Liquid

For other uses, see Liquid (disambiguation).

A liquid will usually assume the shape of its container

A diagram of how the configuration of molecules/atoms differs for the solid, liquid, and gas phases.

A typical phase diagram. The dotted line gives the anomalous behaviour of water. The green lines show how the freezing point can vary with pressure, and the blue line shows how the boiling point can vary with pressure. The red line shows the boundary where sublimation or deposition can occur.

A liquid is one of the main states of matter. It is a fluid whose shape is usually determined by the container it fills. Liquid particles (normally molecules or clusters of molecules) are free to move within the liquid volume, but their mutual attraction limits the ability of particles to leave the volume. The volume of a quantity of liquid is fixed by its temperature and pressure. Unless this volume exactly matches the volume of the container, a surface is observed. The surface of the liquid behaves as an elastic membrane in which surface tension appears, allowing the formation of drops and bubbles. Capillarity is another consequence of surface tension. Liquids are generally resistant to compression: water, for example, does not change its density appreciably unless subject to pressure of the order of a gigapascal.

Liquids in a gravitational field exert pressure on the sides of a container as well as on anything within the liquid itself. This pressure is transmitted in all directions and increases with depth.

If a liquid is at rest in a uniform gravitational field, the pressure $\ p$ at any point is given by

$\ p=\rho gz$

where:

• $\ \rho$ = the density of the liquid (assumed constant)
• $\ g$ = gravity
• $\ z$ = the depth of the point below the surface.

Note that this formula assumes that the pressure at the free surface is zero, and that surface tension effects may be neglected.

Liquids generally expand when heated, and contract when cooled. Water between 0 °C and 4 °C is a notable exception; this is why ice floats.

Objects immersed in liquids are subject to the phenomenon of buoyancy, which is also observed in other fluids, but is especially strong in liquids due to their high density.

Liquids at their respective boiling point change to gases (except when superheating occurs), and at their freezing points, change to solids (except when supercooling occurs). Even below the boiling point liquid evaporates on the surface. However, even at a temperature below the boiling point, a liquid will evaporate until the concentration of the vapors belonging to the liquid reach an equilibrium partial pressure in the gas. Therefore no liquid can exist permanently in a vacuum.

Liquid components in a mixture can often be separated from one another via fractional distillation.

Liquid measures

Quantities of liquids are commonly measured in units of volume. These include the litre, not an SI unit, and the cubic metre (m³) which is an SI unit.

See also

• Ripple effect
• Multiphasic liquid
• Viscosity
• Surface tension
• Sonoluminescence, emission of short bursts of light from imploding bubbles in a liquid when excited by sound.
• Fluid entropy