View source for Properties of water

{{dablink|This article describes water from a scientific and technical perspective. See [[Water]] for its importance for life and humanity, and [[Water (film)]] for the movie of the same name.}}
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{| align="right" border="1" cellspacing="0" cellpadding="3" style="margin: 0 0 0 0.5em; background: #FFFFFF; border-collapse: collapse; border-color: #C0C090;"
! {{chembox header}} | Water
|-
| align="center" colspan="2" | [[Image:Water molecule dimensions.svg|135px|The water molecule has this basic geometric structure.]][[Image:Dihydrogen monoxide.png|100px|Hydrogen monoxide, or water, has this molecular structure.]]
|-
! {{chembox header}} | General
|-
| [[IUPAC nomenclature|Systematic names]]
| Water<br/>Oxane<sup>1</sup><br/>Hydrogen oxide
|-
| Other names
| Aqua<br/>Dihydrogen monoxide
|-
| [[Chemical formula|Molecular formula]]
| H<sub>2</sub>O
|-
| [[Molar mass]]
| 18.02 g/mol
|-
| Appearance
| transparent, almost<br/>colorless liquid with<br/>a slight hint of blue [http://www.dartmouth.edu/~etrnsfer/water.htm]
|-
| [[CAS registry number|CAS number]]
| [7732-18-5]
|-
| {{chembox header}} | <small>1 This name has been criticized because "oxane"<br>is also the name of a cyclic [[ether]].
|-
! {{chembox header}} | Properties
|-
| [[Density]] and [[Phase (matter)|phase]]
| 1 g/cm<sup>3</sup>, liquid
|-
|
| 0.917 g/cm<sup>3</sup>, solid
|-
<!-- | Other solvents e.g. [[ethanol]], [[acetone]] -->
<!-- | solubility info on other solvents -->
<!-- |- -->
| [[Melting point]]
| 0 [[Celsius|&deg;C]], 32 [[Fahrenheit|&deg;F]] (273.15 [[kelvin|K]])
|-
| [[Boiling point]]
| 100 &deg;C, 212 &deg;F (373.15 K)
|-
| [[Heat capacity]] (liquid)
| 4186 J/(kg·K)
|-
| Heat capacity (gas)
| ''c<sub>p</sub>''= 1850 J/(kg·K)<br/>''c<sub>v</sub>''= 3724 J/(kg.K)
|-
| Heat capacity (solid 0 &deg;C)
| 2060 J/(kg·K)
|-
| [[Acid dissociation constant|Acidity]] (p''K''<sub>a</sub>)
| 13.995
|-
| [[Acid dissociation constant|Basicity]] (p''K''<sub>b</sub>)
| 13.995
|-
| [[Viscosity]]
| 1 [[pascal second|mPa·s]] at 20 &deg;C
|-
! {{chembox header}} | Structure
|-
| [[Orbital_hybridisation#Molecule_shape|Molecular shape]]
| non-linear bent
|-
| [[Crystal structure]] <!-- omit if not a solid -->
| Hexagonal<br/>''See [[ice]]''
|-
| [[Dipole#Molecular_dipoles|Dipole moment]]
| 1.85 [[Debye|D]]
|-
! {{chembox header}} | Hazards
|-
| [[Material safety data sheet|MSDS]]
| [[Water (data page)#Material Safety Data Sheet|External MSDS]]
|-
| Main [[Worker safety and health|hazard]]s
| No known hazard
|-
| [[NFPA 704]]
| [[Image:nfpa_h0.png]][[Image:nfpa_f0.png]][[Image:nfpa_r0.png]]
|-
| [[RTECS]] number
| ZC0110000
|-
! {{chembox header}} | [[Water (data page)|Supplementary data page]]
|-
| [[Water (data page)#Structure and properties|Structure and<br/>properties]]
| [[Refractive index|''n'']], [[Dielectric constant|''&epsilon;<sub>r</sub>'']], etc.
|-
| [[Water (data page)#Thermodynamic properties|Thermodynamic<br/>data]]
| Phase behaviour<br/>Solid, liquid, gas
|-
| [[Water (data page)#Spectral data|Spectral data]]
| [[UV/VIS spectroscopy|UV]], [[Infrared spectroscopy|IR]], [[NMR spectroscopy|NMR]], [[Mass spectrometry|MS]]
|-
! {{chembox header}} | Related compounds
|-
| Related [[solvent]]s
| [[acetone]]<br/>[[methanol]]
|-
| Related compounds
| [[ice]]<br/>[[heavy water]]
|-
| {{chembox header}} | <small>Except where noted otherwise, data are given for<br> materials in their [[standard state|standard state (at 25 &deg;C, 100 kPa)]]<br/>[[wikipedia:Chemical infobox|Infobox disclaimer and references]]</small>
|-
|}
'''[[Water]]''' has the [[chemical formula]] [[hydrogen|H]]<sub>2</sub>[[oxygen|O]], meaning that one [[molecule]] of water is composed of two [[hydrogen]] [[atom]]s and one [[oxygen]] atom.  It can also be described ionically as HOH, with a hydrogen [[ion]] (H<sup>+</sup>) that is bonded to a [[hydroxide]] ion (OH<sup>-</sup>).  It is in [[dynamic equilibrium]] between the [[liquid]] and [[vapor]] states at [[standard temperature and pressure]]. At [[room temperature]], it is a nearly [[colorless]], [[taste]]less, and [[odorless]] liquid. It is often referred to in the sciences as ''the [[universal]] [[solvent]]'' and the only pure substance found naturally in all three [[states of matter]].

==Forms of water==
:''See the [[:Category:Forms of water]]''
Water can take many forms.  The [[solid state]] of water is commonly known as [[ice]] (while many other forms exist, see [[amorphous solid water]]); the [[gaseous state]] is known as [[water vapor]] (or [[steam]]), and the common liquid [[phase]] is generally taken as simply water.
Above a certain [[critical temperature]] and pressure (647 [[Kelvin|K]] and 22.064 [[pascal|MPa]]), water molecules assume a ''supercritical'' condition, in which liquid-like clusters float within a vapor-like phase.

[[Heavy water]] is water in which the hydrogen atoms are replaced by its heavier [[isotope]], [[deuterium]]. It is ''chemically'' almost identical to normal water.  Heavy water is used in the [[Nuclear reactor|nuclear industry]] to slow down [[neutron]]s.

==A common substance==
===Water in the Universe===
Water has been found in [[interstellar cloud]]s within our [[galaxy]], the [[Milky Way]].  It is believed that water exists in abundance in other galaxies too, because its components, [[hydrogen]] and [[oxygen]], are among the most abundant elements in the [[universe]].

Interstellar clouds eventually condense into [[solar nebula]]e and [[solar system]]s, such as ours.  The initial water can then be found in [[comet]]s, [[planet]]s, and their [[natural satellite|satellite]]s.  In our solar system, water, in ice form, has been found :
* on the [[Moon]],
* on the planets [[Mercury (planet)|Mercury]], [[Mars]], [[Neptune]], and [[Pluto]],
* on satellites of planets, such as [[Triton (moon)|Triton]] and [[Europa (moon)|Europa]].
The liquid form of water is only known to occur on Earth, though strong evidence suggests that it is present just under the surface of [[Saturn (planet)|Saturn]]'s moon [[Enceladus (moon)| Enceladus]].

===Water on Earth===
The [[water cycle]] (known scientifically as the '''hydrologic cycle''') refers to the continuous exchange of water within the [[hydrosphere]], between the [[Earth atmosphere|atmosphere]], [[soil]] water, [[surface water]], [[groundwater]], and [[plant]]s.

Earth's approximate water  volume (the total water supply of the world) is 1,360,000,000 km&sup3; (326,000,000 mi³). Of this volume:

* 1,320,000,000 km³ (316,900,000 mi³ or 97.2%) is in the [[sea water|oceans]]
* 25,000,000 km³ (6,000,000 mi³ or 1.8%) is in [[glaciers]] and icecaps
* 13,000,000 km³ (3,000,000 mi³ or 0.9%) is [[groundwater]].
* 250,000 km³ (60,000 mi³ or 0.02%) is [[fresh water]] in lakes, inland seas, and rivers.
* 13,000 km³ (3,100  mi³ or 0.001%) is atmospheric water vapor at any given time.

Liquid water is found in '''bodies of water''', such as an [[ocean]], [[sea]], [[lake]], [[river]], [[stream]], [[canal]], or [[pond]].  The majority of water on Earth is [[sea water]]. Water is also present in the atmosphere in both liquid and vapor phases. It also exists as groundwater in [[aquifer]]s. Although water normally boils at about 100 &deg;C, in [[deep sea vent]]s the pressurised superheated water reaches a natural temperature of 400 °C, whereas at the top of Mount Everest, the low pressure allows water to boil at a mere 70 °C.

===Water in industry===

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 [[pollution]]. Pollution includes discharged solutes ([[water pollution|chemical pollution]]) and discharged coolant water (thermal pollution). Industry requires pure water for many applications and utilizes a variety of [[Water purification|purification]] techniques both in water supply and discharge.

==Physics and chemistry of water==

===Density of water and ice===
For most substances, the solid form of the ''substance'' is more [[density|dense]] than the liquid [[phase|form]]; thus, a block of pure solid ''substance'' will sink in a tub of pure liquid ''substance''.  But, by contrast, a block of common [[ice]] will float in a tub of water because solid water is ''less'' dense than liquid water.  This is an extremely important characteristic property of water.  At [[room temperature]], liquid water becomes denser with lowering temperature, just like other substances.  But at 4 °C, just above freezing, water reaches its maximum density, and as water cools further toward its freezing point, the liquid water, under standard conditions, expands to become ''less'' dense.  The physical reason for this is related to the [[crystal structure]] of ordinary [[ice]], known as [[Hexagonal (crystal system)|hexagonal]] [[ice Ih]]. Water, [[gallium]], [[bismuth]], [[acetic acid]], [[antimony]] and [[silicon]] are some of the few materials which expand when they freeze; most other materials contract. It should be noted however, that not all forms of ice are less dense than liquid water.  For example [[high density amorphous ice|HDA]] and [[very high density amorphous ice|VHDA]] are both more dense than liquid phase pure water.  Thus, the reason that the common form of ice is less dense than water is a bit non-intuitive, and relies heavily on the unusual properties inherent to the [[hydrogen bond]].

Generally, water expands when it freezes because of its [[molecular structure]], in tandem with the unusual [[elasticity]] of the hydrogen bond and the particular lowest energy hexagonal [[crystal]] conformation that it adopts under standard conditions.  That is, when water cools, it tries to stack in a [[crystalline lattice]] configuration that stretches the [[rotational]] and [[vibration]]al components of the bond, so that the effect is that each molecule of water is pushed further from each of its neighboring molecules.  This effectively reduces the density ''&rho;'' of water when ice is formed under standard conditions.

The importance of this property cannot be overemphasized for its role on the [[ecosystem]] of Earth.  For example, ''if'' water were more dense when frozen, lakes and oceans in a polar environment would eventually freeze solid (from top to bottom). This would happen because frozen ice would settle on the lake and riverbeds, and the necessary warming phenomenon (see below) could not occur in summer, as the warm surface layer would be less dense than the solid frozen layer below.  It is a significant feature of nature that this does not occur naturally in the environment, but under synthetic laboratory conditions where HDA and VHDA form, specialized forms of ice are more dense, and do sink to the bottom in liquid water.

Nevertheless, the unusual expansion of freezing water (in ordinary ''natural'' settings in relevant biological systems), due to the [[hydrogen bond]], from 4 °C above freezing to the freezing point offers an important advantage for freshwater life in winter. Water chilled at the surface becomes denser and sinks, forming [[convection]] [[Current (fluid)|currents]] that cool the whole water body, but when the temperature of the lake water reaches 4 °C, water on the surface, as it chills further, becomes ''less dense'', and stays as a surface layer which eventually [[freezing|freezes]] and forms ice. Since downward convection of colder water is blocked by the density change, any large body of fresh water frozen in winter will have the coldest water near the surface, away from the [[riverbed]] or lakebed. This accounts for various little known phenomenon of ice characteristics as they relate to ice in lakes and "ice falling out of lakes" as described by early 20th century scientist Horatio D. Craft.

The following table gives the density of water in grams per cubic centimeter at various temperatures in degrees Celsius:

{| class="wikitable"
!Temp (°C)!!Density (g/cm<sup>3</sup>)
|-
|30||0.9957
|-
|20||0.9982
|-
|10||0.9997
|-
|0||0.9998
|-
|-10||0.9982
|-
|-20||0.9935
|-
|-30||0.9839
|}
The values below 0 °C refer to supercooled water.

===Density of saltwater and ice===
The situation in salt water is somewhat different. Ice still floats to keep the [[oceans]] from freezing solid (see following paragraph). However, the salt content of oceans both lowers the [[Colligative properties|colligative]] [[freezing point]] by about 2 °C and lowers the temperature of the density maximum of water to be about at the freezing point. Hence, in ocean water, because of the salt content, the downward [[convection]] of colder water is ''not'' blocked by an expansion of water as it becomes colder near the freezing point; thus the oceans' cold water near the freezing point continues to sink.  For this reason, any creature attempting to survive at the bottom of such cold water as the [[Arctic Ocean]] generally lives in water that is 4 °C colder than the temperature at the bottom of frozen-over [[fresh water]] lakes and rivers in winter.

As the [[surface]] of salt water begins to freeze (at &minus;1.9 °C for normal salinity seawater, 35&#137;) the ice that forms is essentially salt free with a density approximately that of freshwater ice. This ice floats on the surface and the salt that is "frozen out" adds to the [[salinity]] and density of the seawater just below it. This more dense saltwater sinks by convection and the replacing seawater is subject to the same process. This provides essentially freshwater ice at &minus;1.9 °C on the surface. The increased density of the seawater beneath the forming ice sinks towards the bottom, thus the deep ocean waters should have a minimum temperature of &minus;1.9 °C also. However the temperature of the deep oceans is about 4 °C.

===Triple point===
The [[temperature]] and [[pressure]] at which solid, liquid, and [[Water vapor|gaseous water]] coexist in equilibrium is called the [[triple point]] of water.  This point is used to define the units of temperature (the [[kelvin]] and, indirectly, the degree [[Celsius]] and even the degree [[Fahrenheit]]).  The triple point is at a temperature of 273.16 K (0.01 °C) by convention, and at a pressure of 611.73 [[Pascal|Pa]]. This pressure is quite low, about 1/166 of the normal sea level barometric pressure of 101,325 Pa. The atmospheric surface pressure on planet [[Mars]] is remarkably close to the triple point pressure, and the zero-elevation or "sea level" of [[Mars]] is defined by the height at which the atmospheric pressure corresponds to the triple point of water.

===Mpemba effect===
The [[Mpemba effect]] is the surprising phenomenon whereby hot water can, under certain conditions, freeze faster than cold water, even though it must pass the lower temperature on the way to freezing.  However, this can be explained with [[evaporation]], [[convection]], [[supercooling]], and the [[Thermal insulation|insulating]] effect of [[frost]].

===Hot ice===
Hot ice is the name given to another surprising phenomenon in which water at room temperature can be turned into ice ''at room temperature'' by supplying an electric field of the order of 10<sup>6</sup> volts per meter. ([http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2005PhRvL..95h5701C&amp;db_key=PHY&amp;data_type=HTML&amp;format=&amp;high=42ca922c9c01734 Choi 2005])

The effect of such electric fields has been suggested as an explanation of cloud formation. The first time cloud ice forms around a clay particle, it requires a temperature of &minus;10 °C, but subsequent freezing around the same clay particle requires a temperature of just &minus;5 °C, suggesting some kind of "ice memory"  ([http://www.ingentaconnect.com/content/rms/qjrms/2005/00000131/00000608/art00019?token=00631ac58ee86ec5294e7247444f6d62222c227e372530542972715a614f7e41225f406a595720414776746770445129932 Connolly, P.J, ''et al'', 2005])

===Surface tension===
Water drops are stable thanks to the high [[surface tension]] of water. This can be seen when small quantities of water are put onto a nonsoluble surface such as glass: the water stays together as drops. This property is important for life. For example, when water is carried through [[xylem]] up stems in plants the strong intermolecular attractions hold the water column together. Strong cohesive properties hold the water column together, and strong adhesive properties stick the water to the xylem, and prevent tension rupture caused by [[transpiration pull]]. Other liquids with lower surface tension would have a higher tendency to "rip", forming vacuum or air pockets and rendering the xylem water transport inoperative.

===Electrical properties===
''Pure'' water is actually a good [[insulator]] (poor [[conductor (material)|conductor]]), meaning that it does ''not'' conduct [[electricity]] well. Because water is such a good solvent, however, it almost always has some [[solute]] dissolved in it, most frequently a [[salt]]. If water has even a tiny amount of such impurities, then it can conduct electricity much better, because impurities such as salt separate into free [[ion]]s in aqueous solution by which an electric current can flow.

Water can be split into its constituent elements, hydrogen and oxygen, by passing a current through it. This process is called [[electrolysis]]. Water molecules naturally dissociate into H<sup>+</sup> and OH<sup>-</sup> ions, which are pulled toward the [[cathode]] and [[anode]], respectively. At the cathode, two H<sup>+</sup> ions pick up electrons and form H<sub>2</sub> gas. At the anode, four OH<sup>-</sup> ions combine and release O<sub>2</sub> gas, molecular water, and four electrons. The gases produced bubble to the surface, where they can be collected. It is known that the theoretical maximum electrical resistivity for water is approximately 182 [[kilohm]]-meters (or 18.2 M&Omega;·cm) at 25 degrees Celsius. This figure agrees well with what is typically seen on reverse osmosis, ultrafiltered and deionized [[ultrapure water]] systems used for instance, in semiconductor manufacturing plants. A salt or acid contaminant level exceeding that of even 100 parts per trillion (ppt) in ultrapure water will begin to noticeably lower its resistivity level by up to several kilohm-meters (a change of several hundred [[siemens (unit)|nanosiemens]] per meter of conductance).

===Dipolar nature of water===
An important feature of water is its [[polar molecule|polar]] nature. The water molecule forms an angle, with hydrogen atoms at the tips and oxygen at the vertex. Since oxygen has a higher [[electronegativity]] than hydrogen, the side of the molecule with the oxygen atom has a partial negative charge. A molecule with such a charge difference is called a [[dipole]]. The charge differences cause water molecules to be attracted to each other (the relatively positive areas being attracted to the relatively negative areas) and to other polar molecules. This attraction is known as [[hydrogen bond]]ing, and explains many of the properties of water.

Although hydrogen bonding is a relatively weak attraction compared to the covalent bonds within the water molecule itself, it is responsible for a number of water's physical properties. One such property is its relatively high [[melting point|melting]] and [[boiling point]] temperatures; more [[heat]] energy is required to break the hydrogen bonds between molecules. The similar compound hydrogen sulfide (H<sub>2</sub>S), which has much weaker hydrogen bonding, is a gas at [[room temperature]] even though it has twice the molecular weight of water. The extra bonding between water molecules also gives liquid water a large [[specific heat capacity]]. This high heat capacity makes water a good heat storage medium.

Hydrogen bonding also gives water its unusual behavior when freezing. When cooled to near freezing point, the presence of hydrogen bonds means that the molecules, as they rearrange to minimize their energy, form the [[hexagonal]] [[crystal structure]] of ice that is actually of lower density: hence the solid form, ice, will float in water. In other words, water expands as it freezes, whereas virtually all other materials shrink on solidification.

An interesting consequence of the solid having a lower density than the liquid is that ice will melt if sufficient pressure is applied. With increasing pressure the melting point temperature drops and when the melting point temperature is lower than the ambient temperature the ice begins to melt. A significant increase of pressure is required to lower the melting point temperature by very much &mdash; the pressure exerted by an ice skater on the ice would only reduce the melting point by something like 0.09 °C.

===Water as a solvent===

Water is also a good [[solvent]] due to its [[polarity]].  When an ionic or polar compound enters water, it is surrounded by water molecules. The relatively small size of water molecules typically allows many water molecules to surround one molecule of [[solute]]. The partially negative dipole ends of the water are attracted to positively charged components of the solute, and vice versa for the positive dipole ends.

In general, ionic and polar substances such as [[acid]]s, [[alcohol]]s, and [[salt]]s are relatively soluble in water, and nonpolar substances such as fats and oils are not. Nonpolar molecules stay together in water because it is energetically more favorable for the water molecules to hydrogen bond to each other than to engage in [[van der Waals force|van der Waals interactions]]  with nonpolar molecules.

An example of an ionic solute is [[sodium chloride|table salt]]; the sodium chloride, NaCl, separates into Na<sup>+</sup> [[cation]]s and Cl<sup>-</sup> [[anion]]s, each being surrounded by water molecules. The ions are then easily transported away from their [[crystal lattice|crystalline lattice]] into solution. An example of a nonionic solute is [[sucrose|table sugar]]. The water dipoles make hydrogen bonds with the polar regions of the sugar molecule (OH groups) and allow it to be carried away into solution.

The solvent properties of water are vital in [[biology]], because many biochemical reactions take place only within aqueous [[solution]]s (e.g., reactions in the [[cytoplasm]] and [[blood]]).

===Amphoteric nature of water===

Chemically, water is [[amphoteric]] &mdash; i.e., it is able to act as either an [[acid]] or a [[base (chemistry)|base]]. Occasionally the term ''hydroxic acid'' is used when water acts as an acid in a chemical reaction. At a pH of 7 (neutral), the concentration of [[hydroxide]] ions (OH<sup>-</sup>) is equal to that of the [[hydronium]] (H<sub>3</sub>O<sup>+</sup>) or [[hydrogen]] (H<sup>+</sup>) ions. If the [[equilibrium]] is disturbed, the solution becomes acidic (higher concentration of hydronium ions) or basic (higher concentration of hydroxide ions).

Water can act as either an acid or a base in reactions. According to the [[Brønsted-Lowry]] system, an acid is defined as a species which donates a proton (an H<sup>+</sup> ion) in a reaction, and a base as one which receives a proton. When reacting with a stronger acid, water acts as a base; when reacting with a stronger base, it acts as an acid. For instance, it receives an H<sup>+</sup> ion from HCl in the equilibrium:

:HCl + H<sub>2</sub>O {{unicode|&#8652;}} H<sub>3</sub>O<sup>+</sup> + Cl<sup>-</sup>

Here water is acting as a base, by receiving an H<sup>+</sup> ion. 

In the reaction with [[ammonia]], NH<sub>3</sub>, water donates an H<sup>+</sup> ion, and is thus acting as an acid:

:NH<sub>3</sub> + H<sub>2</sub>O {{unicode|&#8652;}} NH<sub>4</sub><sup>+</sup> + OH<sup>-</sup>

===Acidity in nature===
In theory, pure water has a [[pH]] of 7 at 298 K. In practice, pure water is very difficult to produce. Water left exposed to air for any length of time will rapidly dissolve [[carbon dioxide]], forming a dilute solution of [[carbonic acid]], with a limiting pH of about 5.7. As cloud droplets form in the atmosphere and as raindrops fall through the air minor amounts of CO<sub>2</sub> are absorbed and thus most rain is slightly acidic. If high amounts of [[nitrogen]] and [[sulfur]] oxides are present in the air, they too will dissolve into the cloud and rain drops producing more serious [[acid rain]] problems.

===Hydrogen bonding in water===
Water molecule can form a maximum of four [[hydrogen bond]]s because it can accept two and donate two hydrogens. Other molecules like [[hydrogen fluoride]], [[ammonia]], [[methanol]] form hydrogen bonds but they do not show anomalous behaviour of [[thermodynamics|thermodynamic]], [[kinetic]] or structural properties like those observed in water. The answer to the apparent difference between water and other hydrogen bonding liquids lies in the fact that apart from water none of the hydrogen bonding molecules can form four hydrogen bonds either due to an inability to donate/accept hydrogens or due to [[steric]] effects in bulky residues. In water local [[tetrahedral]] order due to the four hydrogen bonds gives rise to an open structure and a 3-dimensional bonding network, which exists in contrast to the closely packed structures of simple [[liquid]]s. There is a great similarity between water and [[silica]] in their anomalous behaviour, even though one (water) is a liquid which has a hydrogen bonding network while the other (silica) has a covalent network with a very high melting point.  One reason that water is well suited, and chosen, by life-forms, is that it exhibits its unique properties over a temperature regime that suits diverse [[biological|biological processes]], including [[hydration]].

It is believed that hydrogen bond in water is largely due to electrostatic forces and some amount of covalency. The partial covalent nature of hydrogen bond predicted by [[Linus Pauling]] in 1930s is yet be to proven unambiguously by experiments and theoretical calculations.

===Quantum properties of molecular water===
Although the molecular formula of water is generally considered to be a stable result in molecular thermodynamics, recent work, started in 1995 [http://www.aip.org/enews/physnews/2003/split/648-1.html] has shown that at certain scales, water may act more like H<sub>3/2</sub>O than H<sub>2</sub>O at the subatomic quantum level.  This result could have significant ramifications at the level of, for example, the [[hydrogen bond]] in [[biology|biological]], [[chemistry|chemical]] and [[physics|physical]] systems.  The experiment shows that when [[neutrons]] and [[protons]] collide with water, they scatter in a way that indicates that they only are affected by a ratio of 1.5:1 of [[hydrogen]] to [[oxygen]] respectively.  However, the time-scale of this response is only seen at the level of [[second|attoseconds]] (10<sup>-18</sup> seconds), and so is only relevant in highly resolved [[kinetic]] and [[dynamics (mechanics)|dynamical]] systems.  For more references see
[http://prola.aps.org/abstract/PRL/v79/i15/p2839_1] and [http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=PRLTAO000091000005057403000001&idtype=cvips&gifs=yes].

==History==
In [[1742]], [[Anders Celsius]] defined the Celsius temperature scale with the [[freezing point]] of water at 100 degrees and the [[boiling point]] at [[standard atmospheric pressure]] at 0 degrees.  The scale was reversed in [[1744]].

The first decomposition of water into hydrogen and oxygen, by [[electrolysis]], was done in [[1800]] by [[William Nicholson (chemist)|William Nicholson]], an English chemist.

[[Gilbert Newton Lewis]] isolated the first sample of pure [[heavy water]] in 1933.

[[Polywater]] was a hypothetical [[polymer|polymerized]] form of water that was the subject of much scientific controversy during the late [[1960s]].  The consensus now is that it does not exist.

==Systematic naming==

The accepted [[IUPAC nomenclature of inorganic chemistry|IUPAC]] name of water is simply "water", although there are two other systematic names which can be used to describe the molecule.

The simplest and best systematic name of water is '''hydrogen oxide'''. This is analogous to related compounds such as [[hydrogen peroxide]], [[hydrogen sulfide]], and [[deuterium oxide]] (heavy water). Another systematic name that has been accepted by IUPAC is '''oxane'''. This name, however, has the problem of already being the name of a cyclic ether also known as [[tetrahydropyran]] (similar compounds include [[dioxane]] and [[trioxane]]).

===Systematic nomenclature and humor===
{{main|dihydrogen monoxide hoax}}

[[Chemist]]s sometimes refer to water as '''dihydrogen monoxide''' or '''DHMO''', an overly pedantic systematic covalent name of this molecule, especially in [[parody|parodies]] of chemical research that call for this "lethal chemical" to be banned. In 2004, the town of [[Aliso Viejo, California]] nearly banned foam cups after learning that DHMO was used in their production (see [http://slashdot.org/articles/04/03/16/1419252.shtml?tid=133&tid=186]). In reality, a more realistic systematic name would be '''hydrogen oxide''', since the "di-" and "mon-" prefixes are superfluous. [[Hydrogen sulfide]], H<sub>2</sub>S, is never referred to as "dihydrogen monosulfide", and [[hydrogen peroxide]], H<sub>2</sub>O<sub>2</sub>, is never called "dihydrogen dioxide".

Some overzealous [[material safety data sheet]]s for water list the following:  Caution: May cause drowning!

The systematic acid name of water is '''hydroxic acid''' or '''hydroxilic acid'''. Likewise, the systematic alkali name of water is '''hydrogen hydroxide''' &ndash; both acid and alkali names exist for water because it is able to react both as an acid or an alkali, depending on the strength of the acid or alkali it is reacted with (it is [[amphoteric]]).  None of these names are used widely outside of DHMO sites.

== See also ==
* [[dihydrogen monoxide hoax]]
* [[double distilled water]]
* [[heavy water]]
* [[hydrodynamics]]
* [[Mpemba effect]]
* [[polywater theory]]
* [[water dimer]]

== External links ==
* [http://www.lsbu.ac.uk/water/ Water Structure and Behaviour] A comprehensive and up-to-date NPOV resource maintained by Prof Martin Chaplin of South Bank University, UK
* [http://www.dhmo.org/ A spoof site on the "dangers" of dihydrogen monoxide]
* [http://www.siwi.org/ Stockholm International Water Institute] (SIWI)
* [http://www.btinternet.com/~martin.chaplin/explan.html Explanation of the anomalous properties of water]
* [http://www.compchemwiki.org/index.php?title=Water Computational Chemistry Wiki]

[[Category:Forms of water]]
[[Category:Hydrides]]
[[Category:Hydrogen compounds]]
[[Category:Hydroxides]]
[[Category:Oxides]]
[[Category:Solvents]]
[[Category:Water| ]]

[[de:Wassermolekül]]
[[es:Agua (molécula)]]
[[la:Aqua (moleculum)]]
[[ro:Apă (moleculă)]]
[[scn:Acqua (elimentu)]]
[[sr:Вода (молекул)]]
[[th:น้ำ (โมเลกุล)]]