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1、ToothDecayandFluoridationloothenamelconsistsmainlyofamineralcalledhydroxyapatite,Ca1()(PO4)6(OH)2.Itisthehardestsubstanceinthebody.Toothcavitiesarecausedbythedissolvingactionpfacidsontoothenamel:Cal0(PO4)6(OH)2(s)+8H-(aq)->lOCa2+(aq)+6HPO42-(aq)+2H20(l)TheresultantCa2+a
2、ndHPO42-ionsdiffuseoutofthetoothenamelandarewashedawaybysaliva・Theacidsthatattackthehydroxyapatiteareformedbytheactionofspecificbacteriaonsugarsandothercarbohydratespresentintheplaqueadheringtotheteeth.Fluorideion,presentindrinkingwater,toothpaste,orothersources,canreac
3、twithhydroxyapatitetoformfluoroapatite,CaI0(PO4)6F2・Thismineral,inwhichF-hasreplacedOH-,ismuchmoreresistanttoattackbyacidsbecausethefluorideionisamuchweakerBrensted-Lowrybasethanthehydroxideion.Becausethefluorideionissoeffectiveinpreventingcavities,itisaddedtothepublicw
4、atersupplyinmanyplacestogiveaconcentrationof1mg/L(1ppm).ThccompoundaddedmaybeNaForNa2SiF6・Thelattercompoundreactswithwatertoreleasefluorideionsbythefollowingreaction:SiF62-aq)+2H20(l)-->6F-(aq)+4H+(aq)+SiO2(s)About80percentofalltoothpastesnowsoldintheUnitedStatescontain
5、fluoridecompounds,usuallyatthelevelof0」percentfluoridebymass.Themostcommoncompoundsintoothpastesarestannousfluoride,SnF2,sodiummonofluorophosphate,Na2PO3F,andsodiumfluoride,NaF.DiamondCoatingsBesidesbeingoneofthehardestknownsubstances,diamondisalsohighlyresistanttocorro
6、sion,andhasexceptionalhigh-temperaturestability.Atpresent,commercialdiamondsarewidelyusedtostrengthencuttingandgrindingtools.Tliesediamondsareembeddedinthetoolsandarenotintimatelyanduniformlypartofthematerial.Scientistshaverecentlydevelopedproceduresforapplyinganultrath
7、inlaye『ofpolycrystallinediamondonmanymaterials・Tlicscthinfilmsimpartthehardnessanddurabilityofdiamondtoavarietyofmatcriah-glass,paper,plastics,metals,andsemiconductordevices・Imaginescratchproofglass;cuttingtoolsthatvirtuallyneverneedsharpening;surfacesthatarcchcmical-rc
8、sistant;temperaturesensorsthatoperateathightemperaturesinharshenvironments・Becausediamondiscompatiblewithbiolo
