Life Depends on the Unique Properties of Water生命取决于水的独特性能 ppt课件.ppt

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1、LifeDependsontheUniquePropertiesofWater4.4Alllivingthingsaredependentonwater.Insideyourbody,yourcellsaresurroundedbyafluidthatismostlywater,andyourcellsthemselvesare70to95percentwater.TheabundanceofwaterisamajorreasonEarthcansupportlife.TheStructureofWaterAwatermoleculeatf

2、irstmayseemprettysimple.Itstwohydrogenatomsareeachjoinedtoanoxygenatombyasinglecovalentbond.Thekeytowater'sunusualpropertiesisthattheelectronsofeachcovalentbondarenotsharedequallybetweenoxygenandhydrogenatoms.Oxygenpullselectronsmuchmorestronglythandoeshydrogen.Partofthere

3、asonisthattheoxygennucleushaseightprotons,andthereforehasastrongerpositivechargethanthehydrogennucleus,whichhasoneproton.Thisunequalpullresultsinthesharedelectronsspendingmoreoftheirtimeinthe"neighborhood"oftheoxygenatom.Theunequalsharingofelectronscausestheoxygenendofthem

4、oleculetohaveaslightnegativecharge,whiletheendwiththetwohydrogenatomsisslightlypositive.Amoleculeinwhichoppositeendshaveoppositeelectricchargesiscalledapolarmolecule.Waterisacompoundconsistingofpolarmolecules.Watermoleculesareattractedtooneanotherinaspecificway.Theslightly

5、negativeoxygenendofonemoleculeattractstheslightlypositivehydrogenendsofadjacentwatermolecules,causingthemoleculestobecomearrangedasyouseeinFigure4-12.Thistypeofweakattractionbetweenthehydrogenatomofonemoleculeandaslightlynegativeatomwithinanothermoleculeisatypeofchemicalbo

6、ndcalledahydrogenbond.Becausetheatomswithinthewatermoleculeshavenottransferredanelectron(andthusafullunitofcharge)toanotheratom,theattractioninahydrogenbondisnotasstrongasthatinanionicbond.Water'sLife-SupportingPropertiesThepolarnatureofwaterandtheeffectsofhydrogenbondinge

7、xplainmostofwater'suniqueproperties.Thesepropertiesincludecohesionandadhesion,temperaturemoderation,thelowerdensityoficecomparedtoliquidwater,andwater'sabilitytodissolveothersubstances.CohesionandAdhesionEachhydrogenbondbetweenmoleculesofliquidwaterlastsforonlyafewtrillion

8、thsofasecond.Yet,atanyinstantmostofthemoleculesareinvolvedinhydrogenbondingwithothermolec