Editing State of matter (section)

===Gas===

{{Main|Gas}}
[[File:Gif -AtomosGas 02.gif|thumb|upright=0.75|Simple illustration of particles in the gas state – in reality these particles will be much further apart.]]

A gas is a compressible fluid. Not only will a gas conform to the shape of its container but it will also expand to fill the container.

In a gas, the molecules have enough [[kinetic energy]] so that the effect of intermolecular forces is small (or zero for an [[ideal gas]]), and the typical distance between neighboring molecules is much greater than the molecular size. A gas has no definite shape or volume, but occupies the entire container in which it is confined. A liquid may be converted to a gas by heating at constant pressure to the [[boiling point]], or else by reducing the pressure at constant temperature.

At temperatures below its [[critical temperature]], a gas is also called a [[vapor]], and can be liquefied by compression alone without cooling. A vapor can exist in equilibrium with a liquid (or solid), in which case the gas pressure equals the [[vapor pressure]] of the liquid (or solid).

A [[supercritical fluid]] (SCF) is a gas whose temperature and pressure are above the critical temperature and [[critical temperature|critical pressure]] respectively. In this state, the distinction between liquid and gas disappears. A supercritical fluid has the physical properties of a gas, but its high density confers solvent properties in some cases, which leads to useful applications. For example, [[supercritical carbon dioxide]] is used to [[supercritical fluid extraction|extract]] [[caffeine]] in the manufacture of [[decaffeination|decaffeinated]] coffee.<ref name=Turrell>
{{Cite book
 |author=G. Turrell
 |date=1997
 |title=Gas Dynamics: Theory and Applications
 |url=https://books.google.com/books?id=-6qF7TKfiNIC&pg=PA3
 |publisher=John Wiley & Sons
 |pages=3–5
 |isbn=978-0-471-97573-1
}}</ref>