Liquid is one of the principal
states of matter. A liquid is a fluidthat has the particles loose and can freely form a distinct surface at the boundaries of its bulk material. The surface is a free surfacewhere the liquid is not constrained by a container.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=]
A liquid's shape is confined to, not determined by, the container it fills. That is to say, liquid particles (normally
moleculesor 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 pressurein 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 tensionappears, allowing the formation of drops and bubbles. Capillarityis another consequence of surface tension. Only liquids can display immiscibility. The most familiar mixture of two immiscible liquids in everyday life is the vegetable oiland waterin Italian salad dressing. A familiar set of miscible liquids is water and alcohol. Only liquids display wettingproperties. Liquids at their respective boiling pointchange to gases (except when superheatingoccurs), and at their freezing points, change to solids (except when supercoolingoccurs). Even below the boiling point liquid 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.
volumeof a quantity of liquid is fixed by its temperatureand 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 pressureat any point is given by
where:: = the
densityof the liquid (assumed constant): = gravity: = 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 tensioneffects may be neglected.
Liquids generally expand when heated, and contract when cooled.
Waterbetween 0 °C and 4 °C is a notable exception; this is why icefloats. Liquids have little compressibility: water, for example, does not change its densityappreciably unless subject to pressure of the order of hundreds bar.
Examples of everyday liquids besides water are
mineral oiland 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 (densest liquid), bromine, francium, caesium, galliumand rubidium. [ [http://chemistry.about.com/od/periodictableelements/a/liquidelements.htm Liquid Elements] ] In terms of planetary habitability, liquid water is believed to be a necessity for the existence of life.
Quantities of liquids are commonly measured in units of
volume. These include the litre, not an SIunit, and the cubic metre(m³) which is an SI unit.
Ripple (fluid dynamics)
Sonoluminescence, emission of short bursts of light from imploding bubbles in a liquid when excited by sound.
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