ref name="NASA-20140924">{{cite web |last1=Clavin |first1=Whitney |last2=Chou |first2=Felicia |last3=Weaver |first3=Donna |last4=Villard |first45=Ray |last5=Johnson |first5=Michele |title=NASA Telescopes Find Clear Skies and Water Vapor on Exoplanet |url=http://www.jpl.nasa.gov/news/news.php?release=2014-322&1 |date=24 September 2014 |website=[[NASA]] |access-date=24 September 2014 |archive-url=https://web.archive.org/web/20170114220647/http://www.jpl.nasa.gov/news/news.php?release=2014-322&1 |archive-date=14 January 2017 |url-status=live}}</ref><ref name="Hanslmeier2010">{{cite book |author=Arnold Hanslmeier |title=Water in the Universe |url=https://books.google.com/books?id=Mj5tSld5tjMC&pg=PA159 |year=2010 |publisher=Springer Science & Business Media |isbn=978-90-481-9984-6 |pages=159– |access-date=9 February 2016 |archive-url=https://web.archive.org/web/20160715031920/https://books.google.com/books?id=Mj5tSld5tjMC&pg=PA159 |archive-date=15 July 2016 |url-status=live }}</ref> [[XO-1b]], [[WASP-12b]], [[WASP-17b]], and [[WASP-19b]].<ref name="NASA-20131203">{{cite web |title=Hubble Traces Subtle Signals of Water on Hazy Worlds |url=http://www.nasa.gov/content/goddard/hubble-traces-subtle-signals-of-water-on-hazy-worlds/ |date=3 December 2013 |publisher=[[NASA]] |access-date=4 December 2013 |archive-url=https://web.archive.org/web/20131206012837/http://www.nasa.gov/content/goddard/hubble-traces-subtle-signals-of-water-on-hazy-worlds/ |archive-date=6 December 2013 |url-status=live}}</ref>
* [[Stellar atmosphere]]s: not limited to cooler stars and even detected in giant hot stars such as [[Betelgeuse]], [[Mu Cephei]], [[Antares]] and [[Arcturus]].<ref name="Hanslmeier2010" /><ref name="Lund Observatory">Andersson, Jonas (June 2012). [http://lup.lub.lu.se/luur/download?func=downloadFile&recordOId=2969749&fileOId=2969772 Water in stellar atmospheres "Is a novel picture required to explain the atmospheric behavior of water in red giant stars?"] {{Webarchive|url=https://web.archive.org/web/20150213133956/http://lup.lub.lu.se/luur/download?func=downloadFile&recordOId=2969749&fileOId=2969772 |date=13 February 2015 }} Lund Observatory, Lund University, Sweden</ref>
* [[Circumstellar disk]]s: including those of more than half of [[T Tauri star]]s such as [[AA Tauri]]<ref name="Hanslmeier2010" /> as well as [[TW Hydrae]],<ref>[http://www.nasa.gov/mission_pages/herschel/news/herschel20111020.html Herschel Finds Oceans of Water in Disk of Nearby Star] {{Webarchive|url=https://web.archive.org/web/20150219053556/http://www.nasa.gov/mission_pages/herschel/news/herschel20111020.html |date=19 February 2015 }}. Nasa.gov (20 October 2011). Retrieved on 28 September 2015.</ref><ref>{{Cite web|url=https://jpl.nasa.gov/|archiveurl=https://web.archive.org/web/20120604082809/http://www.jpl.nasa.gov/news/news.cfm?release=2011-327|url-status=dead|title=JPL|archivedate=4 June 2012|website=NASA Jet Propulsion Laboratory (JPL)}}</ref> [[IRC +10216]]<ref>Lloyd, Robin. ''"Water Vapor, Possible Comets, Found Orbiting Star"'', 11 July 2001, [http://www.space.com/searchforlife/swas_water_010711.html Space.com]. Retrieved 15 December 2006. {{webarchive |url=https://web.archive.org/web/20090523025818/http://www.space.com/searchforlife/swas_water_010711.html |date=23 May 2009 }}</ref> and [[APM 08279+5255]],<ref name="Clavin" /><ref name="water vapor cloud" /> [[VY Canis Majoris]] and [[S Persei]].<ref name="Lund Observatory" />

====Liquid water====
Liquid water is present on Earth, covering 71% of its surface.<ref name="WSS" /> Liquid water is also occasionally present in small amounts [[Water on Mars|on Mars]].<ref>{{cite web |title=NASA Confirms Evidence That Liquid Water Flows on Today's Mars |url=https://www.nasa.gov/press-release/nasa-confirms-evidence-that-liquid-water-flows-on-today-s-mars |website=NASA |access-date=22 June 2020 |date=September 28, 2015 |archive-date=28 September 2015 |archive-url=https://archive.today/20150928154622/http://www.nasa.gov/press-release/nasa-confirms-evidence-that-liquid-water-flows-on-today-s-mars/ |url-status=live }}</ref> Scientists believe liquid water is present in the Saturnian moons of [[Enceladus (moon)|Enceladus]], as a 10-kilometre thick ocean approximately 30–40 kilometres below Enceladus' south polar surface,<ref name="NASA-20140403">{{cite web |last1=Platt |first1=Jane |last2=Bell |first2=Brian |title=NASA Space Assets Detect Ocean inside Saturn Moon |url=http://www.jpl.nasa.gov/news/news.php?release=2014-103 |date=3 April 2014 |website=[[NASA]] |access-date=3 April 2014 |archive-url=https://web.archive.org/web/20140403235224/http://www.jpl.nasa.gov/news/news.php?release=2014-103 |archive-date=3 April 2014 |url-status=live}}</ref><ref name="SCI-20140404">{{cite journal |last1=Iess |first1=L. |last2=Stevenson |first2=D. J. |last3=Parisi |first3=M. |last4=Hemingway |first4=D. |last5=Jacobson |first5=R.A. |last6=Lunine |first6=Jonathan I. |last7=Nimmo |first7=F. |last8=Armstrong |first8=J. W. |last9=Asmar |first9=S. W. |last10=Ducci |first10=M. |last11=Tortora |first11=P. |title=The Gravity Field and Interior Structure of Enceladus |date=April 4, 2014 |journal=[[Science (journal)|Science]] |volume=344 |number=6179 |pages=78–80 |doi=10.1126/science.1250551 |bibcode=2014Sci...344...78I |pmid=24700854|s2cid=28990283 |url=https://authors.library.caltech.edu/45462/7/Iess-SM.pdf |access-date=14 July 2019 |archive-url=https://web.archive.org/web/20171202120709/https://authors.library.caltech.edu/45462/7/Iess-SM.pdf |archive-date=2 December 2017 |url-status=live }}</ref> and [[Titan (moon)|Titan]], as a subsurface layer, possibly mixed with [[ammonia]].<ref>{{Cite journal |url=http://www.lpi.usra.edu/meetings/lpsc2013/pdf/2454.pdf |bibcode=2013LPI....44.2454D |author1=Dunaeva, A.N. |author2=Kronrod, V.A. |author3=Kuskov, O.L. |title=Numerical Models of Titan's Interior with Subsurface Ocean |journal=44th Lunar and Planetary Science Conference (2013) |issue=1719 |page=2454 |year=2013 |access-date=23 March 2014 |archive-url=https://web.archive.org/web/20140323033113/http://www.lpi.usra.edu/meetings/lpsc2013/pdf/2454.pdf |archive-date=23 March 2014 |url-status=live}}</ref> Jupiter's moon [[Europa (moon)|Europa]] has surface characteristics which suggest a subsurface liquid water ocean.<ref>{{cite web |url=http://people.msoe.edu/~tritt/sf/europa.life.html |title=Possibility of Life on Europa |last=Tritt |first=Charles S. |access-date=10 August 2007 |publisher=Milwaukee School of Engineering |date=2002 |url-status=dead |archive-url=https://web.archive.org/web/20070609150109/http://people.msoe.edu/~tritt/sf/europa.life.html |archive-date=9 June 2007}}</ref> Liquid water may also exist on Jupiter's moon [[Ganymede (moon)|Ganymede]] as a layer sandwiched between high pressure ice and rock.<ref>Dunham, Will. (3 May 2014) [http://in.reuters.com/article/us-space-ganymede-idINKBN0DJ00H20140503 Jupiter's moon Ganymede may have 'club sandwich' layers of ocean | Reuters] {{Webarchive|url=https://web.archive.org/web/20140503100145/http://in.reuters.com/article/2014/05/03/us-space-ganymede-idINKBN0DJ00H20140503 |date=3 May 2014 }}. In.reuters.com. Retrieved on 28 September 2015.</ref>

====Water ice====
Water is present as ice on:
[[File:South Polar Cap of Mars during Martian South summer 2000.jpg|thumb|South polar ice cap of Mars during Martian south summer 2000]]
* [[Water on Mars|Mars]]: under the regolith and at the poles.<ref>{{cite book |last=Carr |first=M.H. |date=1996 |title=Water on Mars |publisher=Oxford University Press |location=New York |page=197}}</ref><ref>{{cite journal |last1=Bibring |first1=J.-P. |last2=Langevin |first2=Yves |date=2004 |title=Perennial Water Ice Identified in the South Polar Cap of Mars |journal=Nature |volume=428 |issue=6983 |pages=627–630 |doi=10.1038/nature02461|pmid=15024393 |last3=Poulet |first3=François |last4=Gendrin |first4=Aline |last5=Gondet |first5=Brigitte |last6=Berthé |first6=Michel |last7=Soufflot |first7=Alain |last8=Drossart |first8=Pierre |last9=Combes |first9=Michel |last10=Bellucci |first10=Giancarlo |last11=Moroz |first11=Vassili |last12=Mangold |first12=Nicolas |last13=Schmitt |first13=Bernard |last14=Omega Team |first14=the|last15=Erard |first15=S. |last16=Forni |first16=O. |last17=Manaud |first17=N. |last18=Poulleau |first18=G. |last19=Encrenaz |first19=T.|author19-link=Thérèse Encrenaz |last20=Fouchet |first20=T. |last21=Melchiorri |first21=R. |last22=Altieri |first22=F. |last23=Formisano |first23=V. |last24=Bonello |first24=G. |last25=Fonti |first25=S. |last26=Capaccioni |first26=F. |last27=Cerroni |first27=P. |last28=Coradini |first28=A. |last29=Kottsov |first29=V. |last30=Ignatiev |first30=N. |bibcode=2004Natur.428..627B |s2cid=4373206 }}</ref>
* Earth–Moon system: mainly as [[ice sheet]]s on Earth and in Lunar craters and volcanic rocks<ref>[http://www.spiegel.de/wissenschaft/weltall/0,1518,564911,00.html Versteckt in Glasperlen: Auf dem Mond gibt es Wasser – Wissenschaft –] {{Webarchive|url=https://web.archive.org/web/20080710220126/http://www.spiegel.de/wissenschaft/weltall/0,1518,564911,00.html |date=10 July 2008 }} [[Der Spiegel]] – Nachrichten</ref> NASA reported the detection of water molecules by NASA's Moon Mineralogy Mapper aboard the Indian Space Research Organization's Chandrayaan-1 spacecraft in September 2009.<ref>[https://science.nasa.gov/headlines/y2009/24sep_moonwater.htm Water Molecules Found on the Moon] {{webarchive|url=https://web.archive.org/web/20090927092541/https://science.nasa.gov/headlines/y2009/24sep_moonwater.htm |date=27 September 2009 }}, NASA, 24 September 2009</ref>
* [[Ceres (dwarf planet)|Ceres]]<ref name="McCord2005-jgrp">{{cite journal |title=Ceres: Evolution and current state |journal=Journal of Geophysical Research: Planets |date=21 May 2005 |last1=McCord |first1=T.B. |last2=Sotin |first2=C. |volume=110 |issue=E5 |page=E05009 |doi=10.1029/2004JE002244 |bibcode=2005JGRE..110.5009M |doi-access=free |url=https://hal.archives-ouvertes.fr/hal-00116029/file/2004JE002244.pdf |access-date=5 March 2024 |archive-date=18 July 2021 |archive-url=https://web.archive.org/web/20210718171117/https://hal.archives-ouvertes.fr/hal-00116029/file/2004JE002244.pdf |url-status=live }}</ref><ref name="Thomas2005">{{cite journal |first1=P.C. |last1=Thomas |last2=Parker|first2=J.Wm.|last3=McFadden|first3= L.A. |title=Differentiation of the asteroid Ceres as revealed by its shape |year=2005 |journal=Nature |volume=437 |pages=224–226 |doi=10.1038/nature03938 |bibcode=2005Natur.437..224T |pmid=16148926 |issue=7056 |s2cid=17758979}}</ref><ref name="Carey2006">{{cite news|url=http://space.com/scienceastronomy/050907_ceres_planet.html |title=Largest Asteroid Might Contain More Fresh Water than Earth |first=Bjorn |last=Carey |publisher=SPACE.com |date=7 September 2005 |access-date=16 August 2006 |archive-url=https://web.archive.org/web/20101218180330/http://www.space.com/scienceastronomy/050907_ceres_planet.html |archive-date=18 December 2010 |url-status=live}}</ref>
* Jupiter's moons: [[Europa (moon)|Europa]]'s surface and also that of [[Ganymede (moon)|Ganymede]]<ref name="NYT-20150315">{{cite news |last=Chang |first=Kenneth |title=Suddenly, It Seems, Water Is Everywhere in Solar System |url=https://www.nytimes.com/2015/03/13/science/space/suddenly-it-seems-water-is-everywhere-in-solar-system.html |date=12 March 2015 |work=[[New York Times]] |access-date=12 March 2015 |archive-url=https://web.archive.org/web/20180812232556/https://www.nytimes.com/2015/03/13/science/space/suddenly-it-seems-water-is-everywhere-in-solar-system.html |archive-date=12 August 2018 |url-status=live}}</ref> and [[Callisto (moon)|Callisto]]<ref name=Kuskov2005>{{cite journal| last=Kuskov|first=O.L.|author2=Kronrod, V.A.|title=Internal structure of Europa and Callisto| year=2005|volume=177| issue=2|pages=550–369|doi=10.1016/j.icarus.2005.04.014| bibcode=2005Icar..177..550K| journal = Icarus}}</ref><ref name="Showman1999">{{cite journal|last1= Showman|first1=A. P.|last2= Malhotra|first2= R.|title=The Galilean Satellites|journal= Science|volume= 286|issue= 5437|date= 1 October 1999|pages =77–84|doi= 10.1126/science.286.5437.77|pmid=10506564|s2cid=9492520|url= http://pdfs.semanticscholar.org/3e6e/f125bbbafd779a0af6813ba0f5a18edea652.pdf|archive-url= https://web.archive.org/web/20200412142819/http://pdfs.semanticscholar.org/3e6e/f125bbbafd779a0af6813ba0f5a18edea652.pdf|url-status= dead|archive-date= 12 April 2020}}</ref>
* Saturn: in the [[Rings of Saturn|planet's ring system]]<ref name="Sparrow">{{cite book |last=Sparrow |first=Giles |title=The Solar System |publisher=Thunder Bay Press |year=2006 |isbn=978-1-59223-579-7}}</ref> and on the surface and mantle of [[Titan (moon)|Titan]]<ref name="Tobie">{{cite journal |last1=Tobie |first1=G. |last2=Grasset |first2=Olivier |last3=Lunine |first3=Jonathan I. |last4=Mocquet |first4=Antoine |last5=Sotin |first5=Christophe
|date=2005 |bibcode=2005Icar..175..496T |title=Titan's internal structure inferred from a coupled thermal-orbital model |journal=Icarus |volume=175 |issue=2 |pages=496–502 |doi=10.1016/j.icarus.2004.12.007 }}</ref> and [[Enceladus (moon)|Enceladus]]<ref name="Verbiscer et al. 2007">{{cite journal| doi = 10.1126/science.1134681| last1 = Verbiscer| first1 = A.| last2 = French| first2 = R.| last3 = Showalter| first3 = M.| last4 = Helfenstein| first4 = P.| title = Enceladus: Cosmic Graffiti Artist Caught in the Act| journal = Science| volume = 315| issue = 5813| page = 815| date = 9 February 2007| pmid = 17289992| bibcode = 2007Sci...315..815V| s2cid = 21932253| ref = {{sfnRef|Verbiscer French et al.|2007}}| df = dmy-all}} (supporting online material, table S1)</ref>
* [[Pluto]]–[[Charon (moon)|Charon]] system<ref name="Sparrow" />
* [[Comets]]<ref>{{cite journal |bibcode=1998A&A...330..375G |title=Making a comet nucleus |last1=Greenberg |first1=J. Mayo |volume=330 |date=1998 |page=375 |journal=Astronomy and Astrophysics}}</ref><ref>{{cite web |url=http://starryskies.com/solar_system/Comet/dirty_snowballs.html |title=Dirty Snowballs in Space |publisher=Starryskies |access-date=15 August 2013 |url-status=dead |archive-url=https://web.archive.org/web/20130129035627/http://starryskies.com/solar_system/Comet/dirty_snowballs.html |archive-date=29 January 2013}}</ref> and other related [[Kuiper belt]] and [[Oort cloud]] objects<ref>{{cite journal |author=E.L. Gibb |author2=M.J. Mumma |author3=N. Dello Russo |author4=M.A. DiSanti |author5=K. Magee-Sauer |date=2003 |title=Methane in Oort Cloud comets |journal=[[Icarus (journal)|Icarus]] |volume=165 |issue=2 |pages=391–406 |bibcode=2003Icar..165..391G |doi=10.1016/S0019-1035(03)00201-X }}</ref>

And is also likely present on:
* [[Mercury (planet)|Mercury]]'s poles<ref>NASA, "[http://www.nasa.gov/mission_pages/messenger/media/PressConf20121129.html MESSENGER Finds New Evidence for Water Ice at Mercury's Poles] {{Webarchive|url=https://web.archive.org/web/20121130062257/http://www.nasa.gov/mission_pages/messenger/media/PressConf20121129.html |date=30 November 2012 }}", ''NASA'', 29 November 2012.</ref>
* [[Tethys (moon)|Tethys]]<ref>{{cite journal| doi = 10.1016/j.icarus.2007.03.012| last1 = Thomas| first1 = P.C.| last2 = Burns| first2 = J.A.| last3 = Helfenstein| first3 = P.| last4 = Squyres| first4 = S.| last5 = Veverka| first5 = J.| last6 = Porco| first6 = C.| last7 = Turtle| first7 = E.P.| last8 = McEwen| first8 = A.| last9 = Denk| first9 = T.| first10 = B.| last10 = Giesef| first11 = T.| last11 = Roatschf| first12 = T.V.| last12 = Johnsong| first13 = R.A.| last13 = Jacobsong| date = October 2007| title = Shapes of the saturnian icy satellites and their significance| journal = Icarus| volume = 190| issue = 2| pages = 573–584| bibcode = 2007Icar..190..573T| url = http://www.geoinf.fu-berlin.de/publications/denk/2007/ThomasEtAl_SaturnMoonsShapes_Icarus_2007.pdf| access-date = 15 December 2011| ref = {{sfnRef|Thomas Burns et al.|2007}}| archive-url = https://web.archive.org/web/20110927220431/http://www.geoinf.fu-berlin.de/publications/denk/2007/ThomasEtAl_SaturnMoonsShapes_Icarus_2007.pdf| archive-date = 27 September 2011| url-status=live| df = dmy-all}}</ref>

====Exotic forms====
Water and other [[Volatile (astrogeology)|volatiles]] probably comprise much of the internal structures of [[Uranus]] and [[Neptune]] and the water in the deeper layers may be in the form of [[ionic water]] in which the molecules break down into a soup of hydrogen and oxygen ions, and deeper still as [[superionic water]] in which the oxygen crystallizes, but the hydrogen ions float about freely within the oxygen lattice.<ref name="newscientist.com">[https://www.newscientist.com/article/mg20727764.500-weird-water-lurking-inside-giant-planets.html Weird water lurking inside giant planets] {{Webarchive|url=https://web.archive.org/web/20150415160045/http://www.newscientist.com/article/mg20727764.500-weird-water-lurking-inside-giant-planets.html |date=15 April 2015 }}, ''New Scientist'', 1 September 2010, Magazine issue 2776.</ref>

===Water and planetary habitability===
{{Further|Water distribution on Earth|Planetary habitability}}

The existence of liquid water, and to a lesser extent its gaseous and solid forms, on Earth are vital to the existence of [[Organism|life on Earth]] as we know it. The Earth is located in the [[habitable zone]] of the [[Solar System]]; if it were slightly closer to or farther from the [[Sun]] (about 5%, or about 8 million kilometers), the conditions which allow the three forms to be present simultaneously would be far less likely to exist.<ref>{{cite book |chapter=J.C.I. Dooge. "Integrated Management of Water Resources" |editor1=Ehlers, E. |editor2=Krafft, T |title=Understanding the Earth System: compartments, processes, and interactions |publisher=Springer |year=2001 |page=116}}</ref><ref>{{cite web |title=Habitable Zone |url=http://www.daviddarling.info/encyclopedia/H/habzone.html |website=The Encyclopedia of Astrobiology, Astronomy and Spaceflight |access-date=26 April 2007 |archive-url=https://web.archive.org/web/20070523143747/http://www.daviddarling.info/encyclopedia/H/habzone.html |archive-date=23 May 2007 |url-status=live }}</ref>

Earth's [[gravity]] allows it to hold an [[Celestial body atmosphere|atmosphere]]. Water vapor and carbon dioxide in the atmosphere provide a temperature buffer ([[greenhouse effect]]) which helps maintain a relatively steady surface temperature. If Earth were smaller, a thinner atmosphere would allow temperature extremes, thus preventing the accumulation of water except in [[polar ice cap]]s (as on [[Mars]]).{{Citation needed|date=May 2018}}

The surface temperature of Earth has been relatively constant through [[geologic time]] despite varying levels of incoming solar radiation ([[insolation]]), indicating that a dynamic process governs Earth's temperature via a combination of greenhouse gases and surface or atmospheric [[albedo]]. This proposal is known as the [[Gaia hypothesis]].{{Citation needed|date=May 2018}}

The state of water on a planet depends on ambient pressure, which is determined by the planet's gravity. If a planet is sufficiently massive, the water on it may be solid even at high temperatures, because of the high pressure caused by gravity, as it was observed on exoplanets [[Gliese 436 b]]<ref>{{cite news |magazine=New Scientist |url=https://www.newscientist.com/article/dn11864-strange-alien-world-made-of-hot-ice-and-steam.html |title=Strange alien world made of "hot ice" |date=6 May 2007 |first=David |last=Shiga |access-date=28 March 2010 |url-status=dead |archive-url=https://web.archive.org/web/20080706143705/http://space.newscientist.com/article/dn11864-strange-alien-world-made-of-hot-ice-and-steam.html |archive-date=6 July 2008}}</ref> and [[GJ 1214 b]].<ref>{{cite web |url=http://www.cfa.harvard.edu/news/2009/pr200924.html |title=Astronomers Find Super-Earth Using Amateur, Off-the-Shelf Technology |author=Aguilar, David A. |date=16 December 2009 |publisher=Harvard-Smithsonian Center for Astrophysics |access-date=28 March 2010 |archive-url=https://web.archive.org/web/20120407045343/http://www.cfa.harvard.edu/news/2009/pr200924.html |archive-date=7 April 2012 |url-status=live}}</ref>

==Law, politics, and crisis==
{{Main|Water law|Water right|Water scarcity}}
{{update|section|date=June 2022}}
[[File:Access to drinking water in third world.svg|thumb|upright=1.35|An estimate of the proportion of people in developing countries with access to [[potable water]] 1970–2000]]

[[Water politics]] is politics affected by water and [[water resources]]. Water, particularly fresh water, is a strategic resource across the world and an important element in many political conflicts. It causes health impacts and damage to biodiversity.

Access to safe drinking water has improved over the last decades in almost every part of the world, but approximately one billion people still lack access to safe water and over 2.5 billion lack access to adequate [[sanitation]].<ref name=UN /> However, some observers have estimated that by 2025 more than half of the [[world population]] will be facing water-based vulnerability.<ref>{{cite journal |last=Kulshreshtha |first=S. N |year=1998 |title=A Global Outlook for Water Resources to the Year 2025 |journal=Water Resources Management |volume=12 |issue=3 |pages=167–184 |doi=10.1023/A:1007957229865|s2cid=152322295 }}</ref> A report, issued in November 2009, suggests that by 2030, in some developing regions of the world, water demand will exceed supply by 50%.<ref>{{cite web |url=http://www.mckinsey.com/App_Media/Reports/Water/Charting_Our_Water_Future_Full_Report_001.pdf |title=Charting Our Water Future: Economic frameworks to inform decision-making |access-date=25 July 2010 |url-status=dead |archive-url=https://web.archive.org/web/20100705072816/http://www.mckinsey.com/App_Media/Reports/Water/Charting_Our_Water_Future_Full_Report_001.pdf |archive-date=5 July 2010 }}</ref>

1.6 billion people have gained access to a safe water source since 1990.<ref>[http://mdgs.un.org/unsd/mdg/Resources/enwiki/static/Products/Progress2008/MDG_Report_2008_En.pdf#page=44 "The Millennium Development Goals Report"]. {{Webarchive|url=https://web.archive.org/web/20100827045721/http://mdgs.un.org/unsd/mdg/Resources/enwiki/static/Products/Progress2008/MDG_Report_2008_En.pdf#page=44 |date=27 August 2010 }}, United Nations, 2008</ref> The proportion of people in [[Developing country|developing countries]] with [[WASH|access to safe water]] is calculated to have improved from 30% in 1970<ref name=lomborg>{{cite book |last=Lomborg |first=Björn |year=2001 |title=The Skeptical Environmentalist |publisher=[[Cambridge University Press]] |isbn=978-0-521-01068-9 |url=http://www.lomborg.com/dyn/files/basic_items/69-file/skeptenvironChap1.pdf |page=22 |url-status=dead |archive-url=https://web.archive.org/web/20130725173040/http://www.lomborg.com/dyn/files/basic_items/69-file/skeptenvironChap1.pdf |archive-date=25 July 2013}}</ref> to 71% in 1990, 79% in 2000, and 84% in 2004.<ref name=UN>{{cite web |url=http://mdgs.un.org/unsd/mdg/Resources/enwiki/static/Products/Progress2008/MDG_Report_2008_En.pdf#page=44 |title=MDG Report 2008 |access-date=25 July 2010 |archive-url=https://web.archive.org/web/20100827045721/http://mdgs.un.org/unsd/mdg/Resources/enwiki/static/Products/Progress2008/MDG_Report_2008_En.pdf#page=44 |archive-date=27 August 2010 |url-status=live}}</ref>

A 2006 United Nations report stated that "there is enough water for everyone", but that access to it is hampered by mismanagement and corruption.<ref>[[UNESCO]], (2006), [http://unesdoc.unesco.org/images/0014/001444/144409E.pdf "Water, a shared responsibility. The United Nations World Water Development Report 2"]. {{Webarchive|url=https://web.archive.org/web/20090106144926/http://unesdoc.unesco.org/images/0014/001444/144409E.pdf |date=6 January 2009 }}</ref> In addition, global initiatives to improve the efficiency of aid delivery, such as the [[Paris Declaration on Aid Effectiveness]], have not been taken up by water sector donors as effectively as they have in education and health, potentially leaving multiple donors working on overlapping projects and recipient governments without empowerment to act.<ref>Welle, Katharina; Evans, Barbara; Tucker, Josephine; and Nicol, Alan (2008). [http://www.odi.org.uk/resources/download/1894.pdf "Is water lagging behind on Aid Effectiveness?"] {{Webarchive|url=https://web.archive.org/web/20110727024835/http://www.odi.org.uk/resources/download/1894.pdf |date=27 July 2011 }}</ref>

The authors of the 2007 [[Comprehensive Assessment of Water Management in Agriculture]] cited poor governance as one reason for some forms of water scarcity. Water governance is the set of formal and informal processes through which decisions related to water management are made. Good water governance is primarily about knowing what processes work best in a particular physical and socioeconomic context. Mistakes have sometimes been made by trying to apply 'blueprints' that work in the developed world to developing world locations and contexts. The Mekong river is one example; a review by the [[International Water Management Institute]] of policies in six countries that rely on the Mekong river for water found that thorough and transparent cost-benefit analyses and environmental impact assessments were rarely undertaken. They also discovered that Cambodia's draft water law was much more complex than it needed to be.<ref>{{cite web |url=http://www.iwmi.cgiar.org/Publications/Water_Issue_Briefs/index.aspx |title=Search Results |website=International Water Management Institute (IWMI) |access-date=3 March 2016 |archive-url=https://web.archive.org/web/20130605124732/http://www.iwmi.cgiar.org/Publications/Water_Issue_Briefs/index.aspx |archive-date=5 June 2013 |url-status=live}}</ref>

In 2004, the UK charity [[WaterAid]] reported that a child dies every 15 seconds from easily preventable water-related diseases, which are often tied to a lack of adequate sanitation.<ref name="Burrows 2004 e724">{{cite web | last=Burrows | first=Gideon | title=Clean water to fight poverty | website=The Guardian | date=March 24, 2004 | url=https://www.theguardian.com/environment/2004/mar/24/water.comment | access-date=February 16, 2024 | archive-date=16 February 2024 | archive-url=https://web.archive.org/web/20240216033934/https://www.theguardian.com/environment/2004/mar/24/water.comment | url-status=live }}</ref><ref name="Morris_2004">{{cite journal |last1=Morris |first1=Kelly |date=2004-03-20 |title="Silent emergency" of poor water and sanitation |url=https://www.thelancet.com/journals/lancet/article/PIIS0140-6736(04)15825-X/fulltext |journal=Medicine and Health Policy |volume=363 |issue=9413 |pages=954 |doi=10.1016/S0140-6736(04)15825-X |s2cid=29128993 |access-date=2024-02-16 |archive-date=22 February 2024 |archive-url=https://web.archive.org/web/20240222045218/https://www.thelancet.com/journals/lancet/article/PIIS0140-6736(04)15825-X/abstract |url-status=live |url-access=subscription }}</ref>

Since 2003, the [[UN World Water Development Report]], produced by the [[UNESCO]] [[World Water Assessment Programme]], has provided decision-makers with tools for developing sustainable [[Water politics|water policies]].<ref name="unesco">{{Cite web |title=Home {{!}} UN World Water Development Report 2023 |url=https://www.unesco.org/reports/wwdr/2023/en |access-date=2023-06-05 |website=www.unesco.org |language=en |archive-date=5 June 2023 |archive-url=https://web.archive.org/web/20230605100146/https://www.unesco.org/reports/wwdr/2023/en |url-status=live }}</ref> The 2023 report states that two billion people (26% of the population) do not have access to [[drinking water]] and 3.6 billion (46%) lack access to safely managed sanitation.<ref>{{Cite web |date=2023-03-29 |title=UN World Water Development Report 2023 |url=https://www.rural21.com/english/publications/detail/article/un-world-water-development-report-2023.html |access-date=2023-06-05 |website=www.rural21.com |language=en-GB |archive-date=5 June 2023 |archive-url=https://web.archive.org/web/20230605100140/https://www.rural21.com/english/publications/detail/article/un-world-water-development-report-2023.html |url-status=live }}</ref> People in urban areas (2.4 billion) will face [[water scarcity]] by 2050.<ref name="unesco" /> Water scarcity has been described as endemic, due to [[Overconsumption (economics)|overconsumption]] and [[pollution]].<ref>{{Cite web |date=2023-03-22 |title=UN warns 'vampiric' water use leading to 'imminent' global crisis |url=https://www.france24.com/en/americas/20230322-un-warns-vampiric-water-use-leading-to-imminent-global-crisis |access-date=2023-06-05 |website=France 24 |language=en |archive-date=5 June 2023 |archive-url=https://web.archive.org/web/20230605100139/https://www.france24.com/en/americas/20230322-un-warns-vampiric-water-use-leading-to-imminent-global-crisis |url-status=live }}</ref> The report states that 10% of the world's population lives in countries with high or critical water stress. Yet over the past 40 years, water consumption has increased by around 1% per year, and is expected to grow at the same rate until 2050. Since 2000, [[flooding]] in the tropics has quadrupled, while flooding in northern mid-latitudes has increased by a factor of 2.5.<ref>{{Cite news |date=2023-03-22 |title=New UN report paints stark picture of huge changes needed to deliver safe drinking water to all people |language=en-AU |work=ABC News |url=https://www.abc.net.au/news/2023-03-22/un-26-per-cent-of-world-lacks-clean-drinking-water-46-sanitation/102132320 |access-date=2023-06-05 |archive-date=5 June 2023 |archive-url=https://web.archive.org/web/20230605100633/https://www.abc.net.au/news/2023-03-22/un-26-per-cent-of-world-lacks-clean-drinking-water-46-sanitation/102132320 |url-status=live }}</ref> The cost of these floods between 2000 and 2019 was 100,000 deaths and $650 million.<ref name="unesco" />

Organizations concerned with water protection include the [[International Water Association]] (IWA), WaterAid, Water 1st, and the American Water Resources Association. The [[International Water Management Institute]] undertakes projects with the aim of using effective water management to reduce poverty. Water related conventions are [[United Nations Convention to Combat Desertification]] (UNCCD), [[International Convention for the Prevention of Pollution from Ships]], [[United Nations Convention on the Law of the Sea]] and [[Ramsar Convention]]. [[World Day for Water]] takes place on 22 March<ref>{{Cite web|title=World Water Day|url=https://www.un.org/en/observances/water-day|access-date=2020-09-10|website=United Nations|language=en|archive-date=9 September 2020|archive-url=https://web.archive.org/web/20200909130649/https://www.un.org/en/observances/water-day|url-status=live}}</ref> and [[World Oceans Day]] on 8 June.<ref>{{Cite web|title=About |website=World Oceans Day Online Portal|url=https://www.unworldoceansday.org/about|access-date=2020-09-10|archive-date=20 September 2020|archive-url=https://web.archive.org/web/20200920045516/https://unworldoceansday.org/about|url-status=live}}</ref>

==In culture==

===Religion===
{{Main|Water and religion}}
{{See also|Sacred waters}}
[[File:Inda Abba Hadera holy water.jpg|thumb|People come to Inda Abba Hadera spring ([[Inda Sillasie]], [[Ethiopia]]) to wash in holy water.]]

Water is considered a purifier in most religions. Faiths that incorporate ritual washing ([[Ritual purification|ablution]]) include [[Christianity]],<ref>{{cite book|title=The Hand Book: Surviving in a Germ-Filled World|first=Miryam |last=Z. Wahrman|year= 2016| isbn=978-1-61168-955-6| pages =46–48 |publisher=University Press of New England|quote=Water plays a role in other Christian rituals as well. ... In the early days of Christianity, two to three centuries after Christ, the lavabo (Latin for “I wash myself”), a ritual handwashing vessel and bowl, was introduced as part of Church service.}}</ref> [[Hinduism]], [[Islam]], [[Judaism]], the [[Rastafari movement]], [[Shinto]], [[Taoism]], and [[Wicca]]. Immersion (or [[aspersion]] or [[affusion]]) of a person in water is a central [[Sacrament]] of Christianity (where it is called [[baptism]]); it is also a part of the practice of other religions, including Islam (''[[Ghusl]]''), Judaism (''[[mikvah]]'') and [[Sikhism]] (''[[Amrit Sanskar]]''). In addition, a ritual bath in pure water is performed for the dead in many religions including Islam and Judaism. In Islam, the five daily prayers can be done in most cases after washing certain parts of the body using clean water (''[[wudu]]''), unless water is unavailable (see ''[[Tayammum]]''). In Shinto, water is used in almost all rituals to cleanse a person or an area (e.g., in the ritual of ''[[misogi]]'').

In Christianity, [[holy water]] is water that has been sanctified by a priest for the purpose of [[baptism]], the [[Blessing (Roman Catholic Church)|blessing]] of persons, places, and objects, or as a means of repelling evil.<ref>''Chambers's encyclopædia'', Lippincott & Co (1870). p. 394.</ref><ref>Altman, Nathaniel (2002) ''Sacred water: the spiritual source of life''. pp. 130–133. {{ISBN|1-58768-013-0}}.</ref>

In [[Zoroastrianism]], water (''[[aban|āb]]'') is respected as the source of life.<ref>{{cite web |title=ĀB i. The concept of water in ancient Iran |url=http://www.iranicaonline.org/articles/ab-i-the-concept-of-water-in-ancient-iranian-culture |website=www.iranicaonline.org |publisher=[[Encyclopedia Iranica]] |access-date=19 September 2018 |language=en |archive-url=https://web.archive.org/web/20180516223930/http://www.iranicaonline.org/articles/ab-i-the-concept-of-water-in-ancient-iranian-culture |archive-date=16 May 2018 |url-status=live}}</ref>

===Philosophy===
[[File:Icosahedron-spinoza.jpg|alt=Icosahedron as a part of Spinoza monument in Amsterdam.|thumb|[[Icosahedron]] as a part of [[Baruch Spinoza|Spinoza]] monument in [[Amsterdam]]]]
The Ancient Greek philosopher [[Empedocles]] saw [[Water (classical element)|water]] as one of the four [[classical elements]] (along with fire, earth, and [[Air (classical element)|air]]), and regarded it as an [[ylem]], or basic substance of the universe. [[Thales]], whom Aristotle portrayed as an astronomer and an engineer, theorized that the earth, which is denser than water, emerged from the water. Thales, a [[monist]], believed further that all things are made from water. [[Plato]] believed that the shape of water is an [[icosahedron]] – flowing easily compared to the cube-shaped earth.<ref>Lindberg, D. (2008). ''The beginnings of western science: The European scientific tradition in a philosophical, religious, and institutional context, prehistory to A.D. 1450'' (2nd ed.). Chicago: University of Chicago Press.</ref>

The theory of the [[Humorism|four bodily humors]] associated water with [[phlegm]], as being cold and moist. The [[Water (classical element)|classical element of water]] was also one of the [[Five elements (Chinese philosophy)|five elements]] in traditional [[Chinese philosophy]] (along with [[earth (classical element)|earth]], [[fire (classical element)|fire]], [[wood (classical element)|wood]], and [[metal (classical element)|metal]]).

Some traditional and popular [[Asian philosophy|Asian philosophical systems]] take water as a role-model. [[James Legge]]'s 1891 translation of the ''[[Dao De Jing]]'' states, "The highest excellence is like (that of) water. The excellence of water appears in its benefiting all things, and in its occupying, without striving (to the contrary), the low place which all men dislike. Hence (its way) is near to (that of) the [[Tao]]" and "There is nothing in the world more soft and weak than water, and yet for attacking things that are firm and strong there is nothing that can take precedence of it—for there is nothing (so effectual) for which it can be changed."<ref>{{cite book |url= http://www.sacred-texts.com/tao/taote.htm |via=Internet Sacred Text Archive Home |title=Tao Te Ching |access-date=25 July 2010 |archive-url=https://web.archive.org/web/20100712103909/http://www.sacred-texts.com/tao/taote.htm |archive-date=12 July 2010 |url-status=live}}</ref> ''[[Guanzi (text)|Guanzi]]'' in the "Shui di" 水地 chapter further elaborates on the symbolism of water, proclaiming that "man is water" and attributing natural qualities of the people of different Chinese regions to the character of local water resources.<ref>[http://ctext.org/guanzi/shui-di "Guanzi : Shui Di"]. Chinese Text Project. {{Webarchive|url= https://archive.today/20141106133901/http://ctext.org/guanzi/shui-di|date=6 November 2014}}. Retrieved on 28 September 2015.</ref>

=== Folklore ===
"Living water" features in Germanic and Slavic [[Folklore|folktales]] as a means of bringing the dead back to life. Note the [[Grimms' Fairy Tales|Grimm fairy-tale]] ("[[The Water of Life (German fairy tale)|The Water of Life]]") and the Russian dichotomy of {{ill|living water (folklore)|lt=living|ru|живая вода}} and {{ill|dead water (folklore)|lt=dead water|ru|ru:мёртвая вода}}. The [[Fountain of Youth]] represents a related concept of [[Magic (supernatural)|magical]] waters allegedly preventing aging.

===Art and activism===
Painter and activist [[Fredericka Foster]] curated ''The Value of Water'', at the [[Cathedral of St. John the Divine]] in New York City,<ref>{{cite news |last1=Vartanian |first1=Hrag |title=Manhattan Cathedral Explores Water in Art |url=https://hyperallergic.com/36682/the-value-of-water-cathedral-of-st-john-the-divine/ |access-date=14 December 2020 |publisher=Hyperallergic |date=October 3, 2011 |archive-date=3 February 2021 |archive-url=https://web.archive.org/web/20210203190158/https://hyperallergic.com/36682/the-value-of-water-cathedral-of-st-john-the-divine/ |url-status=live }}</ref> which anchored a year-long initiative by the Cathedral on our dependence on water.<ref>{{cite web|last1=Kowalski|first1=James A.|title=The Cathedral of St. John the Divine and The Value of Water|url=http://huffingtonpost.com/rev-dr-james-a-kowalski/the-value-of-water_1_b_994166.html|website=huffingtonpost.com|date=6 October 2011|publisher=Huffington Post|access-date=14 December 2020|archive-date=6 August 2015|archive-url=https://web.archive.org/web/20150806061621/http://www.huffingtonpost.com/rev-dr-james-a-kowalski/the-value-of-water_1_b_994166.html|url-status=live}}</ref><ref>{{cite web|url=http://vimeo.com/38030959/|title=The Value of Water at St John the Divine|last1=Foster|first1=Fredericka|website=vimeo.com|publisher=Sara Karl|access-date=14 December 2020|archive-date=1 March 2021|archive-url=https://web.archive.org/web/20210301114643/https://vimeo.com/38030959|url-status=live}}</ref> The largest exhibition to ever appear at the Cathedral,<ref>{{cite news|last1=Miller|first1=Tom|title=The Value of Water Exhibition|url=http://artsci.ucla.edu/events/value-water-exhibition|access-date=14 December 2020|publisher=UCLA Art Science Center|archive-date=3 February 2021|archive-url=https://web.archive.org/web/20210203213346/http://artsci.ucla.edu/events/value-water-exhibition|url-status=live}}</ref> it featured over forty artists, including [[Jenny Holzer]], [[Robert Longo]], [[Mark Rothko]], [[William Kentridge]], [[April Gornik]], [[Kiki Smith]], [[Pat Steir]], [[Alice Dalton Brown]], [[Teresita Fernandez]] and [[Bill Viola]].<ref>{{cite news |last1=Madel |first1=Robin |title=Through Art, the Value of Water Expressed |url=https://www.huffpost.com/entry/through-art-the-value-of-water-expressed_b_985997 |access-date=16 December 2020 |publisher=Huffington Post |date=December 6, 2017 |archive-date=1 December 2020 |archive-url=https://web.archive.org/web/20201201064707/https://www.huffpost.com/entry/through-art-the-value-of-water-expressed_b_985997 |url-status=live }}</ref><ref>{{cite web|last1=Cotter|first1=Mary|title=Manhattan Cathedral Examines 'The Value of Water' in a New Star-Studded Art Exhibition|url=http://inhabitat.com/nyc/manhattan-cathedral-examines-the-value-of-water-in-a-new-star-studded-art-exhibition/|website=Inhabitat|date=4 October 2011|access-date=14 December 2020|archive-date=8 July 2019|archive-url=https://web.archive.org/web/20190708195254/https://inhabitat.com/nyc/manhattan-cathedral-examines-the-value-of-water-in-a-new-star-studded-art-exhibition/|url-status=live}}</ref> Foster created Think About Water,<ref>{{Cite web |url=https://www.thinkaboutwater.com/ |title=Think About Water |access-date=15 December 2020 |archive-date=26 November 2020 |archive-url=https://web.archive.org/web/20201126122615/https://www.thinkaboutwater.com/ |url-status=live }}</ref>{{full citation needed|date=November 2022}} an ecological collective of artists who use water as their subject or medium. Members include Basia Irland,<ref>{{Cite web |url=https://www.basiairland.com/ |title=Basia Irland |access-date=19 August 2021 |archive-date=14 October 2021 |archive-url=https://web.archive.org/web/20211014204543/https://www.basiairland.com/ |url-status=live }}</ref>{{full citation needed|date=November 2022}} [[Aviva Rahmani]], [[Betsy Damon]], [[Diane Burko]], [[Leila Daw]], [[Stacy Levy]], Charlotte Coté,<ref>{{cite web |title=Influential Figures Dr. Charlotte Cote |url=https://tseshaht.com/history-culture/influential-figures/dr-charlotte-cote-2/ |website=Tseshaht First Nation [c̓išaaʔatḥ] |access-date=19 August 2021 |archive-date=19 August 2021 |archive-url=https://web.archive.org/web/20210819181901/https://tseshaht.com/history-culture/influential-figures/dr-charlotte-cote-2/ |url-status=live }}</ref> [[Meridel Rubenstein]], and [[Anna Macleod]].

To mark the 10th anniversary of access to water and sanitation being declared a human right by the UN, the charity WaterAid commissioned ten visual artists to show the impact of clean water on people's lives.<ref>{{cite news |title=10 years of the human rights to water and sanitation |url=https://www.unwater.org/10-years-of-the-human-rights-to-water-and-sanitation/ |access-date=19 August 2021 |agency=UN – Water Family News |publisher=United Nations |date=February 27, 2020 |archive-date=19 August 2021 |archive-url=https://web.archive.org/web/20210819190212/https://www.unwater.org/10-years-of-the-human-rights-to-water-and-sanitation/ |url-status=live }}</ref><ref>{{cite news |title=Water is sacred': 10 visual artists reflect on the human right to water |url=https://www.theguardian.com/global-development/2020/aug/04/water-is-sacred-10-visual-artists-reflect-on-the-human-right-to-water |access-date=19 August 2021 |work=The Guardian |date=August 4, 2020 |archive-date=19 August 2021 |archive-url=https://web.archive.org/web/20210819033524/https://www.theguardian.com/global-development/2020/aug/04/water-is-sacred-10-visual-artists-reflect-on-the-human-right-to-water |url-status=live }}</ref>

===Dihydrogen monoxide parody===
{{Main|Dihydrogen monoxide parody}}
'Dihydrogen monoxide' is a technically correct but rarely used [[chemical name]] of water. This name has been used in a series of [[hoaxes]] and [[pranks]] that mock [[scientific illiteracy]]. This began in 1983, when an [[April Fools' Day]] article appeared in a newspaper in [[Durand, Michigan]]. The false story consisted of safety concerns about the substance.<ref>{{cite web |url=http://www.dictionary.com/e/tech-science/dihydrogen-monoxide/ |title=dihydrogen monoxide |date=March 2018 |access-date=2 May 2018 |archive-url=https://web.archive.org/web/20180502140130/http://www.dictionary.com/e/tech-science/dihydrogen-monoxide/ |archive-date=2 May 2018 |url-status=live}}</ref>

===Music===

The word "Water" has been used by many [[Florida]] based [[Rapping|rappers]] as a sort of catchphrase or adlib. Rappers who have done this include [[BLP Kosher]] and [[Ski Mask the Slump God]].<ref>{{Cite web |date=2021-12-27 |title=What Does Water Mean In Rap? (EXPLAINED) | work = Lets Learn Slang |url=https://letslearnslang.com/what-does-water-mean-in-rap/ |access-date=2023-08-06 |language=en-US |archive-date=6 August 2023 |archive-url=https://web.archive.org/web/20230806233317/https://letslearnslang.com/what-does-water-mean-in-rap/ |url-status=live }}</ref> To go even further some rappers have made whole songs dedicated to the water in Florida, such as the 2023 [[Danny Towers]] song "Florida Water".<ref>{{Citation |title=Danny Towers, DJ Scheme & Ski Mask the Slump God (Ft. Luh Tyler) – Florida Water |url=https://genius.com/Danny-towers-dj-scheme-and-ski-mask-the-slump-god-florida-water-lyrics |access-date=2023-08-06 |archive-date=6 August 2023 |archive-url=https://web.archive.org/web/20230806234634/https://genius.com/Danny-towers-dj-scheme-and-ski-mask-the-slump-god-florida-water-lyrics |url-status=live }}</ref> Others have made whole songs dedicated to water as a whole, such as [[XXXTentacion]], and Ski Mask the Slump God with their hit song "H2O".

==See also==
{{Portal|Oceans|Renewable energy||Water|Weather}}
* {{annotated link|Outline of water}}
* {{annotated link|Water (data page)}} is a collection of the chemical and physical properties of water.
* {{annotated link|Aquaphobia}}
* {{annotated link|Blue roof}}
* {{annotated link|Catchwater}}
* {{annotated link|Human right to water and sanitation}}
* {{annotated link|Hydroelectricity}}
* {{annotated link|Marine current power}}
* {{annotated link|Marine energy}}
* {{annotated link|Mpemba effect}}
* {{annotated link|Oral rehydration therapy}}
* {{annotated link|Osmotic power}}
* {{annotated link|Properties of water}}
* {{annotated link|Thirst}}
* {{annotated link|Tidal power}}
* {{annotated link|Water pinch analysis}}
* {{annotated link|Wave power}}
* {{annotated link|Water filter}}
* [[Oxyhydrogen]]

==Notes==
{{notelist|colwidth=30em}}

==References==
{{reflist|colwidth=30em|refs=
<ref name = Braun_1993_612>{{Cite journal|last1=Braun|first1=Charles L.|last2=Smirnov|first2=Sergei N.|date=1993-08-01|title=Why is water blue?|journal=Journal of Chemical Education|volume=70|issue=8|pages=612|bibcode=1993JChEd..70..612B|doi=10.1021/ed070p612|issn=0021-9584|url=http://inside.mines.edu/fs_home/dwu/classes/CH353/study/Why%20is%20Water%20Blue.pdf|access-date=13 September 2023|archive-date=1 December 2019|archive-url=https://web.archive.org/web/20191201000418/http://inside.mines.edu/fs_home/dwu/classes/CH353/study/Why%20is%20Water%20Blue.pdf|url-status=live}}</ref>
}}

===Works cited===
{{Refbegin}}
* {{cite book |last1=Ball |first1=Philip |title=Life's matrix : a biography of water |date=2001 |publisher=Farrar, Straus, and Giroux |isbn=978-0-520-23008-8 |edition=}}
* {{cite book |last1=Franks |first1=Felix |title=Water : a matrix of life |date=2007 |publisher=Royal Society of Chemistry |isbn=978-1-84755-234-1 |edition=2nd}}
*{{Cite book|url=https://books.google.com/books?id=kTnxSi2B2FcC|title=CRC Handbook of Chemistry and Physics|edition=84th|last=Lide|first=David R.|date=2003|series=[[CRC Handbook]]|publisher=CRC Press|isbn=978-0-8493-0484-2|language=en|access-date=14 December 2023|archive-date=4 February 2024|archive-url=https://web.archive.org/web/20240204075307/https://books.google.com/books?id=kTnxSi2B2FcC|url-status=live}}
*{{cite book|first1=Hermann|last1=Weingärtner|first2=Ilka|last2=Teermann|first3=Ulrich|last3=Borchers|first4=Peter|last4=Balsaa|first5=Holger V.|last5=Lutze|first6=Torsten C.|last6=Schmidt|first7=Ernst Ulrich|last7=Franck|first8=Gabriele|last8=Wiegand|first9=Nicolaus|last9=Dahmen|first10=Georg|last10=Schwedt|first11=Fritz H.|last11=Frimmel|first12=Birgit C.|last12=Gordalla|chapter=Water, 1. Properties, Analysis, and Hydrological Cycle|title = Ullmann's Encyclopedia of Industrial Chemistry|publisher=Wiley-VCH Verlag GmbH & Co. KGaA|isbn=978-3-527-30673-2|doi=10.1002/14356007.a28_001.pub3|year = 2016|ref = {{harvid|Weingärtner et al.|2016}}|title-link=Ullmann's Encyclopedia of Industrial Chemistry}}
{{Refend}}

==Further reading==
{{Refbegin}}
* Debenedetti, PG., and HE Stanley, "Supercooled and Glassy Water", ''Physics Today'' '''56''' (6), pp.&nbsp;40–46 (2003). [http://polymer.bu.edu/hes/articles/ds03.pdf Downloadable PDF (1.9 MB)] {{Webarchive|url=https://web.archive.org/web/20181101114735/http://polymer.bu.edu/hes/articles/ds03.pdf |date=1 November 2018 }}
* Gleick, PH., (editor), ''The World's Water: The Biennial Report on Freshwater Resources''. Island Press, Washington, D.C. (published every two years, beginning in 1998.) [http://www.worldwater.org/ The World's Water, Island Press] {{Webarchive|url=https://web.archive.org/web/20090226224320/http://www.worldwater.org/ |date=26 February 2009 }}
* {{cite journal |last1=Jones |first1=Oliver A. |last2=Lester |first2=John N. |last3=Voulvoulis |first3=Nick |title=Pharmaceuticals: a threat to drinking water? |journal=Trends in Biotechnology |volume=23 |issue=4 |year=2005 |pages=163–167 |doi=10.1016/j.tibtech.2005.02.001|pmid=15780706 }}
* [http://ucowr.org/journal-of-contemporary-water-research-and-education Journal of Contemporary Water Research & Education] {{Webarchive|url=https://web.archive.org/web/20160303210459/http://ucowr.org/journal-of-contemporary-water-research-and-education |date=3 March 2016 }}
* Postel, S., ''Last Oasis: Facing Water Scarcity''. W.W. Norton and Company, New York. 1992
* Reisner, M., ''Cadillac Desert: The American West and Its Disappearing Water''. Penguin Books, New York. 1986.
* [http://www.unesco.org/water/wwap/wwdr/ United Nations World Water Development Report] {{Webarchive|url=https://web.archive.org/web/20090222101824/http://www.unesco.org/water/wwap/wwdr/ |date=22 February 2009 }}. Produced every three years.
* St. Fleur, Nicholas. [https://www.nytimes.com/2016/04/16/science/the-water-in-your-glass-might-be-older-than-the-sun.html The Water in Your Glass Might Be Older Than the Sun] {{Webarchive|url=https://web.archive.org/web/20170115152336/https://www.nytimes.com/2016/04/16/science/the-water-in-your-glass-might-be-older-than-the-sun.html |date=15 January 2017 }}. "The water you drink is older than the planet you're standing on." ''The New York Times'' (15 April 2016)
{{Refend}}

==External links==
{{Sisterlinks|water}}
* [http://www.worldwater.org/ The World's Water Data Page]
* [http://www.fao.org/nr/water/aquastat/main/index.stm FAO Comprehensive Water Database, AQUASTAT]
* [http://worldwater.org/conflict.html The Water Conflict Chronology: Water Conflict Database] {{Webarchive|url=https://web.archive.org/web/20130116181835/http://www.worldwater.org/conflict.html |date=16 January 2013 }}
* [http://ga.water.usgs.gov/edu/ Water science school] (USGS)
* [http://water.worldbank.org/ Portal to The World Bank's strategy, work and associated publications on water resources]
* [http://www.awra.org/ America Water Resources Association] {{Webarchive|url=https://web.archive.org/web/20180324205603/http://awra.org/ |date=24 March 2018 }}
* [https://www.waterontheweb.org Water on the web]
* [http://www1.lsbu.ac.uk/water/ Water structure and science] {{Webarchive|url=https://web.archive.org/web/20141228024506/http://www1.lsbu.ac.uk/water/ |date=28 December 2014 }}
* [https://www.youtube.com/watch?v=mPpKhxtFf1Q "Why water is one of the weirdest things in the universe"], ''Ideas'', [[BBC]], Video, 3:16 minutes, 2019
* [https://www.nsf.gov/news/special_reports/water/ The chemistry of water] {{Webarchive|url=https://web.archive.org/web/20200619074258/https://www.nsf.gov/news/special_reports/water/ |date=19 June 2020 }} (NSF special report)
* [http://www.iapws.org/index.html The International Association for the Properties of Water and Steam]
* [https://www.pbs.org/wgbh/molecule-that-made-us ''H2O: The Molecule That Made Us''], a 2020 [[PBS]] documentary

{{Water|expand}}
{{Food chemistry}}
{{Natural resources}}
{{Molecules detected in outer space}}
{{Authority control}}

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[[Category:Liquids]]
[[Category:Materials that expand upon freezing]]
[[Category:Nuclear reactor coolants]]
[[Category:Oxides]]
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