CBSENCERTSolutionsforClass10scienceChapter2

Exercises

Q.1. Asolutionturnsredlitmusblue,itspHislikelytobe:14510

Solution: Asolutionthatturnsthecolourofredlitmustobluemustbebasicinnature.AbasicsolutionhasapHvalueofgreaterthan7.TheonlyoptionwithpHgreaterthan7is10whichwillbethecorrectanswer.

Q.2. EquallengthsofmagnesiumribbonsaretakenintesttubesAandB.Hydrochloricacid(HCl)isaddedtotesttubeA,whileaceticacid(CH3COOH)isaddedtotesttubeB.Amountandconcentrationtakenforboththeacidsaresame.Inwhichtesttubewillthefizzingoccurmorevigorouslyandwhy?

Solution: ThefizzingwilloccurstronglyintesttubeA,inwhichhydrochloricacid(HCl)isadded.ThisisbecauseHClisastrongeracidthanCH3COOHandthereforeproduceshydrogengasataquickerrateduetowhichfizzingoccursmorevigorouslyintesttubeA.

Mg+2HCl→MgCl2+H2↑(faster)Mg+2CH3COOH→Mg(CH3COO)2+H2(slower)

Q.3. FreshmilkhasapHof6.HowdoyouthinkthepHwillchangeasitturnsintocurd?Explainyouranswer.

Solution: ThepHofmilkis6.Asitgetsconvertedtocurd(pHbetween4.5to5.5)thepHwillreducebecausecurdhaslacticacidwhichismoreacidicinnature.Thus,thepHdropsbelow6thenmilkgetsconvertedtocurd.

Q.4. Amilkmanaddsaverysmallamountofbakingsodatofreshmilk.WhydoesheshiftthepHofthefreshmilkfrom6toslightlyalkaline?

Solution: ThemilkmanaddsasmallamountofbakingsodatoshiftthepHofthefreshmilkfrom6toslightlyalkaline.Inalkalinecondition,milkdoesnotsetascurdeasilyandthishelpsinpreventingthemilkfromgettingconvertedtocurd.

Q.5. Amilkmanaddsaverysmallamountofbakingsodatofreshmilk.Whydoesthismilktakealongtimetosetascurd?

Solution: Milkmanaddsasmallamountofbakingsodatofreshmilktomakethemilkslightlyalkaline.Inthisalkalinecondition,theacidsproducedtosetthecurdareneutralizedbythebase.Therefore,ittakeslongertimeforthecurdtoset.

Q.6. PlasterofParisshouldbestoredinamoisture-proofcontainer.Explainwhy?

Solution: PlasterofParis(POP)shouldbestoredinamoisture-proofcontainerbecauseplasterofParis,apowderymass,absorbswater(moisture)toformahardsolidknownasgypsum.

CaSO4.12H2OPlaster of Paris+112H2OWater→CaSO4.2H2OGypsum [Hard solid]

Q.7. Whatisaneutralizationreaction?Givetwoexamples.

Solution: Areactioninwhichanacidandabasereactwitheachothertogivesaltandwateristermedasaneutralizationreaction.Inthisreaction,energyisevolvedintheformofheatandthus,suchreactionsareusuallyexothermicinnature.

Forexample,sodiumhydroxidereactswithhydrochloricacidtogivesodiumchloridesaltandwater:NaOH(Base)+HCl(Acid)→NaCl(Salt)+H2O(Water)

Duringindigestion(causedduetotheproductionofexcessofhydrochloricacidinthestomach),weadministeranantacid(generallymilkofmagnesia,Mg(OH)2whichisbasicinnature).Itneutralizestheexcessacidandthusgivesrelieffromindigestion.

Mg(OH)2+2HCl→MgCl2+2H2O

NCERTScience Chapter2Acids,basesandsalts

PracticemoreonAcids,basesandsalts Page1 www.embibe.com

Q.8. Givetwoimportantusesofwashingsodaandbakingsoda.

Solution: Twoimportantusesofwashingsodaareasfollows:i.Washingsodaisusedinglass,soapandpaperindustries.

ii.Washingsodaisusedtoremovethepermanenthardnessofwater.

Twoimportantusesofbakingsodaareasfollows:i.Bakingsodaisusedinbakingindustries.Bakingpowderisamixtureofbakingsodaandamildacidknownastartaricacid.WhenitisheatedormixedwithwateritreleasesCO2thatmakesbreadorcakefluffy.

ii.Bakingsodaisusedinsoda-acidfireextinguishers.

Q.9. Asolutionreactswithcrushedegg-shellstogiveagasthatturnslime-watermilky.Thesolutioncontains:NaClHCl

Solution: Eggshellsaremadeupofcalciumcarbonate,whichonreactionwithHCl(acid)liberatesCO2gas.WhenCO2reactswithlimewaterCaOH2,itformscalciumcarbonatewhichgivesthesolutionamilkyappearance.CaCO3+2HCl→CaCl2+H2O+CO2Ca(OH)2+CO2→CaCO3+H2O

LiClKCl

Q.10. 10mlofasolutionofNaOHisfoundtobecompletelyneutralizedby8mlofagivensolutionofHCl.Ifwetake20mlofthesamesolutionofNaOH,theamountofHClsolution(thesamesolutionasbefore)requiredtoneutralizeitwillbe

4ml8ml12ml16ml

Solution: Theamountofhydroxideionsandhydrogenionsaredirectlyproportionaltothevolumeoftheirrespectivesolutions.Hence,theproblemcanbesolvedbyunitarymethod.Given, 10 mlNaOHreactswith=8mlHClTherefore, 20 mlNaOHreactswith=(8/10)×20=16mlHCl∴16mlofHClsolutionwillberequiredtocompletelyneutralize20mlofNaOHsolution.

Q.11. Whichoneofthefollowingtypesofmedicineisusedfortreatingindigestion?AntibioticAnalgesicAntacid

Solution: Anantacidisusedforthetreatmentofindigestionwhichiscausedduetoexcessiveacidityinthestomach.Theantacidscontainbasicsubstanceslikemagnesiumhydroxideandaluminiumhydroxide,whichneutralisestheexcessacidinthestomach.

Antiseptic

Q.12. Writewordequationsandthenbalancedequationsforthereactiontakingplacewhen(i)Dilutesulphuricacidreactswithzincgranules.(ii)Dilutehydrochloricacidreactswithmagnesiumribbon.(iii)Dilutesulphuricacidreactswithaluminiumpowder.(iv)Dilutehydrochloricacidreactswithironfilings.

Solution: (i)Thewordequationfortheabovereactionis:Sulphuricacid+Zinc→Zincsulphate+HydrogenThebalancedchemicalequationfortheabovereactionis:H2SO4aq+Zns→ZnSO4aq+H2g

(ii)Thewordequationfortheabovereactionis:Hydrochloricacid+Magnesium→Magnesiumchloride+Hydrogen.Thebalancedchemicalequationfortheabovereactionis:2HClaq+Mgs→MgCl2aq+H2g

(iii)Thewordequationfortheabovereactionis:Sulphuricacid+Aluminium→Aluminiumsulphate+HydrogenThebalancedchemicalequationfortheabovereactionis:3H2SO4aq+2Als→Al2(SO4)3aq+3H2g

(iv)Thewordequationfortheabovereactionis:Hydrochloricacid+Iron→Ferrouschloride+HydrogengasThebalancedchemicalequationfortheabovereactionis:2HClaq+Fes→FeCl2aq+H2g

NCERTScience Chapter2Acids,basesandsalts

PracticemoreonAcids,basesandsalts Page2 www.embibe.com

Q.13. Compoundssuchasalcoholsandglucosealsocontainhydrogenbutarenotcategorizedasacids.Describeanactivitytoproveit.

Solution: Twonailsarefittedonacorkandarekeptina100mlbeaker.Thenailsarethenconnectedtothetwoterminalsofa6-voltbatterythroughabulbandaswitch.SomediluteHClispouredintothebeakerandthecurrentisswitchedon.Thesameexperimentisthenperformedwithaglucosesolutionandanalcoholsolution.

Observations:ItisobservedthatthebulbglowsintheHClsolutionanddoesnotglowintheglucoseoralcoholsolution.

Result:HCldissociatesintoH+andCl-ions.Theseionsconductelectricityinthesolutionresultingintheglowingofthebulb.Ontheotherhand,neithertheglucosesolutionnorthealcoholsolutiondissociatesintoions.Therefore,thesetwosolutionsdonotconductelectricity.Conclusion:Fromthisactivity,itcanbeconcludedthatallacidsgeneratehydrogenionsbutnotallcompoundscontaininghydrogenareacids.Thatiswhy,thoughalcoholsandglucosecontainhydrogen,theyarenotcategorisedasacids.

Q.14. Whydoesdistilledwaternotconductelectricity,whereasrainwaterdoes?

Solution: DistilledwaterisapureformofwateranditspHis7.Itisdevoidofanyionicspecies.Therefore,itdoesnotconductelectricity.

Rainwater,beinganimpureformofwater,containsCO2,SO2,NO.Theseoxidegasesreactwithwaterandgetchemicallyconvertedintoacidswhichareresponsibleforthereleaseofionsthatconductelectricity.Itcontainsmanyionicspecies(acids)whosepHislessthan7andthereforeitconductselectricity.

Formationofsulphuricacid:SO2+12O2+H2O→H2SO4sulphuricacid

Formationofcarbonicacid:CO2+H2O→H2CO3carbonicacid

Q.15. Whydoacidsnotshowacidicbehaviourintheabsenceofwater?

Solution: AsubstanceissaidtobeacidicwhenitcangeneratehydrogenionsH+.Thehydrogeniongenerallycomesfromtheacidwhichdissociatesinthepresenceofwater.Thus,foracidtodissociateintohydrogenionandtherespectiveanion,watermustbepresent.Hence,anacidfailstoshowitsacidicbehaviourintheabsenceofwater.

Q.16. FivesolutionsA,B,C,D,andE,whentestedwithuniversalindicator,showedpHas 4,1,11,7and9,respectively.Whichsolutionis(i)Neutral?(ii)Stronglyalkaline?(iii)Stronglyacidic?(iv)Weaklyacidic?(v)Weaklyalkaline?ArrangethepHinincreasingorderofhydrogen-ionconcentration.

NCERTScience Chapter2Acids,basesandsalts

PracticemoreonAcids,basesandsalts Page3 www.embibe.com

Solution: At25oCpHofneutralsolutions=7.AsthepHfallsbelow7,itdenotesacidiccharacterwithapHof1beinghighlyacidic.WhenthepHgoesabove7,itimpliesthatthesolutionisbasicwith14beinghighlybasic.

(i)HeretheneutralsolutionisDwithpH7.

(ii)HerethestronglyalkalinesolutionisCwithpH11.

(iii)HerethestronglyacidicsolutionisBwithpH1.

(iv)HeretheweaklyacidicsolutionisAwithpH4.

(v)HeretheweaklyalkalinesolutionisEwithpH9.

ThepHinincreasingorderofhydrogen-ionconcentrationisasfollows:11<9<7<4<1.

NCERTScience Chapter2Acids,basesandsalts

PracticemoreonAcids,basesandsalts Page4 www.embibe.com

Questions

Q.1. Youhavebeenprovidedwiththreetesttubes.Oneofthemcontainsdistilledwaterandtheothertwocontainanacidicsolutionandabasicsolutionrespectively.Ifyouaregivenonlyredlitmuspaper,howwillyouidentifythecontentsofeachtesttube?

Solution: Wehavetoaddfewdropsofsolutionfromallthreetesttubesontheredlitmuspaperseparately.Thesolutionwhichturnsredlitmustoblueisbasicsolution.

Usethisbluelitmuspapertotestthesolutionsinothertwotesttubes.Thesolutionfromthetesttubewhichturnsbluelitmuspapertoredwillbetheacidicsolutionandthesolutionofthetesttubewhichdoesnotchangeeitherredorbluelitmuspapercontainsdistilledwater.

Q.2. Whyshouldcurdandsoursubstancesnotbekeptinbrassandcoppervessels?

Solution: Curdandsoursubstancescontainacids.Theseacidscanreactwithbrassandcoppervesselstoformtoxiccompounds.Thismakesthesubstancesunfitforhumanconsumption.Hence,theyarenotkeptinbrassandcoppervessels.

Q.3. WhydoHCl,HNO3,etc.,showacidiccharactersinaqueoussolutionswhilesolutionsofcompoundslikehigheralcoholsandglucosedonotshowacidiccharacter?

Solution: WhenHClorHNO3aremixedwithwater,theydissolveinwatertoformH+orH3O+ionswhichshowtheiracidiccharacter.Thatis,HCl(aq)→H+(aq)+Cl-(aq)H+aq+H2Ol→H3O+aqSolutionsofcompoundslikehigheralcoholsandglucosedonotdissolvetoformanysuchions.Hence,theydonotshowanyacidiccharacter.

Q.4. Youhavetwosolutions,AandB.ThepHofsolutionAis6andpHofsolutionBis8.

Whichsolutionhasmorehydrogenionconcentration?Whichofthisisacidicandwhichoneisbasic?

Solution: pHofsolutionA=6H+=10−6pHofsolutionB=8H+=10−810−6H+ofA>10−8H+ofB

H+ionsconcentrationinsolutionAismorethanthatinsolutionB.

AhasmoreconcentrationthanB.pHofA<pHofB.SolutionwhosepHislesshasmoreH+;therefore,thatismoreacidic.Hence,solutionAismoreacidicthansolutionBandsolutionBismorebasicthansolutionA.

Q.5. WhatisthecommonnameofthecompoundCaOCl2?

Solution: CaOCl2(chemicalname-calciumoxychloride)iscommonlycalledbleachingpowder.Itisusedforbleachingpurpose.

Q.6. Whichgasisusuallyliberatedwhenanacidreactswithametal?

Illustratewithanexample.Howwillyoutestforthepresenceofthisgas?

Solution: Hydrogengasisusuallyliberatedwhenanacidreactswithametal.

Forexample,Zn(s)+2HCl(aq)→ZnCl2(aq)+H2(g)

Theequationgivenaboveillustrateshowzincreactswithdilutehydrochloricacidresultingintheliberationofhydrogengasandtheformationofthesaltzincchloride.

Aburningmatchstick,whenbroughtnearthemouthofthetesttubewhereH2gasisbeingreleasedmakesapopsound.Thisconfirmsthepresenceofhydrogengas.

NCERTScience Chapter2Acids,basesandsalts

PracticemoreonAcids,basesandsalts Page5 www.embibe.com

Q.7. Whydoesanaqueoussolutionofacidconductelectricity?

Solution: AnaciddissolvesinwateranddissociatestoformH+orH3O+ions.Electricityisconductedthroughthesemovingions.

Q.8. WhateffectdoestheconcentrationofH+aqionshaveonthenatureofthesolution?

Solution: ThehighertheconcentrationofH+aqions,themoreisitsacidicnature.AcidswillhavelesserpHvalues.

Q.9. Namethesubstancewhichontreatmentwithchlorineyieldsbleachingpowder.

Solution: BleachingpowderispreparedbytreatingCalciumhydroxideCa(OH)2withchlorine.Thechemicalequationforthereactionis:

Ca(OH)2+Cl2→CaOCl2+H2O

Q.10. MetalcompoundAreactswithdilutehydrochloricacidtoproduceeffervescence.Thegasevolvedextinguishesaburningcandle.Writeabalancedchemicalequationforthereactionifoneofthecompoundsformediscalciumchloride.

Solution: MetalcompoundAisCalciumcarbonate(CaCO3).WhenAreactswithdilutehydrochloricaciditproduceseffervescence.Thechemicalequationisgivenas:CaCO3(s)+2HCl(aq)→CaCl2(aq)+CO2(g)+H2O(l)

ThegasevolvedisCO2thatextinguishesaburningcandle.

Q.11. WhydoesdryHClgasnotchangethecolourofthedrylitmuspaper?

Solution: DryHClgasdoesnotdissociateintoH+ionsandhence,theacidicpropertyofgasisnotimpaired.SodryHClgasdoesnotchangethecolourofthedrylitmuspaper.

Q.12. DobasicsolutionsalsohaveH+aqions?Ifyes,thenwhyarethesebasic?

Solution: Yes,basicsolutionsalsohaveH+ions,buttheyarebasicinnatureduetoagreaternumberofOH-ionsthantheH+ions.

Q.13. Namethesodiumcompoundwhichisusedforsofteninghardwater.

Solution: Sodiumcarbonatedecahydrateiscommonlyusedforsofteninghardwater.Itisalsocalledwashingsoda.Itisalsocalledassodaash.

Q.14. Whiledilutinganacid,whyisitrecommendedthattheacidshouldbeaddedtowaterandnotwatertotheacid?

Solution: Theprocessofaddingwatertoacidishighlyexothermic.Ifnotdonecarefullytheacidmaysplash,causeburnsandeventhebottle/containercanbreakduetothevigorousreaction.Toavoidthisandtoallowtheheatevolvedtobeabsorbedbywaterslowly,theacidisaddedtowaterfordilutingit.Thissignificantlyreducestheriskofsplashing.

Q.15. Underwhatsoilconditiondoyouthinkafarmerwouldtreatthesoilofhisfieldswithquicklime(calciumoxide)orslakedlime(calciumhydroxide)orchalk(calciumcarbonate)?

Solution:Quicklime(calciumoxide),slakedlime(calciumhydroxide)andchalk(calciumcarbonate)areallbasicinnature.Whenthesoilbecomesmoreacidicinnature,thefarmeraddsquicklime(CaO),slakedlimeCa(OH)2orchalkCaCO3tomakeitneutral.Thishelpsthefarmertomakethesoilfittogrowhiscrops.

Q.16. Whatwillhappenifasolutionofsodiumhydrogencarbonateisheated?

Givetheequationofthereactioninvolved.

Solution: WhensodiumhydrogencarbonateNaHCO3isheated,sodiumcarbonate,waterandcarbondioxidegasesareobtained.

Thechemicalequationforthereactionis:2 NaHCO3→HeatNa2CO3+H2O+CO2

NCERTScience Chapter2Acids,basesandsalts

PracticemoreonAcids,basesandsalts Page6 www.embibe.com

Q.17. HowistheconcentrationofhydroniumionsH3O+affectedwhenasolutionofanacidisdiluted?

Solution: WhenthesolutionofacidisdilutedthentheconcentrationofhydroniumionH3O+perunitvolumedecreases.Onaddingwater,lessH+ionsoftheacidareavailabletoformhydroniumions.

Q.18. WriteanequationtoshowthereactionbetweenPlasterofParisandwater

Solution: PlasterofParisisCaSO4.12H2O.ThereactionbetweenthePlasterofParisandwaterisgivenas:CaSO4.12H2OPlaster of Paris(white powder)+32H2O→CaSO4.2H2OGypsum(solid mass)

Q.19. HowistheconcentrationofhydroxideionsOH-affectedwhenexcessbaseisdissolvedinasolutionofsodiumhydroxide?

Solution: Whenabaseismixedwithsodiumhydroxidesolutionthereisanincreaseinthenumberofhydroxideions,whereasthevolumeremainsalmostthesame.ThisleadstoanincreaseintheconcentrationofOH-ionsperunitvolume.

NCERTScience Chapter2Acids,basesandsalts

PracticemoreonAcids,basesandsalts Page7 www.embibe.com

NCERTScience Chapter2Acids,basesandsalts

PracticemoreonAcids,basesandsalts Page8 www.embibe.com