teaching secondary chemistry · 9781444124323_teaching_secondary_chemistry_2e.indb 3 18/05/2012...
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I S BN 978-1-444-12432-3
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T h e O p e n u n i v e r s i t y S e T B o o k
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This book will provide invaluable support whether you are a newly-qualified science teacher, an experienced teacher of chemistry who wants to extend the range of strategies and approaches used, a biologist or physicist who has to teach chemistry, or a student training to be a teacher. Each chapter covers a broad section of the curriculum and is divided into topics. For each topic the book covers:
• Students’ likely Previous knowledge• A suggested Teaching sequence with activities
necessary to cover the basic chemistry• Advice about students’ misconceptions, common problems
with individual activities, and safety issues• Further activities that develop the students’
understanding of the topic• Enhancement ideas that relate the science to everyday
contexts and provide new ideas for experienced teachers• Suggestions for using ICT
This second edition reflects changes in curricula, ideas from recent curriculum development projects, and the current availability of ICT.
This book draws on the experience of a wide range of teachers and those involved in science education. It has been produced as part of the Association for Science Education’s commitment to supporting science teachers by disseminating best practice and new ideas to enhance teaching.
Series editor: David Sang
teaching secondaryA S e S c i e n c e P r A c T i c e
ed iTor: keitH tAber
ChEmISTry
tAb
er
n e w e d i T i o n
A S e S c i e n c e P r A c T i c e
www.ase.org.uk
teaching secondaryA S E S c i E n c E P r A c t i c E
Ed itor: Ke ith S. taber
CHEMISTRYS E c o n d E d i t i o n
9781444124323_TEACHING_SECONDARY_CHEMISTRY_2E.indb 1 18/05/2012 13:38
Titles in this series:Teaching Secondary Biology 9781444124316Teaching Secondary Chemistry 9781444124323Teaching Secondary Physics 9781444124309
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©AssociationforScienceEducation2012Firstpublishedin2012byHodderEducation,AnHachetteUKCompany338EustonRoadLondonNW13BH
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ISBN:9781444124323
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ContentsContributors iv
introduction viKeithS.Taber
1 Key concepts in chemistry 1 KeithS.Taber
2 introducing particle theory 49 PhilipJohnson
3 introducing chemical change 75 KeithS.Taber
4 Developing models of chemical bonding 103 KeithS.Taber
5 extent, rates and energetics of chemical change 137 VanessaKind
6 acids and alkalis 183 JohnOversby
7 Combustion and redox reactions 199 VickyWong,JudyBrophyandJustinDillon
8 electrolysis, electrolytes and galvanic cells 253 GeorgiosTsaparlis
9 inorganic chemical analysis 279 KimChweeDanielTan
10 Organic chemistry and the chemistry of natural products 303 VanessaKind
11 earth science 343 ElaineWilson
12 Chemistry in the secondary curriculum 369 KeithS.Taber
index 379
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iv
ContributorsJudy BrophygraduatedinchemistryfromManchesterUniversitybeforetrainingasateacherattheInstituteofEducation.Shetaughtscienceandchemistryformanyyearsininnercity(11–18)Londoncomprehensives,whilsttakinganactiveroleintheASE.Judyspecialised,successfully,inencouragingherALevelstudents,especiallygirls,toaimhigher.AsavisitingteacheratKing’sCollegeLondonshepassesonherexperiencetosecondarysciencePGCEstudents.
Justin DillonisprofessorofscienceandenvironmentaleducationandHeadoftheScienceandTechnologyEducationGroupatKing’sCollegeLondon.AfterstudyingforadegreeinchemistryhetrainedasateacheratChelseaCollegeandtaughtinsixinnerLondonschoolsuntil1989whenhejoinedKing’s.Justinisco-editoroftheInternational Journal of Science Educationandin2007waselectedPresidentoftheEuropeanScienceEducationResearchAssociationforafour-yearterm.
Philip JohnsontaughtchemistryuptoALevelforthirteenyearsin11–18comprehensiveschoolsbeforejoiningDurhamUniversitySchoolofEducationin1992.Hebeganresearchingintothedevelopmentofstudents’understandinginchemistrywhileteachinginschoolsandcontinuestodoso.Hisworkispublishedininternationalscienceeducationresearchjournals.
Vanessa KindisSeniorLecturerinEducationatDurhamUniversityandDirectorofScienceLearningCentreNorthEast.ShehasextensiveexperienceofchemicaleducationgainedthroughteachinginLondonandHull,andapreviouslectureshipattheInstituteofEducation,UniversityofLondon.VanessawastheRoyalSocietyofChemistry’sTeacherFellow2001–2002.Hercurrentresearchinterestsincludepedagogicalcontentknowledgeforscienceteachingandpost-16chemistryeducation.
John OversbyhasbeenateacherofsciencesandmathematicsinGhanaandtheUKforover20years,andateachereducatoratTheUniversityofReadingfor20years.HeisalsoanactivememberofTheRoyalSocietyofChemistryandChairoftheASEResearchCommittee.Hehasinterestsinmodellinginscienceeducation.HehasbeentheUKcoordinatorofahistoryandphilosophyinscienceteachingprojectandisnowinternationalcoordinatorofaComeniusClimateChangeEducationnetwork.
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Keith S. Tabertaughtsciences,mainlychemistryandphysics,insecondaryschoolsandfurthereducation,beforejoiningtheFacultyofEducationattheUniversityofCambridge,whereheismostlyworkingwithhigherdegreestudents.Whilstteaching,heundertookdoctoralresearchexploringstudentunderstandingofthechemicalbondconcept.HewastheRoyalSocietyofChemistry’sTeacherFellowin2000–2001.Hehaswrittenagooddealaboutchemistryandscienceeducation,andiseditorofthejournalChemistry Education Research and Practice.
Kim Chwee Daniel Tanstartedhiscareerasachemistryteacherin1990.Hehasbeenafacultymemberofthe(Singapore)NationalInstituteofEducationsince1998.Heteacheshigherdegreecoursesaswellaschemistrypedagogycoursesinthepre-serviceteachereducationprogrammes.Hisresearchinterestsarechemistrycurriculum,translationalresearch,ICTinscienceeducation,students’understandingandalternativeconceptionsofscience,multimodalityandpracticalwork.
Georgios TsaparlisisprofessorofscienceeducationatUniversityofIoannina,Greece.Heholdsachemistrydegree(UniversityofAthens)andanM.Sc.,andaPh.D.(UniversityofEastAnglia).Heteachesphysicalchemistry(includingelectrochemistry)andscience/chemistryeducationcourses.Hehaspublishedextensivelyinscienceeducation,hisresearchfocusbeingonstructuralconcepts,problemsolving,teachingandlearningmethodology,andchemistrycurricula.Hewasfounderandeditor(2000–2011)ofChemistry Education Research and Practice(CERP).
Elaine WilsonisaSeniorLecturerinscienceeducationandaFellowofHomertonCollegeattheUniversityofCambridge.ElainewasformerlyasecondaryschoolchemistryteacherwhowasawardedaSalters’MedalforChemistryteaching.Shenowteachesundergraduates,secondarysciencePGCEstudents,coordinatesa‘blendedlearning’ScienceEducationMasterscourseandhashelpedsetupanewEdDcourse.ElainehasreceivedtwocareerawardsforteachinginHigherEducation,aUniversityofCambridgePilkingtonTeachingPrizeandaNationalTeachingFellowship.
Vicky WongtaughtScienceandChemistryintheUK,SpainandNewZealandfortenyears.ShewastheRoyalSocietyofChemistryTeacherFellowin2004–2005andnowworksasanindependentscienceeducationconsultantrunningtrainingcoursesforteachers,writingcurriculummaterialsandundertakingresearch.
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IntroductionKeith S. Taber
ThisbookispartofaseriesofhandbooksforscienceteacherscommissionedbytheAssociationforScienceEducation.Thisparticularhandbookisintendedtosupporttheteachingofchemistryatsecondarylevel(takenhereasages11–16years),whetherasadiscretesubjectoraspartofabroadersciencecourse.Thebookhasbeenwrittenwithaparticularawarenessoftheneedsofnewteachersandofthoseteachingchemistrywhowouldnotconsiderittheirspecialismwithinthesciences.However,thebookshouldprovetobeofinterestandvaluetoanyoneteachingchemistrytopicsatsecondarylevel.Thebookhasbeenwrittenbyateamofauthorswhocollectivelyhaveawiderangeofexperienceinteachingchemistry,supportinganddevelopingteachersofchemistry,andundertakingresearchintoteachingandlearninginchemistrytopics. Itissometimeseasiertocharacterisesomethingbyexplainingwhatitisnot.Thisbookisnotachemistrytextbookforteachers,althoughinevitablyitdiscusseschemistrycontentaspartoftheprocessofdescribingandrecommendingapproachestoteachingthesubject.Therearemanygoodchemistrybooksavailableatvariouslevels,andanyteacherwhoisconcernedabouttheirknowledgeandunderstandinginaspectsofchemistryshouldfirstdosomeworktodeveloptheirownsubjectknowledgebeforeconsideringapproachestoteaching.So-called‘pedagogiccontentknowledge’willonlybesoundwhenwearebuildingonsubjectknowledgethatissound(elsewebecomeveryeffectiveatteachingpoorchemistry). Thehandbookdoesnotsetouttoactasateachingguideforanyparticularcurriculumorsyllabus.Weintendthebooktobeequallyusefulacrossdifferentcourses:whetherthecourseisarrangedasasetoftraditionaltopicsororganisedinsomeotherway,forexampleteachingconceptsthroughthecontextsofmajorareasofapplicationofchemistrysuchasfood,transport,fabrics,etc.Thebookisnottiedtoaparticularstreamorabilitylevelofstudent.Insomeplaceschaptersmakeexplicitsuggestionsfordifferentiatingbetweendifferentgroupsofstudents,butyoushouldalwaysconsiderhowtheadvicegivenherecanbestinformtheteachingofyourparticularclasses. Nordoesthebooksetouttobeamanualforteachingchemistryinthesenseofprovidingcomprehensivecoverageofallcontentthatmightpotentiallybeincludedinasecondarychemistrycourse.Suchamanualwouldinevitablybebothvoluminousandveryquicklyoutofdateaschemistryandchemistryteachingmoveon.
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ideas for effective chemistry teachingThephilosophybehindthepresentbookisthatitismoreimportanttoinformteachersabouteffectiveteachingapproachesthantotrainthemtoapplyspecifiedteachingschemesinparticulartopics.Someoftherecommendationsweofferinthehandbookarebasedonagooddealofclassroomexperienceanddrawonspecificresearchintostudents’learningdifficultiesandeffectiveinnovationsinteaching.Thespecificsuggestionsmadeforteachingtheseaspectsofthesubjectcancertainlybeconsideredasthebestresearch-informedadvicecurrentlyavailabletoteachers. However,teachingisacontextualisedprocess:students,facilities,curriculumrequirementsandsomuchmorecanmakeadifferencetowhatcountsasgoodteachinginaparticularclassroomonaparticularday.Sojustasimportantasthespecificsuggestionsforteachingparticularconceptsistherangeofapproachesadoptedbyauthorsacrossthechapters.Drawingupontherangeofteachingandlearningactivitiesdescribedherecaninformthedevelopmentofteachingthatisbothvariedandresponsivetotheneedsofparticularstudentsandclasses.Althoughnoteverythingthatcouldpossiblybecoveredisincludedinthebook,thethinkingbehindtheexamplesthatarepresentedhereoffersthebasisfordevelopingeffectiveteachingthatcanbeadoptedacrosschemistryteaching(andoftenwellbeyond).
teaching about the nature of chemistryInaccordancewiththephilosophyoutlinedabove,thereaderwillfindavariedsetofchaptersincludedinthebook.Forexample,featuresofthenatureofchemistryasasciencearehighlightedinsomechapters.Teachingaboutthenatureofscience(whichhassometimesbeenreferredtoas‘howscienceworks’),whichrecogniseshowallstudentsneedtounderstandscience(i.e.shouldbescientificallyliterate)fortheirroleascitizens(asconsumers,voters,etc.),hasbecomeincreasinglyimportant.Suchaperspectiveneedstoinformallscienceteachingandreadersmaywishtothinkhowtheideashighlightedinanumberofthechaptershere(suchasthenatureandroleofmodelsindevelopingunderstandingoftheworld)canbemorewidelyapplied. Anotherkeydifferencebetweenthechaptersisthecentralityofthetopicsdiscussed.Thefirstfivechapterssetoutkeyideasthatareneededtounderstandanyareaofchemistry,whereastheotherchaptersstandalonetoamuchgreaterextent.Itisalsousefultonotethatwhilealmostallofthechaptersareclearlyaboutchemistry,the
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chapteraboutEarthscienceoverlapsstronglywithseveralothersciences,andinparticulardrawsongeology.Giventheincreasingimportanceofinterdisciplinaryworkinscienceand–evenmoreso–theimportanceofunderstandingtheenvironmentinscienceandinsocietymorewidely,thischapterremindsusthatchemistrylinkswith,buildsupon,andfeedsinto,awiderangeofscientificwork. Afinaldifferencebetweenchaptersthatwillbeveryobvioustoreadersistheextenttowhichpracticalworkfeatures.Itissaidthatchemistryisapracticalsubject.Thisiscertainlytrue,but–asascience–chemistryisjustasmuchatheoreticalsubject:itistheinterplayoftheoryandevidencethatisattheheartofscientificwork.Chemistryasascienceisbaseduponobservationsthatleadtothedevelopmentofcategoriesandtheidentificationofpatterns,andtowaysofmakingsenseof,andunderstanding,thephenomena.Thisinvolvestheconstructionofmodelsandtheories,whichcanmotivateempiricalinvestigationsthatcantheninformfurtherroundsoftheorisingandexperimentation.Inparticular,muchofchemistryasasciencecanbeconsideredtobeaboutbuildingmodelsandselectingthosethatareusefultoscientistsdespiteinevitablyhavinglimitedrangesofapplication.Thenotionsofacids(Chapter6)oroxidation(Chapter7)certainlyreflectthis,beingconceptsthataretheproductsofhumanimagination,designedtoreflectthepatternsfoundinnatureandtohaveutilitytochemistsastoolsforthinking,explaining,predictingandsosupportingpracticalandtechnologicalwork.Understandingchemicalideasinthisway,ascreativeproductsofscientificwork(ratherthansimplybeingdescriptionsofthewaytheworldis),shouldhelpstudentsmakesenseofhowchemistshavemodifiedtheseconceptsovertime(asdescribedinthecaseofacidsinChapter6),andwhysometimesweseemtooperatewitharangeofnotentirelyconsistentmodels(oxidationintermsofoxygenorelectrons:Chapter7;oxidationintermsofoxidationstates:Chapter9). Anactivitywhichillustratesthisgeneralpointinrelationtoparticletheory(thetopicofChapter2)isincludedinapublicationavailablefromSEP(theScienceEnhancementProgramme,detailsofwhicharegiveninthe‘Otherresources’sectionattheendofthisintroduction).Thefirstoftwogroup-worktasksinthisactivity‘Judgingmodelsinscience’asksstudentstoconsidertwotypesofparticlemodels–particlesliketinyhardbilliardballs;particlesasmoleculeswith‘soft’electronclouds–andtoconsiderwhichmodelbetterexplainsarangeofevidencebasedontheobservablepropertiesofmatter.Studentswillfindthateachmodelisusefulforexplainingsomephenomena,butneitherfitsalltheevidence–andofcoursebothmodelsarestillfoundusefulinscience.
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AkeyaspectofthisJanus-facednatureofchemistry(lookingtoboththephenomenaandthetheories)isthatchemistryisdiscussedintermsofthemacroscopic/molar/phenomenallevelandintermsofsubmicroscopicparticlemodelswhichareusedtomakesenseofthosephenomena.Thesemodelsexemplifythenatureofscience(offeringanextremelypowerfulexplanatoryschemeformakingsenseofthenatureofthematerialworld),butareknowntobechallengingforstudents. Acentralfeatureofthewaychemistryispresentedanddiscussedinclassroomsisthesetofrepresentations(suchasformulaeandchemicalequations)used.Thisisoftenseenasathird‘level’distinctfromthemolarandsubmicroscopiclevels,butismorehelpfullyunderstoodasaspecialisedlanguagethatallowsustoshiftbetweenthosetwolevels(seeChapter3).Translatingbetweenobservablephenomena,symbolicrepresentationsandtheoreticalmodelsisakeypartbothofteaching,andlearning,chemistry.
Practical work and ‘experiments’ in chemistry teaching
Giventhenatureofchemistryasadiscipline,andofschoolchemistryasacurriculumsubject,itisessentialthatstudentsareintroducedtothephenomenathatthetheoriesandmodelsaremeanttohelpusunderstand.Withoutexperiencingthesephenomena,thereislittlemotivationforadoptingthetheoreticalideas.Withinschoolscience,practicalworkhaslongbeenseenasakeypartofchemistrylessonsinmanycountries(andespeciallyintheUK).Yetresearchalsosuggeststhatagooddealofthepracticalworkundertakenbystudentshaslimitedimpactonlearning(orevenonstudentmotivationtocontinuestudyingthesubject). Readersshouldconsidercarefullywhetherpracticalworkundertakenisgoingtobethemosteffectivewayofmeetingthepurposesofteaching.Insomecasesthiswillcertainlybeso.Sothechapteroninorganicanalysis,anareaofsecondarychemistrythatisnotalwaysgivenasmuchattentionasitoncewas,islargelydevelopedaroundpracticalworkthatstudentscancarryout.Thismakessense,astheaimsofteachingthistopicincludetheabilitytocarryoutanalyticalproceduresandinterpretbenchobservationstoidentifyparticularchemicalspeciespresent.Otherchaptersvaryconsiderablyintheamountoflaboratorypracticalworkrecommended.Insomecasesteacherdemonstrationsarerecommendedasbeingmoreeffectivewaysofensuringstudentscanbehelpedtoappreciatethechemicalinterpretationsofobservations
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–asateacheryoucandrawstudents’attentiontothechemicallyrelevantandawayfromthesalientbutincidental.Insometopicsalternativeformsofengaging,activelearningaresuggestedtohelpstudentslearnconceptsnoteasilyappreciatedfromthelaboratoryworkpossibleinschoolcontexts.Ultimately,however,theoryandpracticalexperiencebothhaveimportantrolestoplayandneedtobecoordinatedacrosssecondaryschoollearning. Onespecificissuethathasvexedmeinthiscontextistheuseoftheterm‘experiment’todescribeschoolsciencepracticalwork.Veryfewchemistrypracticalscarriedoutinmostschoolshaveanygenuineclaimtobeexperiments(authenticinvestigationstotestouthypotheses,ratherthantoillustratewhatisalreadysetoutastargetknowledgetobelearnt),andusingthetermexperimentlooselymayunderminelearningaboutthenatureofscience,whenweknowthatstudentscommonlyusetechnicaltermssuchas‘experiment’,‘theory’,‘proof’andsoforthinveryvagueandinconsistentways.However,withinthecultureofschoolchemistrythetermexperimentisoftenusedforanylaboratorypracticalactivity,especiallyonethatisundertakenbystudents(ratherthanbeingademonstration).Someauthorsherehavefollowedthecommonusageandreferredtolaboratorypracticalactivitiesasexperiments.Thisisfineintalkamongteachersandtechnicians,butitmaybewisetoconsidercarefullywhichpracticalactivitiesshouldbepresentedtostudentsasexperiments.Itisprobablybestlimitedtothosewheretheyaregenuinelytestingsomekindofhypothesis–wherestudentsareseekingtodiscoversomethingtheydonotactuallyalreadyknow.
MM ensuring safety during practical workManystandardchemistrypracticalshavebeencarriedoutsafelyinschoolsfordecadesandtherehaveseldombeenseriousaccidents.Yetclearlythereareparticularpotentialhazardsinvolvedinsomepracticalwork.Itisimportanttokeepthepotentialrisksofpracticalactivities(whetherobservedby,orundertakenby,thestudents)inperspective,yettorememberthatstudentsafetymustbeyourparamountconcern.Inthisbook,themarginiconshownhereisusedtoalertyoutoparticularsafetyissuesthatyoushouldbeawareof.Recommendedactivitiesareconsideredsuitableforclassroomusebytheauthors.However,assessingriskisnotjustabouttheactivity,butalsothepeopleandtheconditions.Thesamepracticalmaybeviablewithsometeachinggroupsandnotothers. Itis,therefore,veryimportanttoundertakecarefulriskassessmentsbeforedecidingtodoanypracticalactivitiesinthe
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classroom.Yourriskassessmentneedstoconsidertheexperienceandskillsoftheperson(s)carryingouttheactivity(teacherorstudents),howresponsibletheparticularstudentsareandthefacilitiesavailableintheteachingroom(presenceofafumecupboard,adequateventilation,sufficientaccesstosinks,plentyofspaceforstudentstomovearound,supportofteachingassistants,assistanceofaqualifiedtechnician,etc.) Foranypracticalactivity(includingteacherdemonstrations)theteachershouldcarryoutariskassessment.Thismightinclude:
MM checkingthemodelriskassessmentsuppliedbytheiremployer(thismaybesuppliedbyanotheragency,suchasCLEAPPSorSSERCintheUK,towhichtheemployersubscribes)andadjustingthemodelriskassessmentasappropriate.
MM referringtothehazardsofthestartingmaterialandtheproductsbyconsultingsafetydatasheetsprovidedbythesupplier,andadjustingthemodelriskassessmentasappropriate,
MM consultingmoreexperiencedteachersorexperiencedtechnicians.
Asaresult,thesignificantfindingsshouldberecordedandtheappropriatecontrolmeasuresimplemented.Whatevertheoutcomesofariskassessment,includingtheriskofannoyinghard-workingtechnicalstaff,youshouldalwaysbepreparedtosuspendorcancellaboratoryworkduringalessonifatanypointyoujudgethatstudentbehaviourorsomeotherfactormakescontinuingunwise.
the structure of the bookThebookcontainstwelvechapters.Thefirstthreediscusstheteachingofthemostbasicchemicalideas,whichunderpinallotherchemistrytopics.Theseareideasthatwillneedtobemetearlyoninthesecondaryschool,butwillbedevelopedinmoresophisticatedwaysthroughoutthesecondaryyears.Chapters4and5discussotherfundamentalideaswhichbuilduponthosediscussedinChapters1–3,andprovidethebasisfortheleveloftheoreticalexplorationofchemistryexpectedofmanyupper-secondarylevelclasses(aswellassettingoutthetheoreticalbasisthatisdevelopedinpost-secondarystudy).Thenextsixchaptersdrawupon,andoftenassumeknowledgeof,thesebasicideas,butneednotnecessarilybetaughtintheordertheyappearinthehandbook.Thefinalchapterhasaslightlydifferentflavour;itreflectsuponhowdecisionsaremadeaboutwhichchemistrytopicsshouldbeapartofthesecondaryeducationfordifferentgroupsofstudents.Forreadersinsomecontextssuchdecisionswillbemadecentrallybycurriculumauthorities,butinmanyteachingcontexts
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theremaybeimportantdecisionstobemadeaboutwhichsciencetopicsshouldbepartofacorecurriculum,andwhichcanbeseenaseitherpossibleenrichmentorasimportantforsome,butnotall,groupsofstudents.
2 Introducing particle theory
1 Key concepts in chemistry
3 Introducing chemical change
4 Developing models of chemical bonding
5 Extent, rates and energetics of chemical change
6 Acids and alkalis
8 Electrolysis, electrolytes and galvanic cells
7 Combustion and redox reactions
10 Organic chemistry and the chemistry of natural products
12 Chemistry in the secondary curriculum
9 Inorganic chemical analysis
11 Earth science
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MM the structure of the chaptersAssuggestedabove,thenatureoftheeleventopic-basedchaptersinthisbookreflectsthediversityoftopicsfoundinachemistrycourse,aswellasthedifferentteachingapproachesjudgedworthrecommendingbytheauthorswhohaveparticularexpertiseinthosetopics.However,allofthesechaptersincludeanumberofcommonfeatures.Eachchapterisdividedintosectionsaccordingtomainsubsectionsofthechaptertopic,andeachincludesaschematicdiagramofthechapterstructure.Eachchapteroffersguidanceonaroutethroughthetopic.Thesechaptersalsodiscussthepriorknowledgeandrelevantexperiencethatstudentswillbringtothestudyofthetopicatsecondarylevel,aswellashighlightingthecommonlearningdifficultiesandalternativeconceptionsthatstudentsareknowntohaveinthetopic.Althoughtheauthorsherecanofferguidanceonthelikelyrangeofpriorknowledgeandcommonalternativeconceptionstobefoundinmanyclasses,eachgroupofstudentsisdifferent,and–indeed–eachlearnerisanindividualwiththeirownpersonalunderstandingoftheworldandtheirownwayofinterpretingwhattheyaretaughtinchemistryclasses. ThisissomethingIamwellawareoffrommyownwork,whereIhavespentagooddealoftimeaskingsecondary-agestudentstotellmeabouttheirunderstandingofvarioussciencetopics.Therearesomeverycommonalternativeideas(alternativeconceptionsormisconceptions)thatarefoundinjustaboutanyclass.Forexample,itisverylikelythatinanyuppersecondaryscienceorchemistryclassyouteach,atleastsome(andinmanyclassesitwillbemost)ofthestudentsthinkthatchemicalreactionsoccursothatindividualatomscanfilltheirelectronshells.(Ifyoucannotseewhatiswrongwiththatidea,thenyoumayfindChapter4quiteinteresting.) However,itisalsothecasethatifyouspendenoughtimeaskingstudentstotellyoutheirideasrelatingtosciencetopics,youarelikelytouncoversomeidiosyncraticnotiontheyholdwhichisatoddswithacceptedscientificknowledgeandwhichyouhaveneverheardsuggestedbefore.Studentindividualityandcreativityissuchthatthisislikelytoremainthecaseevenwhenyouhavebeenteachingchemistryforsomedecades!SomeexamplesofstudentthinkingaboutchemistrytopicscanbefoundontheECLIPSEprojectwebsite(fulldetailsofwhicharegiveninthe‘Otherresources’sectionattheendofthisintroduction),offeringatasteofthediverseandoftenunexpectedwaysthatdifferentlearnersmakesenseofthetopicstheymeetinschoolsciencelessons.(Note,theiconhereisusedinthemarginwherereferencesaremadetousefulwebsites.)
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Aswhatstudentsalreadythinkisastrongdeterminantofwhattheywilllearninyourlessons,itisworthspendingsometimeexploringtheirideasandinparticularundertakingdiagnosticassessmentatthestartofmajortopicstocheckonpriorlearningandelicitanystronglyheldalternativeconceptions.Inanidealworldteacherswouldhavetimetotalkatlengthtoeachoftheirstudents.Morerealistically,ideascanbeexploredthroughopen-endedclassroomdiscussionthatinvitesstudents’ideasinanacceptingwayandexplorestheirconsequences(andrelationshipwithavailableevidence),before‘closingdown’discussiontopresentthescientificmodels.Thispresentationofthe‘scientificstory’canthenbedoneinwaysthatanticipatestudentobjectionsandemphasisethereasonsforthescientificexplanationsbeingadoptedinthescientificcommunity–especiallywheretheseexplanationscontradictstudents’ownthinking.Classroommaterialsfordiagnosticassessmenthavebeenpublishedinmanysciencetopicsandsomearereferredtointhisvolume. Thechaptersheredrawuponresearchevidence,althoughincommonwithotherhandbooksintheseries,thechaptersdonotincludeinthetextreferencestotheresearchliterature.However,recommendationsforfurtherreadingandsuggestionsforresourceslikelytobefoundusefulbyteachersarelistedattheendofchapters.
KeithS.TaberCambridge,2012
Other resources
MM booksTheASE Guide to Secondary Science Education(MartinHollins,editor)offersabroadrangeofadviceandinformationforthoseteachingscienceinsecondaryschools.ASEPublications. Anintroductiontohowstudentslearninchemistryandthenatureandconsequencesoftheiralternativeideas,aswellasarangeofclassroomresourcestodiagnosestudentideas,canbefoundinChemical Misconceptions – Prevention, Diagnosis and Cure.Taber,K.S.(2002).London:RoyalSocietyofChemistry.Volume1:Theoreticalbackground;Volume2:Classroomresources. Toexplorefurtherideasforteachingaboutthenatureofscience,readersmightrefertothecompanionhandbookinthisseries,Teaching Secondary How Science Works.VanessaKind andPerMorten Kind(2008).London:HodderEducation.
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CLEAPSSprovidessupportforpracticalwork,andinparticularhealthandsafelyinformationforschoolscience.CLEAPSSisthesourceforsuchusefulresourcesastheSecondary Science Laboratory HandbookandSecondary Science Hazcards(providingsafetyinformationandmodelriskassessmentsforhandlingchemicals).www.cleapss.org.uk Foraresearch-baseddiscussionofeffectivepracticalworkinschoolscienceseePractical Work in School Science: A Minds-on Approach. Abrahams,I.(2011).London:Continuum. Thegroup-workactivity‘Judgingmodelsinscience’isincludedinEnriching School Science for the Gifted Learner.Taber,K.S.(2007).London:GatsbyScienceEnhancementProgramme.Availablefrom‘MindsetsOnline’:www.mindsetsonline.co.uk
MM Websites Anintroductiontothecommonalternativeideas(‘misconceptions’)thatstudentsoftendevelopinchemistryisprovidedinKind,V.(2004).Beyond Appearances: Students’ Misconceptions about Basic Chemical Ideas(2ndedn).London:RoyalSocietyofChemistry.Availableat:www.rsc.org/Education/Teachers/Resources/Books/Misconceptions.asp
TheRoyalSocietyofChemistryprovidesawiderangeofresourcestosupportchemistryteaching,whichmaybesearchedat:www.rsc.org/learn-chemistry
Examplesofhowstudentsthinkaboutandexplainchemistry(andotherscience)topicscanbefoundattheECLIPSE(ExploringConceptualLearning,IntegrationandProgressioninScienceEducation)projectwebsite:www.educ.cam.ac.uk/research/projects/eclipse
Arangeofresourcesforchemistryteachersrecommendedbycolleaguesontwoemaildiscussionlists([email protected]@mailer.uwf.edu)arelistedat:http://camtools.cam.ac.uk/access/wiki/site/~kst24/teaching-secondary-chemistry-resources.html
ThemagazineEducation in Chemistry,publishedbytheRoyalSocietyofChemistry,includesaregularfeaturecalled‘ExhibitionChemistry’offering‘ideasforchemistrydemonstrationstocapturethestudent’simagination’.Theseareaccompaniedbyavideoofthedemonstrationwhichcanbeviewedontheweb:www.rsc.org/Education/EiC/topics/Exhibition_chemistry.asp
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Introducingparticletheory
Philip Johnson
2.1 Melting and solidifyingMM Melting pointsMM Defining a substance
2.2 A ‘basic’ particle modelMM The solid and liquid statesMM Introducing the basic particle model
2.3 Boiling and condensingMM Predicting the gas state MM MBoiling pointsMM Boiling water MM M‘Gases’
2.6 More on the gas state: pressure, weight and diffusion
MM The gas state and pressureMM Mass and weight of the gas stateMM Diffusion involving the gas state
2.7 What, no ‘solids, liquids and gases’?MM Three types of matter?MM The concept of a substance is important
2.4 DissolvingMM Recognising and explaining dissolvingMM Solutes in the liquid and gas statesMM Empty space in the liquid and solid statesMM Intrinsic motion and the liquid stateMM Solubility
2.5 Evaporation into and condensation from the air
MM Evaporation and boilingMM Temperature and energyMM Explaining evaporation below boiling pointMM Factors affecting the rate of evaporationMM Reconciling boiling and evaporationMM Condensation from a water–air mixture
2
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MM Why teach particle theory?Chemistryisallaboutsubstances.Substancescanbeinvolvedinthreekindsofchange:changeofstate,mixingandchemicalchange.Ourdescriptionsofphenomenaareintermsofsubstances.Whenalumpofleadchangestoarunnyliquid,itisstillsaidtobethesamesubstance.Whenasugarcrystaldissolvesinwater,thesubstancesugarisstillthoughttobethere.Thesubstanceswaxandoxygenchangetothesubstanceswaterandcarbondioxideinacandleflame.Whydosuchdescriptionsmakesense?Liquidisverydifferentfromsolid,sohowcanliquidleadbethesamesubstanceassolidlead?Thecrystaldisappears,whynotthesugar?Howisitthatsubstancescanceasetoexistwithnewonescreatedintheirplace? Thesedescriptionsowetheirsensetoparticletheory,whereasubstanceisidentifiedwithaparticle.Observationsaretheoryled,andparticletheoryatvaryinglevelsofsophisticationdeterminesthewaychemistsspeakandthinkaboutthematerialworld.Therefore,ifwewantstudentstoreallyunderstandchemistry,weneedtodeveloptheirunderstandingofparticletheory.Atentrylevel,a‘basic’modelreferringtotheparticlesofasubstanceisgoodenoughforchangesofstateandmixing.Atgreaterresolution,identifyingasubstancewithan‘atomstructure’(whichatomsarebondedtowhich)accommodateschemicalchange.Thischapterconfinesitselftotheintroductionofa‘basic’particlemodel,leavingatomstoChapter3.
MM Previous knowledge and experienceMO the concept of a substance
Thefollowingpictureofstudents’thinkingisbasedonextensiveresearch.Totheuninitiated,whatchemistsmeanbyasubstanceisnotatallobvious.(Chapter1discussesthisatlength.)Mostmaterialsineverydayexperiencearemixturesofsubstances;fewarerelativelypuresamplesofsubstances.Moreover,everydaythinkingassignsidentitytoasampleofmaterialbyitshistoryratherthanitscurrentproperties.Thefocusisonwheresomethinghascomefromandwhathashappenedtoit.Althoughtheideaofmixingisreadilyappreciated,materials‘asfound’arenotnecessarilycategorisedaseithersubstancesormixturesofsubstances.Forexample,therewouldbenodistinctionbetweenmilk(amixtureofsubstances)andsugar(asubstance,sucrose)asingredients.Rustisstillthoughttobeiron,butinadifferentform(oramixtureofironandsomethingelse);thepreoccupationiswithitsorigin.Whereas,fromthechemist’spointofview,rustisanewsubstanceinitsownrightbecauseofitsproperties;thesubstanceironnolongerexists.
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Previousknowledgeandexperience
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Studentsaregenerallymystifiedbythegasstateandareveryfarfromthinkingthat‘gases’areevenmateriallikethatwhichpresentsinthesolidorliquidstates.Difficultieswiththegasstatearenotsurprising.Historically,theideaofagasbeingasubstancewasnotestablisheduntiltheearlyeighteenthcenturywithBlack’sworkoncarbondioxide.RecognisinggasesassubstanceswasanimportantprecursortoLavoisier’sexperimentswithoxygenandhisfoundingofmodernchemistry.Muchofschoolchemistryinvolvesoneormorereactantsand/orproductsinthegasstate.Understandingthegasstateispivotalinunderstandingchemistry. Theconceptofasubstanceissomethingthatneedstobelearntandgoeshandinhandwithparticletheory.Withoutsubstances,particleshavenoidentity.Thedistinctionbetweenpuresamplesandmixturesisalsocrucial;thereareno‘milk’particlesinthesamesenseastherearesugar(sucrose)particles.(Notethatweshouldnottalkabout‘puresubstances’sincethetermismisleading.Thequestionofpurityrelatestothesamplenottotheideaofasubstance.Forexample,inthisbeakeristhereonlywater?Substancesarejustsubstances–therearenotpureandimpuresubstances.)
MO alternative particle modelsThe‘basic’particlemodelposeschallenges.Totheeye,matterappearstobecontinuousandimaginationisneededtothinkintermsofextremelysmall,discreteparticles.Initially,somestudentsmayconstructanimageofparticlesembeddedincontinuousmatter.Unfortunately,textbookssometimesshowsuchimagesandtalkaboutparticles‘in’asolid/liquid/gas,whichcouldleadstudentsastray.Here,theparticlesarenotthesubstance,theyareextratoit.Ideasofparticlemovementareconsistentwiththismodelsincemovementwillbedeterminedbythestateofthecontinuousmatter(forexample,particlescanmovearound‘in’aliquid).Identifyingtheparticleswiththesubstancehelpstoavoidsuchmisconceptions,forexample,‘sugarparticles’,‘waterparticles’and‘oxygenparticles’. Thosestudentswhodoseetheparticlesasbeingthesubstanceoftenstartbyattributingmacroscopicpropertiestotheparticles.Forexample,studentsmaythinkthatacopperparticleishard,malleable,conductselectricityandiscoppercoloured.Ittakestimetoappreciatethatthephysicalnatureoftheparticlesisirrelevant,withthecharacteristicpropertiesofthestatesdeterminedonlybymovementandspacing,notbywhatindividualparticlesarelike. Thedifferentkindsofmovementassociatedwiththedifferentstatesarenotsoproblematicforstudents.However,theintrinsicnatureofthemovementismoredemanding,especiallyforthesolid
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state.Themostchallengingaspectofthemodelisthenotionofemptyspace.Logically,iftheparticlesarethesubstance,theremustbenothingbetweentheparticles–anythingofamaterialnaturewouldbemoreparticles.However,studentsareinclinedtothinkthereissomethinginbetween.Manywillsay‘air’,althoughwhatismeantisoftenveryvague.
MM Choosing a routeThechapteroverviewpresentsasequencewhichdevelopsunderstandinginacumulativeprogressionwhereeachstepbuildsonprevioussteps.Itseekstoavoidintroducingtoomanyideasatoncebutalsoensuresthatthekeyideasinvolvedinaneventarenotoverlooked.Whatisnecessaryandsufficientforacoherentaccountatthedesiredlevelofunderstandingistheaim. Assumingstudentsdonotalreadyholdtheconceptofasubstance,meltingbehaviourisanaccessiblefirststep.Meltingataprecisetemperatureindicatesapuresampleofasubstanceandthetemperatureidentifieswhichsubstance.The‘basic’particlemodelcanthenbeintroducedtoexplainmeltingandwhydifferentsubstanceshavedifferentmeltingpoints.Onceestablishedforthesolidandliquidstates,themodelisusedtopredict thegasstate(andboiling).Thisapproachrecognisesthatstudentswillnotknowwhatagasisand,importantly,demonstratesthepredictivefunctionofscientificmodels. Thebasicmodelisthenusedtoexplaindissolving,whichincludeslookingatdiffusionwithintheliquidstate.Toaccountforevaporationintoandcondensationfromtheairbelowboilingpoint,ideasofenergydistributionareintroduced.Intermsofunderstandingthenatureofmodels,thisillustratesdevelopmentofthemodeltoaccommodatefurtherphenomenawithoutundoingthemainideas.Totidyupwereturntothegasstateandconsiderpressureandweightandsomewellknowndiffusionexperimentsforillustratingintrinsicmotionwhichdrawonotherideastodifferingdegrees.Finally,weconsidertheadvantagesofintroducingparticleswithinasubstance-basedframeworkovera‘solids,liquidsandgases’framework. Toalargeextent,thesequenceisgovernedbythelogicalstructureofthecontent.However,thebasicmodelcouldbeconsolidatedforthesolidandliquidstatesinthecontextofdissolvingbeforemovingontothegasstate.Eachstageisnowconsideredinmoredetail.
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2.1Meltingandsolidifying
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2.1melting and solidifyingMM melting points
Therearemanyaspectstomelting,atvaryingdepthsofunderstanding.Tobeginwith,introducetheideaofameltingpointaswhensomethingis‘hotenoughtomelt’.Asaclassexperiment,studentscouldbechallengedtofindthemeltingpointofcandlewaxbythemethodshowninFigure2.1.(Notethatasampleofcandlewaxisnotjustonesubstancebut,withinthelimitsoftheexperiment,thereisapreciseenoughmeltingpoint.Candlewaxhastheadvantagesofbeingfamiliarandreadilycuttoconvenientsize.Octadecanoicacid(stearicacid,meltingpoint70°C)or1-hexadecanol(cetylalcohol,meltingpoint56°C)arealternatives.) Discussionshoulddrawattentiontotheindependencefromsamplesize–ifmeltingtemperaturedependedonamount,theexperimentwouldnotbeworthdoing.(Ofcourse,thetimeittakesforasampletomeltdoesdependonamount,forexamplesnowflakeversusiceberg.)
Demonstrationsofmeltinglead(meltingpoint328°C)andsodiumchloride(meltingpoint801°C)canfollow.(ForthelatteruselargecrystalsfromrocksaltandthreeBunsenflames.)Thetemperaturescannotbemeasured,butdrawattentiontothedistinctchangefromsolidtorunnyliquidwhichischaracteristicofaprecisemeltingpoint.
MM Defining a substanceAsampleofasubstancecannowbedefinedassomethingwhichhasaprecisemeltingpoint,andsolidandliquidcanbedefinedas
thermometer to gently stir and monitor temperature next to boat
water
gentle heat
wax (rice-grain size) in a foil boat
02_01 ASE Teaching Secondary Chemistry
Figure 2.1 Finding the melting point of candle wax
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twoofthestatesasubstancecanbein.Figure2.2highlightsmeltingpointasa‘switching’temperature.Onheating,asubstancemeltswhenitreachesthistemperature.Oncooling,asubstancesolidifiesonfallingtothistemperature.
Manystudentsthinkthetemperatureneedstofallwellbelowmeltingpointbeforesolidifyingoccurs;perhapstheresultofanassociationof‘freezing’with0°C.(Thisiswhytheword‘solidify’ispreferredtotheterm‘freeze’inthelanguageofchemistry.)Moreofthestoryisrevealedinheating/coolingcurveexperimentsandtheirinterpretationintermsoflatentheats.Howeverthesearedifficultideasgoingwellbeyondrequirementsatthisstage.Thinkingthatsolidifyingtakesplaceatjustbelowmeltingpointrepresentssignificantprogressformoststudentsandisapositiontobuildonlater.Otherpointsforlateraretheabsenceofaprecisemeltingpointformixtures(suchaschocolate)and,afterchemicalchange,thedecompositionofsomesubstancesbeforemelting(suchascalciumcarbonate).
2.2a ‘basic’ particle model Particletheoryinvolvesanumberofideaswhichworktogethertoformamodel.Theminimumcorefora‘basic’particlemodelis:
MM Asampleofasubstanceisacollectionofidenticalparticles;theparticlesarethesubstance.
MM Theparticlesholdontoeachother;the‘holdingpower’isdifferentfordifferentsubstances.
MM Theparticlesarealwaysmovinginsomeway–theyhaveenergyofmovement.Heatinggivestheparticlesmoreenergyofmovement–theyaremoreenergetic.
‘Holdingpower’ispreferredtotalkofforcessincethestrengthsofforcesdependonthedistancesbetweenparticles,anddistanceschange.Holdingpowerbelongstotheparticleanddoesnotchange.However,holdingpowerappliesonlytoparticlesofthesamesubstance–itcannotbeusedtopredictthestrengthofholdbetweenparticlesofdifferentsubstances.
precisemelting point
temperature
LIQUIDSOLID
Figure 2.2 A substance has a precise melting point.
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MM the solid and liquid statesThestateofasubstancedependsonthebalancebetweenholdingpowerandenergyofmovement(whichdependsonthetemperatureofthesample).Forthesolidstate,theholdingpowercankeeptheparticlesclosetogetherinfixedplacesandtheparticlescanonlyvibrate.Fortheliquidstate,theholdingpowerkeepstheparticlesclosetogetherbutnotinfixedplaces.Theparticlesmovearoundfromplacetoplace.Themeltingpointofasubstancedependsontheholdingpower–thestrongerthehold,thehigheritis. Indiagrams,werecommendusingshapesotherthancirclesforsubstanceparticles.Thishastheadvantageofmakingiteasiertoshowthedisorderoftheliquidstate.Withcirclesthereisatendencytomoveparticlestoofarapart.Furthermore,ifcirclesarereservedforatoms,thedistinctionbetweenthesubstanceandsub-substancelevelsoftheoryisemphasised.
MM introducing the basic particle modelWithrespecttounderstandingthenatureofmodelsitshouldbenotedthatthephenomenonofmeltingofitselfisnotdirectevidencefortheexistenceofparticles.Themodelcanbepresentedasausefulwayofthinking.AsnotedinChapter1,everydaydiscourseoftenuses‘particle’todescribesmallyetvisiblepiecesofmaterialanditisimportanttoemphasisethespecialscientificmeaningwithintheparticletheory.Asmalldropofwater(0.05cm3)containstheorderof1.7×1021waterparticles(molecules).Employingtermssuchas‘grains’,‘pieces’,‘bits’,‘lumps’,‘specks’,‘droplets’and‘globules’inothercontextshelpstomakethedistinction.Appreciatingtheabsolutescaleofparticlesizeisachallengetoallofus.However,asenseofbeingextremelysmallisgoodenoughtooperatewiththemodel.
Solid stateThe particles hold themselves close together.Each particle is in a fixed position.The particles are vibrating.There is an ordered arrangement.
Liquid stateThe particles hold themselves close together.The particles move around from place toplace, randomly.
Figure 2.3 the same substance in the solid and liquid states
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Thechangeinmovement–fromfixedplacestomovingaround–isthekeypointinexplainingthechangefromsolidtoliquid.Tocounternotionsofindividualparticleshavingastate,emphasisethattheparticlesthemselvesdonotchange–individualparticlesdonotmelt.Pointoutthatwehavenotsaidwhatindividualparticlesarelike,physically.Allwecansayistheyarenotlikeanythingweknow.Theybehavelikerubberballsbuttheyarenotmadeofrubber!Itisalsoworthnotingthatthedistancesbetweentheparticleshardlychanges.Formeltingice,theparticlesactuallymoveclosertogether.Movingfurtherapartdoesnotexplainthechangefromsolidtoliquid.Inconsideringthebalancebetween‘hold’and‘energy’,studentshaveatendencytothinkoftheholdweakeningratherthanincreasedenergyovercomingtheholdenablingtheparticlestomovearound.(Thechangeinaveragedistanceapartisverysmallandhencetheaveragestrengthofforceschangeslittle.Attimes,movingparticleswillbeverycloseandifnotenergeticenoughwillbeheldinplace.)
MO limitations of an energy-only explanationAnenergyexplanationofmeltingisalimitedviewwhichdoesnotaccountforicemeltingattemperaturesdownto–12°Cinthepresenceofsalt.Likeallchanges,amorecompleteexplanationofmeltingconcernsentropywhichconsidersthearrangementofenergyandparticles.Atmeltingpoint,thedecreaseinentropyofthesurroundings(duetolossofenergysuppliedforthemelting)isequaltotheincreaseinentropyassociatedwiththechangefromsolidtoliquidsituation.However,iceinthepresenceofsaltmeltstoformasolution,andsolutionshavehigherentropythanpureliquid.Therefore,theincreaseinentropyfromicetosaltsolutionisgreaterthantheincreaseforicetowater.Thegreaterincreasecompensatesagreaterdecreaseinentropyofthesurroundingsandsoicemeltsatatemperaturebelow0°C(alowertemperaturemeansabiggerchangeinentropyforagivenchangeinenergy).Ofcourse,thistreatmentisveryadvanced,butstudentswillknowaboutsaltonicyroadsandmaywellask.Weneedtoacknowledgethatthereismoretounderstandaboutmelting.Fornowonecouldsaythatitiseasierforicetomelttoformpartofasolutionthanapureliquid.Whenmeasuringmeltingpointsitisimportanttohavepuresamplesofsubstances.
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2.3boiling and condensingMM Predicting the gas state
Havingestablishedthebasicparticlemodelforthesolidandliquidstates,studentscouldbeinvitedtospeculateaboutthecontinuedheatingofsubstancesintheliquidstate.Imagineheatingadropofwaterinasealedplasticbag(withallairexcluded)tohigherandhighertemperatures.Whatmighthappentotheparticles?Withlittleornopromptingsomestudentswillsuggestthattheparticlescouldmoveapartbecausetheyhavetoomuchenergyfortheholdtokeepthemtogether.ObjectsheldtogetherbyVelcro®canbeusedinananalogy.Theabilitytohold–theVelcro–doesnotgoawaybutwithenoughenergytheobjectswillseparate.StudentscouldimaginethemselveswearingsuitsmadefromthetwosidesofVelcroarrangedinachequeredpattern.Theywouldsticktogetherasoppositepatchesmatchedup,butwithenoughenergytheycouldmoveapart.Onlosingenergy,theVelcrowouldbeabletokeepthemtogetheragain.(PatchesofVelcrogluedtopolystyreneorwoodenballsmakeaneffectivemodel.) Whatisobservediftheparticlesmoveapart?Studentsmaynotbesosureaboutthisbuthereisanexperimentworthtrying!Sinceheatingwaterinaplasticbagisnotsoeasy,injectasmallvolumeofwater(0.05cm3)intoasealedgassyringepreheatedtoaround150°Cinstead.Onthebenchoutofitsoven,thesyringeretainsahighenoughtemperaturelongenoughtogivesatisfactoryresults.Explainthatthesmallamountofwaterwillquicklyheatuponcontactwiththehotglass.Theplungermovesoutquickly,theliquidwaterdisappears(somebubblingmaybeseen)andinsidethesyringeisclear(Figure2.4).Practiceisrecommendedsincethevolumeofgasisverysensitivetothevolumeofliquidwater,andspeedisoftheessence.Reasonablyconsistentvolumesaroundthe100cm3markareachievable,butback-upsyringesareadvisableincaseanothergoisnecessary.Oncooling,condensationappearsastheplungermovesin.(Theplungermightnotgorightbackinbecausesomeairmayintrudethroughthepiercedseal.)
Figure 2.4 Water changing to the gas state
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Therearemanyimportantpointstodiscuss.Whenparticlesmoveapartthegasstateiscreated(Figure2.5).Theparticlesarestillwaterparticlessothisiswaterinthegasstate.Drawattentiontothehugechangeinvolume.Iftheparticlesclosetogetheronlytakeup0.05cm3,whatisbetweentheparticleswhentheyareapartinthegasstate?Manystudentswillwanttosayair(itlookslikeairinsidethesyringe),butremindthemthatthereisonlywaterinthesyringe–wherecouldairhavecomefrom?Weareforcedtoconcludethereisnothingbetweentheparticles–emptyspace.Somestudentsmaysuggestthattheparticlesexpand.Thisisconsistentwiththeobservationsbutpointoutthatwewouldhavetoexplainwhyparticlescanexpand.Itissimplertosaytheparticlesdonotchangeandscientistsalwayspreferthesimplestmodelthatworks.Forsolidtoliquidthekeychangeisachangeinmovement(fixedpositionstomovingaround);forliquidtogasthekeychangeisachangeinspacing(fromclosetogethertoapart).Ideally,studentswillnothaveheardaboutwaterbeingmadeofhydrogenandoxygensincethiscanbeasourceofconfusion.Iftheyhave,somewillsaythegasisoxygenand/orhydrogen,becausetheseareknowntobe‘gases’.Again,pointoutthatwedonotneedamorecomplicatedexplanation(andtestsshowitisstillwater).
MM boiling waterThesyringeexperimenthelpsstudentstounderstandthemorecomplexeventofabeakerofboilingwater.Whatarethebigbubbles?Previously,mostwillhavesaid‘air’butnowtheyknowwaterinthegasstatelookslikeair.Theparallelwiththesyringeexperimentcanbemade.Atthebottomofthebeaker,asmallamountofliquidwaterchangestothegasstate(someparticlesmoveapart)toformabubble.Drawattentiontothemistabovetheboilingwater.Isthiswaterinthegasstate?No!Waterinthegasstatecannotbeseen.Themistiswherethegaseouswaterhascondensedtoformsmalldropletsofliquidwater.Tocorroborate,generateanothersyringeofcleargaseouswater,removethecapandexpelthegastogiveasmallpuffofmistoncoolingintheair.
Gas stateThe particles do not hold themselves together.Particles are apart, moving freely in all diections.There is a lot of empty space (nothing) betweenthe particles.
Figure 2.5 the gas state
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MM boiling pointsStudentscouldmeasuretheboilingpointofwater;theboilingpointsofothersubstancessuchasethanol(79°C)andpropanone(56°C)couldbedemonstrated(avoidnakedflames,heatonahotplateinafumecupboard).Aswithmelting,placetheinitialfocusontheideaofboilingpointasa‘switching’temperature.Onheating,asubstancechangesfromtheliquidtothegasstatewhenitreachesthistemperature.Oncooling,asubstancechangesfromthegastotheliquidstatewhenitfallstothistemperature(itcondenses).Again,fulldiscussionoftemperature–timegraphsassociatedwiththechangeofstatecouldbeleftforlater.However,thesteadytemperatureduringboilingisverynoticeableandcouldbeexplainedintermsofneedingenergytochangefromliquidtogas.Thisiswhytheliquiddoesnotchangetogasallatonce(andaparallelcouldbedrawnwithmelting).ThesubstanceanditsstateschartcannowbecompletedasinFigure2.6.
MM ‘gases’Studentsarenowinapositiontounderstandwhat‘gases’are.Thesearesubstanceswithboilingpointsbelowroomtemperatureandthereforeinthegasstateatroomtemperature.Ineffect,theyhavealreadyboiledtothegasstate.Boilingpointsofthesesubstancesarelowbecausetheholdingpowersoftheirparticlesareveryweak.Atroomtemperature,theparticlesareenergeticenoughtobeapart.Likewaterinthegasstate,nearlyallgasesareclearandcolourless.Lookingthesamehardlyhelpswiththenotionofbeingsomanydifferentsubstances.Puttingaburningsplinttosamplesofnitrogen(splintgoesout),oxygen(splintburnsbrighter)andmethane(catchesalight)showstheyareverydifferent.Introducingajarofchlorineshowsthatthegasstatecanhavecolour,butisstillclear.Itisworthstressingtheclarityofthegasstatetocounteranyconfusionaboutmistsandsmokesbeinggases.Thegasstateisclearbecauseparticlesareseparatefromeachotherandwecannotseeindividualparticles.
precisemelting point
temperature
LIQUID
preciseboiling point
GASSOLID
Figure 2.6 A substance and its three states
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MO airAircannowbeunderstoodasamixtureofsubstancesthatareallabovetheirboilingpoints.Translatingthepercentagecompositionintoparticlesisausefulconsolidationexercise.Outofevery10000particles,7800arenitrogen,2100areoxygen,93areargon,fourarecarbondioxideandthreearevariousothers.Givenallthediscussioninthemediaaboutcarbondioxide,studentswillbesurprisedtofindouttherearesofewparticlesofit.However,proportionalityisthepoint.Puttingmorecarbondioxideintotheatmospheremakesasignificantincreasebecausethereisnotmuchtostartwith.
2.4DissolvingDissolvingprovidesacontexttostrengthenanddevelopstudents’understandingofthebasicparticlemodel,butalsotorecogniseitslimitations.
MM recognising and explaining dissolving Forafocalevent,studentscouldwatchasingle,largecrystalofcommonsalt(sodiumchloride)dissolveinwaterongentlestirring.Thecrystal’sgradualdisappearanceisexplainedbysaltparticlesmixinginwithwaterparticles.Wecanseeacrystalbecausetherearelotsofparticlestogetherinalump.Wecannotseeindividualsaltparticlesdispersedamongthewaterparticlessothesolutionlooksclear(norcanweseeindividualwaterparticles,justthewaterasawhole).Thecrystalhasdissolvedbutthesaltparticlesarestillthere.Suspensionsoffinepowdersaremorechallenging.Manystudentswillsaypowderedchalk(calciumcarbonate)dissolvesonstirringbecauseitspreadsthroughoutthewaterturningitwhite.Forsomestudentsitisworthspendingtimeonwhatapowderis–theycouldmakeonebygrindingalumpofchalkinapestleandmortar.Emphasisethateachtinypieceofchalkisstillmadeofbillionsandbillionsofparticles.Oncloseinspectionofthesuspension,thetinypiecesofchalkarevisibleandeventuallysettletothebottom.Thepiecesofchalkhavenotdissolved.Fromthis,clearnessemergesasthekeycriterionforrecognisingdissolving(aswiththegasstate).Coppersulfatecouldbeintroducedtoshowaclearbutcolouredsolution.Comparingtheresultsofpassingsolutionsandsuspensionsthroughfilterpaperreinforcestheirdifferenceandprovidesanotheropportunitytoapplytheparticlemodel.(Makesurethefilterpapergradeisfineenough.)
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MM Solutes in the liquid and gas statesLeststudentsthinkdissolvingonlyappliestosolutesinthesolidstate,thoseintheliquidandgasstates(atroomtemperature)shouldbeincluded.Fortheliquidstate,ethanol,glycerine,oliveoilandvolasilaresuitableexamplestotestinwater.Thefirsttwodissolvereadily,thelasttwodonot–allcanbeusedbystudents.(Althougholiveoilisamixture,itallbehavesinthesameway.) Forasoluteinthegasstate,injecting1cm3ofwaterintoagassyringeofammonia(100cm3)isaneffectivedemonstration.Theplungermovesinquicklyatfirst,slowinguntilalloftheammoniahasdissolvedinthewater.Explainingtheseobservationstestsstudents’understandingofthegasstate.Howcan100cm3ofammoniadissolveinonly1cm3ofwater?Thekeypointisthatmostofthe100cm3isnothing–emptyspace.Weknowthattheparticlesof100cm3ofwaterinthegasstatemakeasmalldropwhentogetherintheliquidstate.So,100cm3ofammoniainthegasstateisequivalentinamounttoadroporasmallcrystal.Adroporsmallcrystaldissolvingin1cm3ofwaterisnotsuchasurprise. Itisworthemphasisingthattheoriginalstateofasolutehasnobearingonthenatureofthesolution.Particlediagramsforsolutionsofsugar,ethanolandammonialookthesamesavethedifferentsoluteparticleshapes(Figure2.7).Adissolvedsubstancedoesnothaveastateassuch.
MM empty space in the liquid and solid statesVolumeandemptyspacecanbecoveredinthecontextofliquidstatesolutes.When50cm3ofethanoland50cm3ofwateraremixed,thetotalvolumeisaround96cm3.Eventhoughparticlesintheliquidstateareclosetogether,therewillbepocketsofspacewheretheshapesdonotfittogethersnugly,especiallysincetheyaremovingaround.Theparticlesofanothersubstancecouldgointosomeofthisspaceandviceversa.Mixingpastashellsandlentils
Figure 2.7 Particle diagrams of sugar, ethanol and ammonia solutions
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makesagoodanalogy.Bythesameargument,therewillbepocketsofemptyspacebetweenparticlesinthesolidstatetoo.Studentscouldbechallengedtofindoutifvolumeis‘lost’withsolidsolutes.
MM intrinsic motion and the liquid state Totargettheideaofintrinsicmotion,studentscouldbeaskedtopredictandthenobservewhetheracrystalofsaltdissolveswithoutstirring.Thisalsomovesthinkingontothemechanismofthechange.Evenforstillwater,individually,theparticlesaremovingaround.Bombardingwaterparticlesknocksaltparticlesoutofthecrystal,graduallydismantlingit.Usingacolouredcrystallikecoppersulfateorpotassiummanganate(VII)showshowtheparticlesofthedissolvedsubstancespreadoutduetotheintrinsicrandommotionoftheliquidstate(aclassicdiffusionpractical).
MO Why does stirring speed up dissolving?Stirringhasabigeffectontherateofdissolving.Why?Moststudentswillsuggestthatstirringgivestheparticlesmoreenergy.Thisisaplausibleexplanation.However,moreenergeticparticleswouldmeanahighertemperatureandstirringdoesnotincreasethetemperatureofthewatermeasurably.Stirringmovesbatchesorswathesofparticlesaroundbutitdoesnotincreasetheirindividual,randommovement.So,whydoesstirringspeedupdissolving?Thediffusionexperimentwithacolouredcrystalgivesaclue.Withoutstirring,ahighconcentrationofsoluteparticlesbuildsupnexttothecrystal.Throughrandomcollisions,someparticlesareknockedbacktothecrystalandrejoin.Therefore,therateatwhichthecrystalactuallydisappearsdependsonthedifferencebetweentherateofparticles‘leaving’andrateofparticles‘rejoining’.Stirringmoves‘dissolved’particlesawayfromthecrystalandhencereducestherateofrejoining(solongastheincomingsolutionhasalowerconcentration).Usinghypotheticalnumberscanhelptomakethepoint.Forexample,iftherateofleavingis100particlespersecondandtherateofrejoiningwithoutstirringis60particlespersecond,therateofdissolvingis40particlespersecond.Onstirring,therateofleavingwillstillbe100buttherateofjoiningwillreduceto,say,20.Therateofdissolvingwillbe80particlespersecond.Youmayfeelthisistooadvancedforyourstudents.However,theideaofrejoiningisnotsodifficult.Importantly,itcounterstheteleologicalthinkingthattendstocreepintoexplanations:particleshavenointentions.Furthermore,thenotionofatwo-wayprocesslaysimportantgroundworkforunderstandingdynamicequilibriumatalaterstage.
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MM SolubilityThebasicmodelexplainsdissolvingbutitcannotdealwithsolubility.Thereisnosimpleexplanationforwhyupto203gofsucrosewilldissolvein100cm3ofwater,comparedto36gofsodiumchlorideandonly0.1gofcalciumhydroxide(allatroomtemperature).Allaresaturatedsolutionsbutsaturationhasgotnothingtodowithfillingupspaces.Similarly,thereisnosimpleexplanationforthedifferenteffectsoftemperatureonthesolubilityofdifferentsubstances,withsomeincreasing(suchaspotassiumnitrate),somedecreasing(carbondioxide)andsomealmostunaffected(sodiumchloride).Entropyisneededtounderstandsolubility.However,solubilityisatopicthatcannotbeignoredinearlysecondaryschoolchemistryandthisisacasewherethelimitationsofourmodelshouldbeopenlyacknowledged.Ourmodelisnotwrong,butotherideasareneededtotellthewholestoryofdissolvingandthiswillhavetowait.Meanwhile,viewingsaturationintermsofequalratesof‘leaving’and‘rejoining’doesaccommodatethewiderangeinsolubilitiesbutdoesnotexplainwhythisarisesatsuchdifferentconcentrations(downtoeffectivelyzero)andthedifferentialeffectoftemperature. Anotheraspectofsolubilityistheuseofdifferentsolvents.Forexample,whywillsodiumchloridedissolveinwaterbutnotinpropanone,whereasoliveoilwilldissolveinpropanonebutnotinwater?Developingthebasicmodelwithnotionsofdifferentwaysofholdingandcompatibilitybetweenwaysgivessomeaccount.So,sodiumchlorideparticlesholdontosodiumchlorideparticlesandpropanoneparticlesholdontopropanoneparticles,butsodiumchlorideandpropanoneparticlesdonothaveagoodholdforeachotherandwillnotmix.Ontheotherhand,sodiumchlorideandwaterparticlesdohaveaholdforeachother.AgeneralideaoftypesofholdpreparesthegroundforthemoredetailedlookatstructureandbondingcoveredinChapter3.
2.5 evaporation into and condensation from the air
MM evaporation and boilingFromeverydayexperience,studentswillknowthatwaterevaporatesattemperatureswellbelowboilingpoint.Thebasicparticlemodelcandealwiththeoveralldisappearance–waterparticlesbecomemixedinamongairparticles–butitcannotexplainhowthiscanhappen.Moreover,toexplainboilingwesaidparticleswere
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energeticenoughtoovercometheholdandmoveapart.Ifwatermustbeat100°Ctoboil,howcanparticlesescapefromeachotherwhenlessenergeticatmuchlowertemperatures?Thereseemstobeaseriouscontradictionherewhichunderminesourmodelratherthanmerelyexposingitslimitationsaswasthecasewithsolubility.Bringinginideasofenergydistributionresolvestheproblem.
MM temperature and energyTheideaofenergydistributionisbestintroducedwiththegasstate.Withoutgoingintodetailsofmomentum,studentscanappreciatethatenergyisexchangedincollisions(snookerisausefulanalogy).Thetotalamountofenergywithinthesamplestaysthesamebutindividualparticlesatanyonetimehavedifferentamountsofenergy.Wecansaythattemperaturerelatestotheaverageenergyoftheparticles.Muchcanbedonewithasimplifiedideaoflow-,medium-andhigh-energyparticles.Athighertemperaturestherearemorehigh-energyparticlesandfewerlow-energyparticles.Thesameargumentsapplytotheliquidandsolidstates(energyisexchangedbetweenneighbouringvibratingparticles).Itisworthemphasisingthatindividualparticlesdonothaveatemperatureinthesamewaythattheydonothaveastate.
MM explaining evaporation below boiling pointEvaporationisexplainedasfollows(Figure2.8).High-energywaterparticlesthathappentobeatthesurfaceofthesample,andthathappentobemovinginanoutwarddirection,willbeabletoovercometheholdandmoveintotheair.Theescapedwaterparticlesmixwiththeairparticles(here,wewillnotdistinguishairitselfasamixtureofsubstances).Atthesametime,high-energyairparticlescollidewithotherwaterparticles(thoseofmediumandlowenergy)atthesurface.Energyistransferredinthecollisionssothewaterparticlesgainenoughenergytoescape.Ineffect,thewaterparticlesare‘knockedout’.Inthiswayallofthewaterparticleswilleventuallyescapefromthesaucerandmixinwiththeairparticles. Viewingtheeventintermsofenergyandtemperature,theinitiallossofhigh-energywaterparticles(thatcanleaveontheirown)willcausethetemperatureoftheremainingsampletodrop(theaverageenergywillbelower).Energywillthentransferfromtheroomtotheremainingwaterinthesaucer(throughcollisionswithhigh-energyairparticles).Thishelpstomaintainthenumberofparticleswithenoughenergytoescape,andsotheparticlesofthesamplegraduallydisperseamongtheairparticles.AlthoughnotasobviousasheatingwithaBunsen,theroomisprovidingtheenergytomaintaintheevaporationofthewater.
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MM Factors affecting the rate of evaporationThemodeleasilyaccountsfortheeffectsofsurfacearea,temperatureandabreezeontherateofevaporation.Theactiontakesplaceatthesurfaceandsowithalargersurfacearea,morecanhappenatonce.Athighertemperaturestherearemorehigh-energyparticlesabout.Theeffectofabreezeparallelstheeffectofstirringondissolving.Intherandommovement,waterparticlescouldreturn.Theactualrateofevaporationisthedifferencebetweentherateofleavingandrateofrejoining.Abreezesweepsawaywaterparticles,thusreducingtherateofrejoining.(Notethatapplyingideasofenergydistributionrefinesourpreviousexplanationofdissolving.Onlythehigh-energysolventparticles‘knock’particlesawayfromacrystal(andhigher-energycrystalparticlesaremoreeasilyknockedout).Rateofdissolvingincreaseswithtemperaturebecausemoreparticleshavehighenergy.)
MM reconciling boiling and evaporationHavingexplainedevaporationbelowboilingpointwithourdevelopedmodel,weshouldreconsiderboiling.Itisworthhighlightingthedifferences.Evaporationtakesplaceatanytemperature(betweenmeltingandboiling)andtherearenobubbles.Boilingtakesplaceataparticulartemperatureandtherearebubblesofwaterinthegasstate.Forevaporation,theparticlesleaveonebyonefromthesurfacetoformamixturewithairparticles.We
this water particleis being knocked
out by a high-energyair particle
low energy
this water particlecan leave on its
own
air particles (notshowing the differentsubstances of the air)medium energy
high energy
Figure 2.8 A particle explanation for evaporation below boiling point
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cannotseeindividualparticlesleavingandatanytemperaturetherewillbesomehigh-energyparticlesforthistohappen.Forboiling,abubbleisapuresampleofwaterinthegasstatethatformsinsidetheliquidwater(whereitisbeingheated).Ittakesmorethanoneparticletoformabubble.Thismeanstherehavetobeenoughhigh-energywaterparticlesabletoovercometheholdatthesametime.Thereareonlyenoughhigh-energywaterparticleswhenthetemperaturereaches100°C.Ourimprovedmodelgivesabetteraccountofwhyboilingtakesplaceataparticulartemperatureandcanalsoexplainwhyboilingpointdependsonexternalpressure.Thiswillbecoveredlaterinthechapter.
MM Condensation from a water–air mixtureTheappearanceofcondensationonacoldobjectisaperplexingeventformanystudents.Theyknowtheliquidiswater,butarenotsurewhereitcomesfrom.Manystudentsthinkthewatergoesstraightuptothecloudsafterevaporatingratherthanoccupyingtheairallover(mostwatercyclediagramsdodepictitgoingstraightup).Ourexplanationofevaporationshowshowwaterparticlescanmixintotheairandthisopensupthepossibilityofsomewaterparticlesbeinganormalpartofair.However,alittlemorethinkingwiththemodelisrequired. Althoughescapingwaterparticleshavehighenergy,allwillnotretainthisstatus.Onmixingandcollidingwithairparticles,adistributionofenergiesamongthewaterparticleswillre-establish.Thedistributionwillmatchthatoftheairparticlessincethewholecollectionofparticlesisatonetemperature.Inthatcase,whenlow-andmedium-energywaterparticlesmeetup(astheywillthroughrandommovement)onecouldaskwhytheydonotclustertogethertoformadropletofwaterintheliquidstate(theywillnothaveenoughenergytoescapetheirhold).Thiscouldhappen,butahigh-energyparticle(waterorair–mostlikelyairbecausetherearemoreairparticlesthanwaterparticles)couldalsoknockthewaterparticlesapart(aswithevaporation).Theoutcomewilldependontherelativeratesorchancesofthetwocompetingprocesses.Thechancesofwaterdropletsbeingbrokenapartwilldependonthenumberofhigh-energyparticles,whichdependsonthetemperature.Thechancesofwaterparticlesjoiningwilllargelydependontheconcentrationofwater,andalsothetemperaturesincethisaffectstheproportionoflow-andmedium-energyparticleswithintheconcentration.Inaclearair–watermixture,likenormalroomtemperatureair,therateofbreakingapartisgreaterthantherateofclusteringandsodropletsdonotform.Oncoolingthemixture,therateofbreakingapartdecreases.Whentherateof
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breakinggoesbelowtherateofclustering,dropletsofwaterwillform.Thetemperatureatwhichthishappenswilldependontheconcentrationofwater. Inexplainingtheappearanceofcondensation,theemphasisisusuallyontemperaturebutconcentrationisanequallyimportantfactor.Forexample,whenajarisplacedoveralightedcandle,condensationformsdespitetheglassbeingwarmed.Thisisbecausetheflamegivesoutwaterandtheconcentrationinsidebecomesveryhigh.Onliftingthejarthecondensationquicklyevaporatesbecausetheconcentrationfalls–thereisnochangeintemperature.
MO Could evaporation and condensation be explained without ideas of energy distribution?
Ifideasofenergydistributionarefelttobetoointricateforsomestudents,asimplertreatmentofwater–airmixturescouldbegiven.Evaporationcouldbeexplainedintermsofbombardingairparticlesknockingoutwaterparticles.Importantly,thisexplainswhyevaporationhappensbelowboilingpoint.Waterparticlesthenbecomemixedwithairparticles,withtheairparticleskeepingthemapart.Themoreenergytheairparticleshave,thebettertheyareatkeepingthewaterparticlesapart.Therefore,oncoolingtherecomesapointwhenthewaterparticlescannotbekeptapartandcondensationforms.Howcoldthemixturehastobeforcondensationtoformdependsontheconcentrationofwaterintheair:themorewater,thelesscolditneedstobe.
2.6 more on the gas state: pressure, weight and diffusion
Ourargumentsforwater–airmixturesalsoapplytopuresamplesofsubstancesinthegasstate.Higher-energyparticlespreventlower-energyparticlesfromclustering.Wheretheholdingpowerisveryweak,onlytheverylowest-energyparticleswouldclusterandthesearefaroutnumberedbytherest.
MM the gas state and pressure Gasstatepressureisreadilytackledbythebasicparticlemodel.Bombardingparticlesexertaforceonthewallsoftheircontainer.Thetotalforceonanareadependsonhowstrongthecollisionsareandthenumberofcollisionsatanyonetime.Foragivensampleof
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gas,theaveragestrengthofcollisionsdependsontheaveragespeed,inotherwords,temperature.Thenumberofcollisionswilldependpartlyontheaveragespeedbutalsoonthenumberofparticlesinagivenvolume(concentration).Thereforepressureincreasesbyeitherraisingthetemperatureorsquashingtoasmallervolume. Pressureconsiderationscouldbeaddressedinsomeoftheeventswehavelookedat.Forthe‘dropofwaterinhotsyringe’experiment,onecouldaskwhytheplungerstopsmovingandexplainthatthisiswhenthepressureofgaseouswaterinsideequalsatmosphericpressureoutside.Forabeakerofboilingwater,thebubblesformwhenthereareenoughhigh-energyparticlestogiveapocketofgaswithapressureequaltoatmosphericpressure.Fromthiswecanpredictthatwaterwillboilatalowertemperatureiftheexternalpressureisreduced.Oninjectingwaterintoasyringeofammonia,theplungerispushedinbyatmosphericpressuresincethepressureinsidefallsastheammoniadissolves.Theclassic‘fountainexperiment’makesasuitablyspectacularfollow-updemonstration.(Thiscanbefoundasno.79inClassic Chemistry Demonstrations,RoyalSocietyofChemistry,1998.)
MM mass and weight of the gas stateThegasstatepresentsmanychallenges,notleasttheideathatasampleofgashasamassandthereforeaweightduetogravity.Understandingthatgasesaresubstancesoughttohelpwiththeideaofhavingmass.Theparticlesofasubstancehavemassandthisdoesnotchangewithachangeofstate.However,understandinghowasampleofgasexertsaweightisnotatallstraightforward.Consideranabsolutelyemptycontainer(strongenoughtoresistatmosphericpressure)standingonabalance.Ifairisletintothecontainer,thebalancereadingincreasestoshowtheweightoftheair.However,sincemostoftheairparticlesarenotincontactwiththebottomofthecontainer,howcanthisbe?Theexplanationlieswiththeconcentrationandhencepressuregradientcausedbygravityactingonthesample.Thepressureactingdownwardsonthebottomisgreaterthanthepressureactingupwardsonthetop,whichgivesanoveralldownwardsforce.Thissohappenstoequaltheweightofallparticles.Suchargumentsmustwaitforlater(perhapsinphysics).Weraisetheissuetopointoutthatitisnotatallobviouswhyasampleofgashasaweight.Theincreaseinweightwhenparticlesfromthegasstateendupcombinedinthesolidstateislessproblematic(suchasoxygeninmagnesiumoxide).
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MM Diffusion involving the gas stateMO ammonia and hydrogen chloride
PerhapsthebestknowndiffusionexperimentisthatshowninFigure2.9.
Oncesolutionsofgaseoussoluteshavebeenaddressed,studentswillbeinthepositiontoappreciatethatsomeammoniaparticlesescapefromconcentratedammoniasolutionandlikewiseforhydrogenchloride.Theappearanceofthewhiteringsshowsthattherespectivesubstanceparticleshavemetandhavethereforemovedbythemselves(makingtheirwayinamongairparticles).Thefasterprogressoftheammoniaparticlesisalsoevidentandthiscouldjustbeleftasanobservation.Explainingthefastermovementwilldrawonideasofkineticenergy(½mv2).Assumingthesameaveragekineticenergy,ammoniaparticleshaveahigherspeedtocompensatetheirsmallermass.
MO PerfumePouringperfumeorasmellysubstancelikepropanoneintoadishinvolvesevaporationandmixingintotheair.Somestudentsthink‘smell’issomethingelseinfusingthesubstancesoitisimportanttoestablishsmellasaninteractionbetweenparticlesandnosereceptorswhichleadstoastimulationinthebrain(theoriessuggesteitherthroughshapeorvibrationalfrequency).Interestingly,someanimalscansmellcarbondioxidewhichenablesthemtocatchpreyinthedark.Alsobeawarethatthisdemonstrationgivesafalseimpressionoftherateofdiffusion.Mostofthespreadingoutisduetoparticlesbeingcarriedbyaircurrentsratherthantheirindividualrandommotion.
02_09 ASE Teaching Secondary Chemistry
otton wool soaked in concentrated hydrochloric acid solution
cotton wool soaked in concentrated hydrochloric acid solution
cotton wool soaked in concentrated ammonia solution
white smoke ring forms
Figure 2.9 diffusion of ammonia and hydrogen chloride in air
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MO Samples of substances in the gas stateThesimplestwaytodemonstratediffusioninthegasstateistousesubstancesinthegasstate.Traditionally,samplesarepositionedinalternateverticalarrangements(Figure2.10a).However,thisaddsthecomplicationofweightanddifferentdensities.Arrangingthetubeshorizontallyavoidstheissue(Figure2.10b).
2.7What, no ‘solids, liquids and gases’?Mostlikely,youwillhavebeenexpectingtoreadabout‘solids’,‘liquids’and‘gases’inachapteronintroducingparticletheory.Thisistheconventionalapproach.Infact,wehaveavoidedallreferenceto‘solids’,‘liquids’and‘gases’becausefundamentallytherearenosuchthings.Forthechemist,thereareonlysubstancesandtheirstates.Theroomtemperaturestateisquitearbitraryandhasnofundamentalsignificance(Figure2.11).
a Tubes arranged vertically
hydrogenor carbondioxide
airhydrogenor carbondioxide
air
5 minutes 5 minutespositive tests
for hydrogen orcarbon dioxide
in both test tubes
b Tubes arranged horizontally
hydrogenor carbondioxide
air
Figure 2.10 diffusion using substances in the gas state
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MM three types of matter?Atfirstsight,basingourteachingon‘solids,liquidsandgases’seemsappropriatesincethisreflectsdirectexperienceandismoreuserfriendly.Classifyingmaterialsintothethreecategoriesmakesaniceactivity.However,thiscarriesthedangerofpromotingtheideathat‘solids’,‘liquids’and‘gases’arethreeseparatekindsofmatter,akintodifferentspecies.Studentscouldcomeoutthinkingsomethingiseither‘asolid’,‘aliquid’or‘agas’:manydo.Changesofstatearethenviewedastheanomalousandsomewhatconfusingbehaviourofsomesubstances(forexample,iswax‘asolid’or‘aliquid’?).Thinkingof‘gases’asathirdkindofmatterpreservestheirmysteryandinhibitstheideathatgasesaresubstancesjustasmuchasanythingelse.
MM the concept of a substance is importantByfocusingonthestatesgenerically,the‘solids,liquidsandgases’approachalsoignorestheconceptofasubstance.Statesdonothaveanidentityandthereforetheparticleslackidentity–thetalkisintermsof‘solidparticles’,‘liquidparticles’and‘gasparticles’.Thissuggestsdifferenttypesofparticle,whichreinforcestheideaofthreedifferentkindsofmatterandthatparticlescarrythemacroscopicproperties–theverycommonmisconceptionnotedearlier. Ignoringtheconceptofasubstanceprecludestheimportantdistinctionbetweenpuresamplesofsubstancesandmixturesofsubstances.All‘solids’aretreatedasoneandsimilarly‘liquids’and‘gases’.Inclassificationactivities,thiscanleadtotheveryunsatisfactorysituationwheredifficultiesassociatedwith
1600
1400
Tem
pera
ture
°C
1200
1000
800
600
400
200
0
–200–273
Oxygen Water Salt
20°C
Figure 2.11 Substances and their room temperature states
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classifyinggelsandpastesareseenaslimitationsoftheframeworkratherthantheirnatureasmixtures.Strictlyspeaking,theideaofastateonlyappliestopuresamplesofsubstancesandinthesecasesdecidingwhichstateisrelativelyunproblematic.Studentsshouldnotbeaskedtodotheimpossibleasifitwerepossible.Thefirstthingachemistwantstoknowaboutasampleofstuffiswhetheritisapuresampleofasubstanceornot.Separationtechniquesaresoimportantbecausepuresamplesarewanted(well,pureenough). Distinguishingbetweenmixturesandsubstanceshelpsexplainthedifferencebetweenmeltinganddissolving.Otherwise,ifsugarsolutionisjust‘aliquid’,whyhasthesugarnotmelted?Similarly,therewouldbenodifferentiationbetweenboilingandevaporationbelowboilingpoint.Inthe‘solids,liquidsandgases’approach,botharetreatedasachangeofstate.However,asnotedearlier,itmakesnosensetosaythatwaterchangestothegasstateundersuchdifferentconditions.Evaporationintotheairbelowboilingpointisbetterviewedasamixingphenomenonakintodissolving. Anotherlimitationofthe‘solids,liquidsandgases’approachisthewayittiesthestrengthsofforcestothestate.Thus‘solids’havestrongforces,‘liquids’mediumforcesand‘gases’veryweakforces.Ofcourse,thispertainsifcomparingdifferentsubstancesat,say,roomtemperature.However,itdoesnotdealwellwithchangesofstate.Onmeltingthereisnotasignificantweakeningofforcessincetheaveragedistancechangeslittle.Furthermore,oxygeninthesolidstatedoesnothavestrongerforcesthanironintheliquidstate.Moreinsidiousisencouragementoftheideathatforcestrengthisaconsequenceofthestate(becauseXisasolidetc.)ratherthanafactorindeterminingthestatealongwithparticleenergy.Again,ideasofthreetypesofmatterareboosted. Introducinga‘basic’particlemodelthroughsubstancescoverseverythingthe‘solids,liquidsandgases’approachseekstodobutavoidstheproblems.Scientifically,itismoreaccurateandcoherent.Asnotedearlier,theconceptofsubstanceisnotobviousandneedstobetaughtalongsideparticletheoryinmutualsupport.Identifyingbasicparticleswiththesubstancealsolaysasecurefoundationforideasofatoms.Forexample,whenthereistalkof‘oxygen’,studentsmustbeabletodistinguishbetweenoxygenatomsandthesubstanceoxygen(O2)–theyarequitedifferent.(ThiswascoveredingreaterdepthinChapter1.)
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Otherresources
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Other resourcesStuff and SubstanceisamultimediapackagedevelopedbytheGatsbyScienceEnhancementProgramme(SEP)whichprovidesteachingmaterialstosupportasubstance-basedapproachtointroducingtheparticletheory.Stuff and Substance: Ten Key Practicals in Chemistryisabookletwhichcomplementsthemultimediapackage.Itlooksathowpracticalworkcanbeusedtosupportthedevelopmentofkeyideas.TheStuff and SubstancematerialsareavailableonlineintheNationalSTEMCentreelibrary(www.nationalstemcentre.org.uk/).RegisteredusersoftheNationalSTEMCentrecanalsodownloadtheindividualvideosandanimations.
Somearticlesdescribingtheresearchbackgroundare:
Johnson(1998).Progressioninchildren’sunderstandingofa‘basic’particletheory:Alongitudinalstudy.International Journal of Science Education,20(4),393–412.
Johnson&Papageorgiou(2010).Rethinkingtheintroductionofparticletheory:asubstance-basedframework.Journal of Research in Science Teaching,47(2),130–150.
Johnson&Tymms(2011).Theemergenceofalearningprogressioninmiddleschoolchemistryrelatingtotheconceptofasubstance.Journal of Research in Science Teaching,48(8),849–984.
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