teaching secondary chemistry - the association for … · titles in this series: teaching secondary...

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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

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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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ISBN:9781444124323


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


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.


vi

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


iNtrODUCiNg PartiCle theOry

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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.


Previousknowledgeandexperience

51

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



52

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.


2.1Meltingandsolidifying

53

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



54

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.


2.2A‘basic’particlemodel

55

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.


2.3Boilingandcondensing

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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


2.3Boilingandcondensing

59

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



60

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.)


2.4Dissolving

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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.


2.5Evaporationintoandcondensationfromtheair

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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.


2.5Evaporationintoandcondensationfromtheair

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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


2.6Moreonthegasstate:pressure,weightanddiffusion

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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).


2.6Moreonthegasstate:pressure,weightanddiffusion

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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


2.7What,no‘solids,liquidsandgases’?

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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.)


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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