ThedifferencebetweenaliquidandagasisobviousundertheconditionsoftemperatureandpressurecommonlyfoundatthesurfaceoftheEarth.Aliquidcanbekeptinanopencontainerandfillittothelevelofafreesurface.Agasformsnofreesurfacebuttendstodiffusethroughoutthe11available;itmustthereforebekeptinaclosedcontainerorheldbyagravitationalfield,asinthe12ofaplanet'satmosphere.Thedistinctionwasa13featureofearlytheoriesdescribingthephasesofmatter.Inthenineteenthcentury,forexample,onetheorymaintainedthataliquidcouldbe"dissolved"inavaporwithoutlosingitsidentity,andanothertheory14thatthetwophasesaremadeupofdifferentkindsofmolecules.Thetheoriesnowprevailingtakeaquitedifferentapproachbyemphasizingwhatliquidsandgaseshavein15Theyarebothformsofmatterthathaveno16structure,andtheybothflowreadily.
Thefundamentalsimilarityofliquidsandgasesbecomesclearlyapparentwhenthetemperatureandpressureare17somewhat.Supposeaclosedcontainer18filledwithaliquidisheated.Theliquidexpands,orinotherwordsbecomeslessdense;someofitevaporates.Incontrast,thevaporabovetheliquidsurfacebecomesdenserastheevaporatedmoleculesare19toit.Thecombinationoftemperatureandpressureatwhichthedensitiesbecome20iscalledthecriticalpoint.