Vapor (source code)
= Vapor
{wiki}
A <vapor> is a substance in a gaseous state at a <temperature> where a condensed phase can also be reached under suitable <pressure> conditions. For a pure substance below its <liquid-gas critical point>, isothermal compression can reach <phase coexistence> and then convert some vapor to <liquid>. At coexistence the equilibrium <vapor pressure> depends on <temperature>.
A sealed cell containing both <water> and water vapor illustrates the distinction between amount and state: at fixed <temperature>, changing the liquid fraction can leave the equilibrium <vapor pressure> unchanged while both phases remain. A dilute vapor can still approximate an <ideal gas>, so vapor does not mean that ideal-gas calculations are automatically invalid. Conversely, the broad term <gas> does not imply ideal behavior. Specifying composition, <temperature> and <pressure> makes the usage physically meaningful.
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