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1、BoilingPointsandDistillationTheactionofboilingisaprocessfamiliartoanybodywhohascookedpastaorbrewedtea.Asheatisappliedtoapanofwater,thetemperatureofthewaterincreasesuntilitreaches100°C(212°F).Atthistemperature,additionalheatcausesthewatertobubblevigorous
2、lyandtransformsliquidwaterintogaseouswater,orsteam.Mostorganicliquidswillbehaveinasimilarfashion.Onheating,thetemperatureoftheliquidincreasesuntilacertaintemperaturecalledtheboilingpointisreached.Additionalheatingcausestheliquidtovaporizeaccompaniedbyvi
3、gorousbubblingoftheliquid.Theboilingpointofasubstanceisaphysicalpropertyofasubstanceandcanbeusefulforcharacterizingthatsubstance.Theprocessofheatingasubstanceuntilitvaporizes,coolingthevapors,andcollectingthecondensedliquidisthebasisofacommonlyusedpurif
4、icationtechniquecalleddistillation.VaporPressureandBoilingPointIfaliquidisplacedinanempty,closedcontainer,somemoleculesatthesurfaceoftheliquidevaporateintotheemptyspaceabovetheliquid.Oncevaporized,someofthemoleculesinthevaporcondenseintotheliquidinacomp
5、etingprocess.Asthespaceabovetheliquidbecomesoccupiedwithmoleculesofvaporizedliquid,thepressureofthevaporabovetheliquidrisesuntilitreachesacertainvalue.Whenthepressurestabilizes,theratesofevaporationandcondensationareequal.Thepressureofthevaporunderthese
6、conditionsiscalledtheequilibriumvaporpressure.Theequilibriumvaporpressureofaliquidistemperaturedependent.Asthetemperatureoftheliquidisraised,moremoleculesvaporizeandtheequilibriumvaporpressureincreases.ThegraphinFigure11showstherelationshipbetweenvaporp
7、ressureandtemperatureforthreesubstances,dichloromethane,water,andd-limonene.Figure11.Vaporpressure-temperaturediagramfordichloromethane,waterandd-limonene.Thenormalboilingpointofasubstanceisdefinedasthetemperatureatwhichthevaporpressureofthatsubstanceeq
8、ualsatmosphericpressure,760mmHg.Thenormalboilingpointsofdichloromethane,water,andd-limoneneare,respectively,40.2°C,100°C,and175°C.Ifthebarometricpressureislessthan760mmHg,thetemperatureatwhichasubstanceboilswillbelessthanthenorma
