Chapter COMBUSTION,FUELSANDFLAME

Weusedifferentkindsofmaterialsasfuelsforvariouspurposesathome.Youmighthaveobservedorheardabouthowpeopleusedtoburnwood,coal,cakesofcowdung,keroseneetc.,forcookingfoodathome.Blacksmithsinvillagesalsousethemforheatingmetals.Bothinurban and rural areas, now a day’s, LPG is used as fuel for cooking the food.We use the light from the burning candle orkerosenelamp,whenthereisnosupplyofelectricity.Youmighthavealsoobservedburningofacandleorcoal.Whatdifferencedoyounoticeintheburningprocess?•Whydoescandlegiveflamewhenitisburntbutwhydoescoalburnwithoutemittingaflame?•Doallfuelsproducesameamountofheatwhentheyareburnt?•Whatdoweneedtoburnamaterial?•Haveyouevertriedburningapieceofpaperorwoodorcoal,asmallrockorapebble?•Doallofthemburn?Letusdoanactivitytoknowwhichofthesematerialsburnandwhichdonot.

Activity-1Doallmaterialsburn?Youwillneedapairoftongs,somemetalorclaydishesandacandleoraspiritlamp.Usingtongs,pickupa smallpieceofpaper andbring it close to the lighted spirit lampandkeep iton flameas shown in

figure-1.Recordyourobservationintable1.

Fig-1

Carryout this experimentwith apieceof charcoal,magnesium ribbon, straw, cotton cloth, nylon cloth, drywood,pebble,wax,plasticpieceetc,andrecordyourobservations.Youcanalsotrytoburnliquids.Take2mlofwaterinsmallplate.Bringlightedstickneartowaterintheplate(seefigure2). Table1

S.No. Nameofthematerial Howdoesitburn burnt Burns immediatelyBurns slowly Does not

burn 1. Magnesium ribbon

2. Pebbles

3. Petrol

4.

5.

•Whatdoyouobserveinyourattemptofburningwater?•Isthereanydifferenceinflameoflightedstick?•Whathappenedtothelightedstickwhenitisbroughtclosertowaterintheplate?

Fig-2

Carryoutthisactivityusingcoconutoil,mustardoil,kerosene,spirit,petroletc.(sticksshouldbelongenoughtopreventanyfireaccidentifthematerialcatchesfire)Recordyourobservationintable1.

•Whatcanweconcludefromthisactivity?Wecanconcludethatsomematerialsburnandothersdon’t.Intheaboveactivityyouobservedthatwhenmaterialsburnin

air,heatandlightareproduced.A chemical process inwhich amaterial reactswith oxygen present in the air to generate heat is called combustion. The

materials which burn when brought near a flame are combustible materials. Some of them can also be used as fuels. Thematerialswhichdonotburnarecallednon-combustiblematerials.•Whichofthematerialintheaboveactivityarecombustible?

Thinkanddiscuss

•Whysomematerialburnandwhysomedonot?Givereasons.•Whysomematerialswhichdonotburnatnormaltemperatureburnathighertemperatures?Whatisneededfortheprocessofcombustion?Weknowthatweneedamatchstickoralightertoburnamaterial.

•Howwillyouprovethatairisneededtoburnamaterial?•Canweburnamaterialintheabsenceofair?

Activity-2Testingthenecessityofairforburning

Takeasmallburningcandleandputitonatable.Invertaglasstumbleroverit.Thecandlecontinuestoburnforsometime.Thenflickersandfinallyflamegoesoff.(Seefigure3)

Fig-3

Removethetumblerandagainlightthecandle.Putthetumblerbackoverthecandle.Whenthecandleflamebeginstoflicker,removethetumbler.Whathappenstothecandle?Noticecarefully.Wefindthatputtingtheglasstumbleroverthecandlecutsoffthesupplyofairandthecandleflamegoesoff.Thisexperimentprovesthatairisneededtoburnamaterial.Someotherexperimentsneedtobeconductedtoprovethatthe

oxygenpresentinairsupportsthecombustion.

Thinkanddiscuss

Ifyoulifttheglasstumbler(Whichisplacedoveraburningcandle)to1cmheightwhathappens?Why?

Wecancarryoutanexperimenttodemonstratethatoxygenhelpsinburning.

LabActivityAim:ToprovethatoxygenhelpsinburningMaterial required: Test tube, test tube holder, spirit lamp, match box, inscence stick (agarbatti), potassium permanganatecrystals.

ProcedureLightascented/incensestick(agarbatti),andletitburnfor10s,thenputouttheflameandkeepitaside.Takepotassiumpermanganateinatesttube.Holdthetesttubewithatesttubeholderandheatitovertheflameofspiritlamp.

Oxygenisreleasedonheatingofpotassiumpermanganate.Inserttheagarbattiwiththeburningstub,intothetesttubeasshownin

figure4.

Fig-4Observations•Howdoesscentedstickstartedburning?

•Whydoesnotitcatchagainfirewhenitiskeptasideinairafterputtingitsflameoff?Youobserve that stickburnswitha flame.Here theoxygensupportscombustionbyhelpingAgarbathi toburnwithbright

flame.

Thinkanddiscuss

•Howdoyousaythatthegasreleasedintheaboveexperimentisoxygen?•Canwereplacepotassiumpermanganatewithanyothersubstancetoreleaseoxygen?•Isthereanyotherproceduretoprovethatoxygenisneededforburning?Afewmoreexamplesofcombustionaregivenbelow.Canyouexplainthereasonsforthechangestakingplace?

•Aslowfireburstsintoaflamewhenairisblownonit,butacandleburningwithflamegoesoffwhenairisblownonit.Why?•Ifalargequantityofdrygrassissetonfireinforeststhenitisverydifficulttoputoffthefire.Why?•Whenanobjectcatchesfire,thefireisputoffbycoveringwithsandorablanket.Why?

Theexamplesandactivitiesdiscussedabove shows that combustion takesplaceonly in thepresenceof air.Weknow thatsomematerialscatchfireassoonastheyarebroughtnearaflame,butsomematerialstakealongtimetostartburningthoughtheyarekeptneartheflame.•Whatcouldbethereasonforthisvariationinburningpatternsamongcombustiblematerials?Letusexplorethisinthefollowingactivities.

IgnitionTemperatureInactivity1,acandleisusedtoburnapieceofpaper.Canweburnpaperwithoutthehelpofflame?

Activity-3BurningapaperwithsunraysOnasunnyday,gooutandfocusthesunraysonapieceofpaperusingamagnifyinglens(Figure-5).Touchthespotafter

sometime.Howdoyoufeel?

Fig-5Youmusthaveheardaboutpeople inancient timesrubbingpiecesofstones together toproducesparks.Haveyoutried it?

Rubtwostoneshardtogetherandtouchthem.Whatdoyoufeel?Nowrecallsomeofyourexperiences:

•Doesamatchstickburnbyitself?•Whydoyourubthematchstickonthesideofthematchboxtoburnit?•Canyouburnapieceofwoodbybringingitclosetoalightedmatchstick?•Whydoweusepaperorkeroseneoiltostartfireinwoodorcoal?Onthebasisofaboveobservationsandpreviousexperiences,wecanconcludethatacombustiblesubstancehastobeinitially

heatedtocatchfireorburn.Thelowesttemperatureatwhichasubstancecatchesfireiscalleditsignitiontemperature.Whenasubstancestartsburningheatisproducedandithelpstoburnthesubstancecontinuously.Theignitiontemperatureisdifferentfordifferentsubstances.Theignitiontemperatureofasubstancedecidesquicknessofcatchingfire.ThesubstanceswhichhaveverylowignitiontemperatureandeasilycatchfirearecalledInflammableSubstances.Examples

ofinflammablesubstancesarepetrol,alcohol,liquifiedpetroleumgas(LPG)etc.,•Canyoumakealistofsomemoreinflammablesubstances?

Activity-4UnderstandingignitiontemperatureTaketwosmallpapercups.Pourwaterinoneofthecups.Putthetwocupsondifferenttripodstandsandheatbothofthem

usingacandleasshowninthefigure-6.

Fig-6•Whichcupburnsfirst?•Doesthewaterinthecupbecomehot?Why?Explainwhyonecupburnsquicklybuttheotherdoesnot,onthebasisofignitiontemperature.Whenheatissuppliedtocups,

the heat received by second cup is transferred to water in it. The water in this cup prevents the paper to reach its ignitiontemperatureandhenceitdoesnotburn.•Whendoesthesecondcupstartburning?Makeaguessanddiscusswithyourteacher.

TypesofCombustionDuringsummertime,drygrasscatchesfireintheforests.Itmayspreadtothetreesandverysoonthewholeforestwillbeon

fire.Itisverydifficulttocontrolsuchforestfires.Iftheheadofamatchstickisrubbedonthesideofamatchbox,itstartsburning.•Whatmakesmatchstickstocatchfire?Amixtureofantimonytrisulphide,potassiumchlorateandwhitephosphoruswithsomeglueandstarchwasappliedonthe

headofamatchstickmadeofsuitablewood.Whenitstruckagainstaroughsurfacewhitephosphorusgotignitedduetotheheatof friction. This starts the combustion of the match stick. However, white phosphorus proved to be dangerous both for theworkersinvolvedinthemanufacturingofmatchboxesandfortheusers.Thesedaystheheadofthesafetymatchstickcontainsonlyantimonytrisulphideandpotassiumchlorate.Therubbingsurface

has powdered glass and a little red phosphorus (which ismuch less dangerous).When thematch stick is struck against therubbingsurface,someredphosphorusgetsconvertedintowhitephosphorus.Thisimmediatelyreactswithpotassiumchlorateinthematchstickheadtoproduceenoughheattoigniteantimonytrisulphideandstartthecombustion.Thetypeofcombustioninwhichmaterialsuddenlyburnsintoflameswithouttheapplicationofanyexternalagentiscalled

spontaneouscombustion.Turnontheknobofthegasstoveinthekitchenandbringaburningmatchstickoragaslighternearit.Thegasburnsrapidly

andproducesheatandlight.Suchcombustionisknownasrapidcombustion.Materialssuchasspirit,petrolandcamphorburnevenwithasparkfroma

gaslighter.

Doyouknow?1.Youmighthavenoticedwordswrittenashighlyinflammableonpetroltankers.Thisisawarningtothepublictokeep

flameawayfromtheinflammablematerial.2.Wegenerallyenjoysoundandlightfromfireworksonfestivaldays.Whenacrackerisignitedasuddenreactiontakes

placewith theevolutionofheat, lightandsound.A largeamountofgas isalso liberated in this reaction.Sucha reaction iscalledexplosion.Explosionscanalsotakeplaceifpressureisappliedonthecrackers.

Thinkanddiscuss

•Whyisphosphoruspreservedinwater?(Hint:thinkabouttheroleofignitiontemperatureincombustion)•WhyKerosenestovesandBunsenburnershavesmallholesinthem?(Hint;Thinkabouttheroleofairincombustion)•Itishardtoignitematchstickinrainydays.Why?

FuelsWe know that combustion gives heat and light. The sources of heat for domestic, automobile and industrial purposes are

mainlywood,charcoal,petrol,kerosene,LPG,CNGetc.Thesesubstancesarefuels.Inthepreviouschapter,westudiedaboutthefossilfuelsandtheiruseinvariousways.Wenotonlyusefossilfuelsbutalsootherkindsoffuelsfordifferentpurposesathome,inautomobilesandinindustries.Listdifferentfuelsthatareusedfor!Domesticpurpose…….................................Automobiles/Aircraft/Trains/Rockets........................................................................Industry.......................................................Classifytheabovefuelsintosolid,liquid,gasesandwritethemintable2.

Table-2 Solid Liquid GasLookatthefuelsinthetable2.

•Canyoudecidethebestfuelamongthem?•Whatisthecriteriatodecideabestfuel?Discusswithyourfriends.Deciding thebest fueldependsupon thepurposeof itsuse.Abest fuel for cookingmaynotbeabest fuel for runningan

automobile.Ingeneral,thereareseveralfactorsthathavetobekeptinmindwhilechoosingafuel,likepurposeofuse,fuelefficiencyand

availability, reasonable price, easy to handle and safe to store, easy to ignite and put off etc. The fuel should also burn at amoderaterateandcauselesspollution.Inaddition,itshouldhaveahighcalorificvalue.

•Whatiscalorificvalue?

Suppose1kgofcoaland1kgofcowdungareburnt.Whichoneproducesmoreheat?Differentsubstancesproducedifferentamountsofheatonburning.Heatismeasuredinkilojoules.Calorificvalueofafuelistheamountofheatenergyproducedoncompletecombustionof1kgofthatfuel.Itismeasuredinkilojoulesperkg(kj/kg).

Doyouknow?

Fuel Calorificvalue(kj/kg)

Cowdung 6000-8000

Wood 17000-22000

Coal 25000-33000

Petrol 45000 Kerosene 45000

Diesel 45000

CNG50000

LPG55000

Biogas35000-40000

Hydrogen150000

FirecontrolYoumusthaveseenorheardaboutfirebreakingoutinhouses,shops,factories,etc.

•Howcanweputoffthefireifitbreaksout?Weusemanymethodstoextinguishafirebut theyallfollowoneprinciple,whichis theprincipleofeliminationof factors

whichsupportthecombustion.

Letusrecallthefactorswhichsupportthecombustion:

(a)Presenceofacombustiblematerialorthefuel

(b)Supplyofairoroxygen

(c)Hightemperature(Morethantheignitiontemperature)

So,eliminationofanyoneofthethreefactorswillhelpincontrollingthefire.Letusseesomeexamples.

Example

Ifafirebreaksoutinahouseorinanybusinessestablishmentthefirebrigadewillimmmediatelyputofftheelectricmainsandthenstartsprayingwateronthefire.

•Whythefirebrigadestarttheworkbyputtingoftheelectricmains?•Howwaterhelpsineliminatingthefactors,whichsupportthecombustion?Initially,thewaterspraycoolsthecombustiblematerialbelowitsignitiontemperature.Thispreventsthefirefromspreading.

Thentheheatturnsthewaterintovapourswhichsurroundtheburningmaterialandpreventsupplyofoxygentotheburningmaterials.So,thefireisextinguished.

Themost common fire extinguisher iswater.Butwaterworks onlywhen things likewood cloth andpaper are on fire. Ifelectricalequipmentisonfirewatermayconductelectricityandharmthosetryingtodousethefire.

Waterisalsonotsuitableforfiresinvolvingoilandpetrol,becausewaterisheavierthantheoil,itsinksbelowtheoilandoilkeepsburningonthetop.

Since it is difficult to remove the combustiblematerial froma fire, cuttingof air supply and lowering the temperature arebettermethods.

Incaseswherewatercannotbeused,carbondioxidegasisthebestchoiceusedwhichisheavierthanoxygen.Itcanbestoredasaliquidincylindersunderhighpressure.Whenreleasedfromthecylinder,itexpandsandbringsdownthetemperature.

Italsocoversthefirelikeablanket.Sincethecontactbetweenthefuelandoxygeniscutoff,thefireiscontrolled.

Thatiswhyitisanexcellentfireextinguisher.Theaddedadvantageofcarbondioxideisthatinmostcasesitdoesnotharmthe electrical equipment. It is mandatory for offices, educational institutions and multistoried buildings to install fireextinguishers.

FlameActivity-5

ObservingthebehaviourofdifferentsolidfuelsCollectsomefuelslikecandle,coal,domesticgas,charcoal,magnesiumribbon,wood,cakesofcow-dung,camphor,wickof

theoillamp,wickofkerosenestove,etc.Burneachofthemonebyonewiththehelpofspiritlampandnotethetimetheytaketocatchfire.Alsoobservehowdotheyburn?•Doallofthemburninthesamemanner?Ifnot,whatdifferencedoyounotice?•Doallofthemformaflamewhiletheyareburning?Recordyourobservationinthefollowingtable

Table-3 Material Forms DoesnotTime flame formflame Candle Magnesium Wickof Kerosenestove Charcoal Domesticgas Camphor Cowdungcake

Youmayobservethatacandleburnswithflamewhereascharcoaldoesnot.Somematerialsburnwithflame,somedonot.Keroseneoilandmoltenwaxrisethroughthewickbecomegasandformflames.Butcharcoalcannotbevapourised.Soitdoesnotproduceaflame.Afuelcatchesfireimmediatelyifitisintheformofgas.Cookinggascatchesfireimmediately.Spiritandpetrolturnintogasatroomtemperature.Hence,theycatchfirequickly.

ThinkanddiscussAwaxcandleburnswithayellowflame.Thedomesticgasburnswithablueflame.Why?

StructureofaflameActivity-6

ObservingthestructureoftheflameLightawaxcandleandwatchtheflame.Carefullynotethedifferentcolouredzonesintheflame.Howmanycoloursarethere

intheflame?

Startingfromthebaseoftheflame,howmanyflamezonesdoyouobserve?Whatisthecolouroftheoutermostzoneoftheflame?

Observetheinnermostzonewhichisdark.Whatdoyouobservethere?Istherecombustiontakesplace?Inthiszonewaxgetsvapourised.Thisisadarkzone.Seefigure7.

Observenearthebaseoftheflame.Vapourisedwaxgetscompletelyoxidizedandburnswithablueflame.Itisbluezone.

Doyouknow?Acandleismainlyasourceoflightbutheatisalsoreleased.Acandleismadeofwaxinwhichathickthreadisinserted.

Wax in the candlemeltswhen it is lighted by amatch stick.A little of thewax forms vapour.This vapour combineswithoxygen in the air to form flame.Theheat of the flamemeltsmorewax from the top of the candle.The liquidwaxmovesupwardthroughthethread.Italsochangestovapourwhenitreachesthetopofthewickandburnswiththeflame.

Activity-7Observingwhathappensindifferentzonesofcandleflame•Lightacandle.Holdaglasstubewithapairoftongsandintroduceitsoneendinthedarkzoneofanonflickeringcandleflame.Keeplightedmatchstickneartheotherendoftheglasstubeasshowninfigure-8.Whatdoyouobserve?Doyouseeaflame?Ifsowhatisitthatproducesaflame?Noticethatthewaxneartheheatedwickmeltsquickly.

Fig-8•Whenthecandle'sflameissteady,introduceacleanglassslideintotheluminouszone(yellowzone)oftheflameandholdfor10seconds.Seefigure9.Whatdoyouobserve?

Fig-9

Ablackishcircularringisformedontheglassslide.Whatisit?Itindicatesthedepositionofun-burntcarbonparticlespresentintheluminouszoneoftheflame.Incompletecombustiontakesplaceinthiszone.Holdathinlongcopperwirejustinsidetheflameforabouthalfaminuteasshowninfigure-10.Whatdoyouobserve?The

copperwirejustoutsidetheflamegetsredhot.Itindicatesthatthenon-luminouszoneoftheflamehashightemperature.Itisthehottestpartoftheflame.Itisblueincolourandcompletecombustiontakesplaceduetogoodsupplyofoxygen.

Keywords

Combustion, combustible and non - combustible materials, ignition temperature, inflammable material, spontaneouscombustion,rapidcombustion,explosion,fuels,calorificvalue.

Whatwehavelearnt?

•Burningamaterialinthepresenceofair(oxygen)iscalledcombustion.•Oxygenorairisneededforcombustiontotakeplace.

•Thelowesttemperatureatwhichasubstancecatchesfireiscalleditsignitiontemperature.

•Thetypeofcombustioninwhichmaterialsuddenlyburnsintoflameswithouttheapplicationofanyexternalagentiscalledspontaneouscombustion.

•Thetypeofcombustioninwhichmaterialburnsrapidlyandproducesheatandlightiscalledrapidcombustion.

•Theamountofheatenergyproducedoncompletecombustionof1kgoffueliscalledthecalorificvalueofthatfuel.

•Waxdoesnotburninthedarkzoneofthecandleflame.

•Inthebluezoneofthecandleflame,vaporizedwaxburnscompleatlyduetogoodsupplyofoxygen.

WHATWEHAVELEARNT?

1.Givefourexamplesofcombustiblematerials.(AS1)

2.Givefourexamplesofnoncombustiblematerials.(AS1)

3.Whyshouldnotwestorespiritorpetrolnearourlivingplace?(AS1)

4.Giveanexampleofagoodfuel.Howdoyouchoosethatfuel?Explain.(AS1)

5.Theoilfiresshouldnotbesprayedwithwater.Why?(AS1)

6.Whatprecautionsaretobetakenwhilepouringwateronfire?(AS1)

7.Whyawickisnotusedingasburners?(AS1)

8.Waterisnotusedtocontrolfiresinvolvingelectricalequipment.Why?(AS1)

9.Itisdifficulttoburnaheapofgreenleavesbutnotaheapofdryleaves.Explainwhy?(AS1)

10.Givesupportingargumentsforboththestatements(1)fireisuseful(2)fireisharmful(AS2)

11.Inafewyearsthefuelsonearthwillbeexhausted.Think,whatwouldhappentohumancivilization?(AS2)

12.Whatwouldhappenifoxygenstopstosupportcombustion?–Makeaguess.Andifitisthesituationforwhatotherworksfuelsareuseful.(AS2)

13.Useofmorefuelsinourdailylifecausesairpollutionanditisharmfultohumanbeingandtheotherlifeonearth.Suggestsomeremediestoavoidthis.(AS2)

14.Letusassumethatyouareonthemoon.Ifyoutrytofocussunlightonapaperusingmagnifyingglass,doesthepapercatchfire?ornot?Why?(AS2)

15.Canyouheatwaterinapapervessel?Howisitpossible?(AS3)

16.“Iscombustionpossiblewithoutthesupplyofoxygen?''Discusswithyourteacher(AS3)

17.Expalingivingreasons:Inwhichofthefollowingsituationswaterwillgetheatedinashortertime?(AS3)

a)Srikarkeptwaterbeakernearthewickintheyellowpartofacandleflame.

b)Sonukeptwaterbeakerintheoutermostpartoftheflame.

18.Projectwork:Collect information about the experiments of JosephPriestly.Write a twopage report describingPriestly’sexperimentsprovingthatoxygenisneededforburning.(AS3)

19.Listthewaysadoptedbyfirefighterstocombatfires.(AS4)

20.Collect information available on different fuels. Find out the cost per kg.Compare the costwith calorific value. Preparereportonthat.(AS4)

21.Collecttheinformationaboutannualfuelconsumptionindifferentpartsoftheworld.Howmanyyearsmore the fossil fuels last?Makeaposterwith this information and issue an appeal to save fuel.(AS4)

22.Drawthediagramofcandleflameandlabelallthezones.(AS5)

23.Wheredoyoufindspontaneouscombustionandrapidcombustioninyourdailylife?(AS7)

24.Howdoyouorganizeyourdailyworkswithfuelstoconservebio-diversity?(AS7)

25.Howdoyoufeelabout“Fuelshavebecomeapartofhumanlife”?(AS7)