ncert exemplar solutions science class 10 · 2018-03-22 · ncert exemplar solutions science class...

NCERT Exemplar Solutions Science Class 10th

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Science NCERT Exemplar Solutions

Chapter – 1: Chemical Reactions and Equations

Multiple Choice Questions (MCQs)

Q. 1 Which of the following is not a physical change?

(a) Boiling of water to give water vapour

(b) Melting of ice to give water

(c) Dissolution of salt in water

(d) Combustion of Liquefied Petroleum Gas (LPG)

Ans. (d)

Explanation: A physical change involves only change in physical state whereas a

chemical change results the formation of new substances. Boiling of water, melting of ice

and dissolution of salt are physical changes as no new products are formed.

Q. 2 The following reaction is an example of a

4NH3 (g) + 5O2 (g) 4NO (g) + 6 H2O (g)

1. Displacement reaction

2. Combination reaction

3. Redox reaction

4. Neutralisation reaction

(a) 1 and 4

(b) 2 and 3

(c) 1 and 3

(d) 3 and 4

Ans. (c)

Explanation: A Redox reaction is combination of oxidation (removal of hydrogen) and

reduction (addition of hydrogen). Displacement reaction involves displacement of some

part of a compound with other atom like H atoms of NH3 are displaced by oxygen.

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Q. 3 Which of the following statements about the given reaction are correct?

3Fe(s) + 4H2O(g) Fe3O4 (s) + 4H2 (g)

1. Iron metal is getting oxidized.

2. Water is getting reduced.

3. Water is acting as reducing agent.

4. Water is acting as oxidizing agent.

(a) 1, 2 and 3

(b) 3 and 4

(c) 1, 2 and 4

(d) 2 and 4

Ans. (c)

Explanation: Addition of oxygen to Fe is an oxidation process whereas lose of oxygen

from H2O to H2 is a reduction step. Since water is source of oxygen therefore it acts as

oxidizing agent.

Q. 4 Which of the following are exothermic processes?

1. Reaction of water with quick lime

2. Dilution of an acid

3. Evaporation of water

4. Sublimation of camphor (crystals)

(a) 1 and 2

(b) 2 and 3

(c) 1 and 4

(d) 3 and 4

Ans. (a)

Explanation: Energy is released in exothermic reactions in the form of heat.

Q. 5 Three beakers labelled as A, B and C each containing 25 mL of water were taken. A

small amount of NaOH, anhydrous CuSO4 and NaCl were added to the beakers A, B and C

respectively. It was observed that there was an increase in the temperature of the solutions

contained in beakers A and B, whereas in case of beaker C, the temperature of the solution

falls. Which one of the following statement(s) is (are) correct?

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1. In beakers A and B, exothermic process has occurred.

2. In beakers A and B, endothermic process has occurred.

3. In beaker C, exothermic process has occurred.

4. In beaker C, endothermic process has occurred.

(a) Only 1

(b) Only 2

(c) 1 and 4

(d) 2 and 3

Ans. (c)

Explanation: For an exothermic reaction, heat releases during reaction so temperature

increases whereas endothermic reactions proceed with absorption of heat which decrease

the temperature of reaction.

Q. 6 A dilute ferrous sulphate solution was gradually added to the beaker containing

acidified permanganate solution. The light purple colour of the solution fades and finally

disappears. Which of the following is the correct explanation for the observation?

(a) KMnO4 is an oxidizing agent, it oxidizes FeSO4

(b) FeSO4 acts as an oxidizing agent and oxidizes KMnO4

(c) The colour disappears due to dilution, no reaction is involved

(d) KMnO4 is an unstable compound and decomposes in the presence of FeSO4 to a

colourless compound

Ans. (a)

Explanation: Potassium permanganate (KMnO4) acts as good oxidizing agent in acidic

medium and reduces itself to manganese (II) cations, Mn2+

. It oxidizes FeSO4 to Fe2(SO4)3.

Q. 7 Which among the following is (are) double displacement reaction(s)?

2 2

2 4 2 4

2 2

4 2 2 2

2

0

20 2 0

i Pb CuCl PbCl Cu

ii Na SO BaCl BaSO NaCl

iii C CO

vi CH CO H

(a) (i) and (iv)

(b) Only (ii)

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(c) (i) and (ii)

(d) (iii) and (iv)

Ans. (b)

Explanation: A double displacement reaction involves displacement of two ions of two

different compounds that results the formation of new compounds.

Q. 8 Which among the following statement(s) is /are true? Exposure of silver chloride to

sunlight for a long duration turns grey due to-

(i) The formation of silver by decomposition of silver chloride.

(ii) Sublimation of silver chloride.

(iii) Decomposition of chlorine gas from silver chloride.

(iv) Oxidation of silver chloride.

(a) (i) Only

(b) (i) and (iii)

(c) (ii) and (iii)

(d) Only (iv)

Ans. (a)

Explanation: In the presence of sunlight, AgCl decompose to form grayish white silver

metal and releases chlorine gas.

Q. 9 Solid calcium Oxide reacts vigorously with water to form calcium hydroxide

accompanied by liberation of heat. This process is called slaking of lime. Calcium

hydroxide dissolves in water to form its solution called lime water. Which among the

following is are true about slaking of lime and the solution formed?

(i) It is an endothermic reaction.

(ii) It is exothermic reaction.

(iii) The pH of the resulting solution will be more than seven.

(iv) The pH of the resulting solution will be less than seven.

(a) (i) and (ii)

(b) (ii) and (iii)

(c) (i) and (iv)

(d) (iii) and (iv)

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Ans. (b)

Explanation: The formation of calcium hydroxide from calcium oxide and water with

heat so it is an exothermic reaction. Calcium hydroxide is a basic compound so pH will be

more than 7.

Q. 10 Barium chloride on reacting with ammonium sulphate forms barium sulphate and

ammonium chloride. Which of the following correctly represents the type of the reaction

involved?

(i) Displacement reaction

(ii) Precipitation reaction

(iii) Combination reaction

(iv) Double displacement reaction

(a) Only (i)

(b) Only (ii)

(c) Only (iv)

(d) (ii) and (iv)

Ans. (d)

Explanation: Double displacement reaction involves displacement of cation and anions of

two different compounds. In given reaction, precipitate of barium sulphate is formed.

Q. 11 Electrolysis of water is a decomposition reaction. The mole ratio of hydrogen and

oxygen gases liberated during electrolysis of water is

(a) 1 : 1

(b) 2 : 1

(c) 4 : 1

(d) 1 : 2

Ans. (b)

Explanation: The balance chemical equation for electrolysis of water is;

Electric current

2 H2O(l) 2H2(g) + O2(g)

Q. 12 Which of the following is (are) an endothermic process(es)?

(i) Dilution of sulphuric acid

(ii) Sublimation of dry ice

(iii) Condensation of water vapours

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(iv) Evaporation of water

(a) Both (i) and (iii)

(b) Only (ii)

(c) Only (iii)

(d) Both (ii) and (iv)

Ans. (d)

Explanation: An endothermic process involves absorption of heat such as sublimation and

evaporation needs heat energy.

Q. 13 In the double displacement reaction between aqueous potassium iodide and aqueous

lead nitrate, a yellow precipitate of lead iodide is formed. While performing the activity if

lead nitrate is not available, which of the following can be used in place of lead nitrate?

(a) Lead sulphate (insoluble)

(b) Lead acetate

(c) Ammonium nitrate

(d) Potassium sulphate

Ans. (b)

Explanation: For the formation of lead iodide some soluble salt of lead is required like

lead acetate which will form lead iodide with potassium acetate.

Q. 14 Which of the following gases can be used for storage of fresh sample of an oil for a

long time?

(a) Carbon dioxide or oxygen

(b) Nitrogen or oxygen

(c) Carbon dioxide or helium

(d) Helium or nitrogen

Ans. (d)

Explanation: Inert gases like Helium and Nitrogen can be used for storage of food items

as they prevent the oxidation of food items.

Q. 15 The following reaction is used for the preparation of oxygen gas in the laboratory

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3 22 2 3Heat

CatalystKClO s KCl s O g

Which of the following statement(s) is/are correct about the reaction?

(a) It is a decomposition reaction and endothermic in nature

(b) It is a combination reaction

(c) It is a decomposition reaction and accompanied by release of heat

(c) It is a photochemical decomposition reaction and exothermic in nature

Ans. (a)

Explanation: Decomposition reaction can be defined as the reaction which involves

decomposition of one compound to more than one substance. Endothermic reactions

require heat energy to form products.

Q. 16 Which one of the following processes involve chemical reactions?

(a) Storing of oxygen gas under pressure in a gas cylinder

(b) Liquefaction of air

(c) Keeping petrol in a China dish in the open

(d) Heating copper wire in the presence of air at high temperature

Ans. (d)

Explanation: Chemical changes involve formation of new compounds from one of more

substances. Reaction of copper wire with oxygen forms copper (II) oxide.

Q. 17 In which of the following chemical equations, the abbreviations represent the correct

states of the reactants and products involved at reaction temperature?

2 2 2

2 2 2

2 2 2

2 2 2

2 0 2 0

2 0 2 0

2 0 2 0

( )

( )

( )

2 ) 0(0 2

a H H g

b H g l H l

c H g g H l

d

l

H H g

l

g g

Ans. (c)

Explanation: At standard conditions, hydrogen and oxygen exist in gaseous state whereas

water in liquid state.

Q. 18 Which of the following are combination reactions?

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3 2

2 2

2 2 3

4 4

2 2 30

0

4 30 2 0

Heati KClO KCl

ii Mg H O Mg OH

iii Al Al

iv Zn FeSO ZnSO Fe

(a) Both (i) and (iii)

(b) Both (iii) and (iv)

(c) Both (ii) and (iv)

(d) Both (ii) and (iii)

Ans. (d)

Explanation: Combination reactions form a new product with the combination of more

than one reactant molecules.

Short Answer Type Questions

Q. 19 Write the balanced chemical equations for the following reactions and identify the

type of reaction in each case.

(a) Nitrogen gas is treated with hydrogen gas in the presence of a catalyst at 773 K to form

ammonia gas.

(b) Sodium hydroxide solution is treated with acetic acid to form sodium acetate and water.

(c) Ethanol is warmed with ethanoic acid to form ethyl acetate in the presence of

concentrated H2SO4.

(d) Ethane is burnt in the presence of oxygen to form carbon dioxide, water and releases

heat and light.

Ans.

Catalyst

(a) N2(g) + 3H2(g) 2NH3(g)

773 K

This is an example of combination reaction.

3 3 2b NaOH aq CH COOH l CH COONa aq H O l

This is an example of neutralization reaction as well as double displacement reaction.

Conc. H2SO4

(c) C2H5OH (l) + CH3COOH (l) CH3COOC2H5 (l) + H2O(l)

This is an example of double displacement reaction and also called as esterification.

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(d) 2 C2H6(g) + 7O2(g) 4CO2(g) + 6H2O(g)

This is an example of combustion reaction.

Q. 20 Write the balanced chemical equation for the following reactions and identify the

type of reaction in each case.

(a) In Thermite reaction, iron (III) oxide reacts with aluminium and gives molten iron and

aluminium oxide.

(b) Magnesium ribbon is burnt in an atmosphere of nitrogen gas to form solid magnesium

nitride.

(c) Chlorine gas is passed in an aqueous potassium iodide solution to form potassium

chloride solution and solid iodine.

(d) Ethanol is burnt in air to form carbon dioxide, water and releases heat.

Ans.

(a)

2 3 2 3

2 2Aluminium oxideIron III oxide AIuminium Molten iron

Fe O s Al S Fe l Al O

This is an example of displacement and Redox reaction.

2 3 2

3 ( ) Magnesium nitrideMagnesium Nitrogen

b Mg s N g Mg N s

This is an example of combination reaction.

2 2

2 2Potassium Chlorine Potassium Iodine

iodide chloride

c KI aq Cl g KCl aq I S

This is an example of Redox and displacement reaction.

2 5 2 2 2

3 2 3Ethanol Carbon dioxide

d C H OH l O g CO g H O Heat

This is an example of combustion reaction.

Q. 21 Complete the missing components/variables given as X and Y in the following.

3 2 32

3 3 2

2 4 4 2

3 2

2 2

2

( )

( )

( )

( )X

a Pb NO aq KI aq PbI X KNO Y

b Cu s AgNO aq Cu NO aq X s

c Zn s H SO aq ZnSO X H Y

d CaCO s CaO s CO g

Ans.

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3 2 32 2 2a Pb NO aq KI aq PbI s KNO aq

(b) Cu(s) + 2AgNO3(aq) Cu(NO3)2(aq) + 2Ag (s)

2 4 4 2 + c Zn s H SO aq ZnSO aq H g

3 2 Heatd CaCO s CaO s CO g

Q. 22 Which among the following changes are exothermic or endothermic in nature?

(a) Decomposition of ferrous sulphate

(b) Dilution of sulphuric acid

(c) Dissolution of sodium hydroxide in water

(d) Dissolution of ammonium chloride in water

Ans.

(a) Decomposition of ferrous sulphate absorbs heat during reaction therefore it is an

example of endothermic reaction.

(b) When we dissolve sulphuric acid in water, large amount of heat is liberated so it is an

exothermic reaction.

(c) When we dissolve sodium hydroxide in water, large amount of heat is liberated so it is

an exothermic reaction.’

(d) Mixing of ammonium chloride in water absorbs heat from reaction mixture therefore it

is an example of endothermic reaction.

Q. 23 Identify the reducing agent in the following reactions.

(a) 4NH3 + 5O2 4NO + 6H2O

(b) 2H2O + 2F2 4HF + O2

(c) Fe2O3 + 3CO 2Fe + 3CO2

(d) 2H2 + O2 2H2O

Ans.

(a) NH3 is the reducing agent as it gives hydrogen to O2.

(b) H2O is the reducing agent.

(c) CO is a reducing agent.

(d) H2 is the reducing agent.

Q. 24 Identify the oxidizing agent (oxidant) in the following reactions.

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3 4 2 2 2

2

4 4

2 5

2 3 4 2

2 2

8 3 4

2 2

5 2 5

3 4 4

a Pb O HCl PbCl Cl H O

b Mg O MgO

c CuSO Zn Cu ZnSO

d V O Ca V CaO

e Fe H O Fe O H

f CuO H Cu H O

Ans.

(a) Since Pb3O4 gives oxygen so it is an oxidizing agent (oxidant).

(b) Since O2 provides oxygen so it is oxidant.

(c) CuSO4 is the oxidizing agent because it gets reduces to Cu.

(d) V2O5 has been reduced to V so it will be the oxidant.

(e) H2O is the oxidant as it is reduced to H2.

(f) CuO is the oxidant as it is reduced to Cu.

Q. 25 Write the balanced chemical equations for the following reactions.

(a) Sodium carbonate on reaction with hydrochloric acid in equal molar concentrations

gives sodium chloride and sodium hydrogen carbonate.

(b) Sodium hydrogen carbonate on reaction with hydrochloric acid gives sodium chloride,

water and liberates carbon dioxide.

(c) Copper sulphate on treatment with potassium iodide precipitates cuprous iodide (Cu2I2),

liberates iodine gas and also forms potassium sulphate.

Ans.

(a) Na2CO3(s) +HCl(aq) NaCl(aq) + NaHCO3(aq)

(b) NaHCO3(s) + HCl(aq) NaCl(aq) + H2O(l) + CO2(g)

(c) 2CuSO4(s) + 4KI (aq) Cu2I2 (s) + I2(g) + 2K2SO4(aq)

Q. 26 A solution of potassium chloride when mixed with silver nitrate solution, an

insoluble white substance is formed. Write the chemical reaction involved and also

mention the type of the chemical reaction.

Ans.

The solution of KCl with AgNO3 form white precipitate of AgCl. This is a double

displacement reaction.

KCl(aq) + AgNO3(aq) AgCl(s) + KNO3(aq)

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Q. 27 Ferrous sulphate decomposes with the evolution of a gas having a characteristic

odour of burning sulphur. Write the chemical reaction involved and identify the type of

reaction.

Ans:

2FeSO4(s) Fe2O3(s) + SO2(g) + SO3(g)

The heating of ferrous sulphate gives ferric oxide and mixture of sulphur dioxide and

sulphur trioxide gas. This is an example of thermal decomposition reaction and the odour

of burning sulphur is due to SO2 gas.

Q. 28 Why do fire flies glow at night?

Ans.

Fire flies have a luminous bag which contains a protein; luciferin. The oxidation of

luciferin occurs in the presence of luciferare enzyme with emission of light which occurs

due to oxidation of magnesium. Hence they glow at night.

Q. 29 Grapes hanging on the plant do not ferment but after being plucked from the plant

can be fermented. Under what conditions do these grapes ferment? Is it a chemical or a

physical change?

Ans.

After plucking grapes from plants, fermentation of sugar is carried out in the presence of

yeast which changes sugar to ethanol and carbon dioxide. This process occurs in the

absence of oxygen means in anaerobic conditions.

Whereas grapes attach to plant involve in aerobic respiration and no fermentation can be

possible under aerobic conditions.

Here fermentation is a chemical change as it results the formation of new substances;

alcohol and carbon dioxide.

Q. 30 Which among the following are physical or chemical changes?

(a) Evaporation of petrol

(b) Burning of Liquefied Petroleum Gas (LPG)

(c) Heating of an iron rod to red hot

(d) Curdling of milk

(e) Sublimation of solid ammonium chloride

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Ans.

(a) Physical change

(b) Chemical change

(c) Physical change

(d) Chemical change

(e) Physical change

Q. 31 During the reaction of some metals with dilute hydrochloric acid, following

observations were made.

(a) Silver metal does not show any change.

(b) The temperature of the reaction mixture rises when aluminium (Al) is added.

(c) The reaction of sodium metal is found to be highly explosive.

(d) Some bubbles of a gas are seen when lead (Pb) is reacted with the acid Explain these

observations giving suitable reasons.

Ans.

(a) The reactivity of silver is very low as it is placed below hydrogen in reactivity series so

it does not react with dilute hydrochloric acid.

(b) Aluminium reacts with dilute HCl to form Aluminium chloride with hydrogen gas. It is

an exothermic reaction so temperature of reaction mixture rises.

(c) Sodium is an alkali metal which is one of the most reactive metals and readily reacts

with dilute HCl to form NaCl and hydrogen gas. The evolution of hydrogen gas cause

explosion.

(d) Reaction of lead metal with dilute HCl forms lead (II) chloride and releases hydrogen

gas in the form of bubbles. Since reaction is quite slow due to less reactivity of lead, only

bubbles of H2 are seen to evolve.

Q. 32 A substance X, which is an oxide of a group 2 element, is used intensively in the

cement industry. This element is present in bones also. On treatment with water it forms a

solution which turns red litmus blue. Identify X and also write the chemical reactions

involved.

Ans.

The substance X is calcium oxide (CaO) which is also known as quick lime. Reaction of

quick lime with water forms calcium hydroxide. It is an alkaline solution and easily turns

red litmus to blue.

Calcium hy

2 2Calci droxide

(slaked lime)redlitmu

um ox

Blue

ide

Wate

s

r

CaO s H O l Ca OH

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Q. 33 Write a balanced chemical equation for each of the following reactions and also

classify them.

(a) Lead acetate solution is treated with dilute hydrochloric acid to form lead chloride and

acetic acid solution.

(b) A piece of sodium metal is added to absolute ethanol to form sodium ethoxide and

hydrogen gas.

(c) Iron (III) oxide on heating with carbon monoxide gas reacts to form solid iron and

liberates carbon dioxide gas.

(d) Hydrogen sulphide gas reacts with oxygen gas to form solid sulphur and liquid water.

Ans:

3 2 32a 2 ( )2Pb CH COO aq HCl dil PbCl s CH COOH aq

This is a double displacement and a precipitation reaction.

2 5 2 5 2b 2 2 2 C H OH l Na s C H O Na H

This is a displacement reaction.

2 3 2c 3 2 3Fe O s CO g Fe s CO g

This is an example of Redox reaction.

2 2 2d 2 2 2 0H S g O g S s H l

This is an example of Redox reaction.

Q. 34 Why do we store silver chloride in dark coloured bottles?

Ans.

Silver chloride can decompose in the presence of light to form silver metal therefore it

must store in dark coloured bottles which can interrupt the path of light and prevent the

decomposition of silver chloride.

The photochemical decomposition of silver chloride is shown below;

(gre

2

(white)y)

2 2 ( )Sunlight

ChlorineSilverSilverChloride

AgCl s Ag s Cl g

Q. 35 Balance the following chemical equation and identify chemical reaction the type of

chemical reaction.

(a) Mg(s) + Cl2(g) MgCl2(s)

Δ

(b) HgO(s) Hg + O2(g)

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NCERT Exemplar Problems AndSolutions : Science Class 10

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