Liquid
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[[Image:solid liquid gas.svg|thumb|A diagram of how the configuration of molecules/atoms differs for the solid, liquid, and gas phases.]]
[[Image:Phase-diag2.svg|thumb|300px|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 (chemistry)|sublimation]] or [[Deposition (physics)|deposition]] can occur.]]
'''Liquid''' is one of the principal [[states of matter]]. A liquid is a [[fluid]] that has the particles loose and can freely form a distinct surface at the boundaries of its bulk material. The surface is a [[free surface]] where the liquid is not constrained by a container.<ref name=White>{{cite book |author=White, Frank |title=Fluid mechanics |publisher=McGraw-Hill |location=New York |year=2003 |pages= p. 4 |isbn=0-07-240217-2 |oclc= |doi=}}</ref>
==Characteristics==
A liquid's shape is confined to, not determined by, the container it fills. That is to say, liquid particles (normally [[molecules]] or clusters of molecules) are free to move about the volume, but they form a discrete surface that may not necessarily be the same as the vessel. The same cannot be said about a [[gas]]; it can also be considered a fluid, but it ''must'' conform to the shape of the container entirely.
At a temperature below the [[boiling point]], a liquid will evaporate until, if in a closed container, 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 complete [[vacuum]]. The surface of the liquid behaves as an elastic membrane in which [[surface tension]] appears, allowing the formation of [[Drop (liquid)|drops]] and [[bubble]]s. [[Capillarity]] is another consequence of [[surface tension]]. Only liquids can display [[immiscibility]]. The most familiar mixture of two immiscible liquids in everyday life is the [[vegetable oil]] and [[water]] in Italian salad dressing. A familiar set of miscible liquids is water and alcohol. Only liquids display [[wetting]] properties. Liquids at their respective [[boiling point]] change to [[gas]]es (except when [[superheating]] occurs), and at their [[freezing point]]s, change to [[solid]]s (except when [[supercooling]] occurs). Even below the boiling point liquid [[evaporation|evaporates]] on the surface. 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. Liquid components in a mixture can often be separated from one another via [[fractional distillation]].
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. Liquids in a gravitational field, like all fluids, 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. In the study of fluid dynamics, liquids are often treated as [[incompressible]], especially when studying [[incompressible flow]].
If a liquid is at rest in a uniform [[gravitational field]], the [[pressure]] <math>\ p</math> at any point is given by
:<math>\ p=\rho g z </math>
where:
:<math>\ \rho</math> = the [[density]] of the liquid (assumed constant)
:<math>\ g</math> = [[gravity]]
:<math>\ z</math> = 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. Liquids have little [[compressibility]] : water, for example, does not change its [[density]] appreciably unless subject to pressure of the order of hundreds bar.
Examples of everyday liquids besides water are [[mineral oil]] and [[gasoline]]. There are also mixtures such as [[milk]], blood, and a wide variety of aqueous solutions such as household [[bleach]]. Only six elements are liquid at or about room temperature and pressure: [[mercury (element)|mercury]] (densest liquid), [[bromine]], [[francium]], [[caesium]], [[gallium]] and [[rubidium]].<ref>[http://chemistry.about.com/od/periodictableelements/a/liquidelements.htm Liquid Elements]</ref> In terms of [[planetary habitability]], liquid water is required for the existence of [[life]].
==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 ==
{{wiktionary}}
* [[Ripple (fluid dynamics)]]
* [[Multiphasic liquid]]
* [[Viscosity]]
* [[Surface tension]]
* [[Sonoluminescence]], emission of short bursts of light from imploding bubbles in a liquid when excited by sound.
==Notes==
{{reflist}}
{{State of matter}}
[[Category:Viscosity]]
[[Category:Volume]]
[[Category:Liquids]]
[[Category:Phases of matter]]
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