CBSE NCERT Solutions for Class 10 science Chapter 2
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CBSE NCERT Solutions for Class 10 science Chapter 2 Exercises Q.1. A solution turns red litmus blue, its pH is likely to be: 14510 Solution: A solution that turns the colour of red litmus to blue must be basic in nature. A basic solution has a pH value of greater than 7. The only option with pH greater than 7 is 10 which will be the correct answer. Q.2. Equal lengths of magnesium ribbons are taken in test tubes A and B. Hydrochloric acid (HCl) is added to test tube A, while acetic acid (CH3COOH) is added to test tube B. Amount and concentration taken for both the acids are same. In which test tube will the fizzing occur more vigorously and why? Solution: The fizzing will occur strongly in test tube A, in which hydrochloric acid (HCl) is added. This is because HCl is a stronger acid than CH3COOH and therefore produces hydrogen gas at a quicker rate due to which fizzing occurs more vigorously in test tube A. Mg+2HCl→MgCl2+H2↑ (faster) Mg+2CH3COOH→Mg(CH3COO)2+H2 (slower) Q.3. Fresh milk has a pH of 6. How do you think the pH will change as it turns into curd? Explain your answer. Solution: The pH of milk is 6. As it gets converted to curd (pH between 4.5 to 5.5) the pH will reduce because curd has lactic acid which is more acidic in nature. Thus, the pH drops below 6 then milk gets converted to curd. Q.4. A milkman adds a very small amount of baking soda to fresh milk. Why does he shift the pH of the fresh milk from 6 to slightly alkaline? Solution: The milkman adds a small amount of baking soda to shift the pH of the fresh milk from 6 to slightly alkaline. In alkaline condition, milk does not set as curd easily and this helps in preventing the milk from getting converted to curd. Q.5. A milkman adds a very small amount of baking soda to fresh milk. Why does this milk take a long time to set as curd? Solution: Milkman adds a small amount of baking soda to fresh milk to make the milk slightly alkaline. In this alkaline condition, the acids produced to set the curd are neutralized by the base. Therefore, it takes longer time for the curd to set. Q.6. Plaster of Paris should be stored in a moisture-proof container. Explain why? Solution: Plaster of Paris (POP) should be stored in a moisture-proof container because plaster of Paris, a powdery mass, absorbs water (moisture) to form a hard solid known as gypsum. CaSO4.12H2OPlaster of Paris+112H2OWater→CaSO4.2H2OGypsum [Hard solid] Q.7. What is a neutralization reaction? Give two examples. Solution: A reaction in which an acid and a base react with each other to give salt and water is termed as a neutralization reaction. In this reaction, energy is evolved in the form of heat and thus, such reactions are usually exothermic in nature. For example, sodium hydroxide reacts with hydrochloric acid to give sodium chloride salt and water: NaOH(Base)+HCl(Acid)→NaCl(Salt)+H2O(Water) During indigestion (caused due to the production of excess of hydrochloric acid in the stomach), we administer an antacid (generally milk of magnesia, Mg(OH)2which is basic in nature). It neutralizes the excess acid and thus gives relief from indigestion. Mg(OH)2+2HCl→MgCl2+2H2O NCERT Science Chapter 2 Acids, bases and salts Practice more on Acids, bases and salts Page 1 www.embibe.com
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Transcript of CBSE NCERT Solutions for Class 10 science Chapter 2
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
