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Revision Question Bank

Chemical Reactions and Equations

1. Give the reaction taking place during respiration in the body cells.

Sol:

The reaction taking place during respiration is as follows

6 12 6 2Glucose Oxygen

C H O aq 6 O aq

2 2Carbon waterdioxide

6 CO aq 6H O l Energy

2. Name the reaction in which one of the reacting species loses, while the other gains

electrons.

Sol:

Redox reaction is that reaction, in which one of the reacting species loses, while the

other gains electrons.

3. Why is decomposition reaction called opposite of combination reaction? Write

equations for these reactions.

Sol:

In a combination reaction, two or more substances

combine together to form a single new substance whereas, in a decomposition reaction,

a single substance is broken into two or more new substances, e.g.

Combination reaction

Burn2 2 2

Hydrogen Oxygen water

2H g O g 2H O g

Decomposition reaction

Electricity2 2

water Hydrogen Oxygen

2H O l 2H O g

It is clear that decomposition reaction is the reverse of combination reaction.

4. Write balanced chemical equation for each of the following.



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(a) Methane burns in air to give carbon dioxide, water and heat.

(b) Ferric oxide reacts with sulphuric acid to give ferric sulphate and water.

Sol:

(a) 4 2 2 2Oxygen waterMethane Carbon dioxide

CH g 2O g CO g 2H O g Heat

(b) 2 3 2 4 2 4 23Iron(III) Sulphuric waterIron(III)oxide acid sulphate hydrate

Fe O s 3H SO aq Fe SO aq 3H O l

5. All single displacement reactions are irreversible reactions. Explain.

Sol:

Single displacement reactions involve the displacement of a less reactive element by a

more reactive element and the less reactive element precipitates out. But the reverse of

this is not possible because the less reactive element cannot

displace back the more reactive element.

For example,

4 4copperBlueIron Green

Fe s CuSO aq FeSO aq Cu

6. Write equations for each decomposition reactions where energy is supplied in the form

of heat, light or electricity.

Sol:

(i) The decomposition reaction, when energy is supplied in the form of heat,

Heat3 3 2

calciumCalcium carbonoxidecarbonate dioxide

CaCO s CaO CO g

(ii) The decomposition reaction, when energy is supplied in the form of electricity

Electricity2 2 2


2H O l 2H g O g

(iii) The decomposition reaction, when energy is supplied in the form of sun-light.

sunlight2

silver brominesilver bromide

2AgBr s 2Ag Br

7. In what way are chemical equations useful for us?

Sol:



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A chemical equation is useful for us because it gives us the following informations

(i) Names of the reactants, which take part in the chemical reaction,

(ii) Names of the products formed,

(iii) Number of moles of each reactant and product,

(iv) Mass of each substance involved in the reaction.

(v) Physical state of all reactants and products,

(vi) Physical conditions of a reaction.

8. Lime water turns milky when carbon dioxide gas is passed through it and further turns

colourless when excess of carbon dioxide is bubbled through it. Explain.

Sol:

Lime water is Ca (OH)2 solution. When CO2 is passed through lime water, it turns milky

due to the formation of insoluble CaCO3 precipitate. But when excess of CO2 is passed

through the solution, calcium bicarbonate is formed, which is soluble in

water and so the solution again turns colourless.

2 3 22carbon watercalciumCalciumdioxide carbonatehydroxide

Milky suspension

Ca OH CO CaCO H O

3 2 2 3 2water carbonCalcium carbonate Soluble

dioxideExcess

CaCO H O CO Ca HCO aq



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Chemical Reaction {Chapter Test}

M:M: 30 M: Time: 40Mins.

1. Why do iron railings on roadside get rusted if not painted? [1]

Solution: If it is not painted, the iron surface will remain in contact with air and

moisture. So, oxide layer called the rust, will deposit on its surface.

2. When the powder of a common metal is heated in an open China dish, its colour turns

black. However, when hydrogen is passed over the hot black substance so formed, it

regains its original colour.

Based on the given information, answer the following questions

(a) Name the metal initially taken in the powder form.

(b) What type of chemical reaction takes place in each of the two given steps? Write

balanced chemical equations for both reactions. [2]

Solution: (a) Copper metal is taken initially in the powder form.

(b) In step I combination reaction/oxidation reaction takes place.

Heat2 Oxidation

Copper (brown) Oxygen Copper oxide (Black)

2Cu s O g 2CuO s

In step n, reduction reaction takes place.

Reduction2 2

Copper oxide Hydrogen Copper (Brown) Water

CuO s H g Cu s H O g

3. Draw the apparatus for electrolysis of water. Why is the amount of hydrogen produced,

double the amount of oxygen? [3]

Solution: The apparatus for electrolysis of water is shown below



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Number of molecules of hydrogen produced are double the molecules of oxygen

produced, according to the reaction.

Electrolysis2 2 2


2H O l 2H g O g

4. A piece of sodium is dropped in water. What happens in this process? Which gas is

evolved in the process? How is it detected? [4]

Solution: When sodium is dropped in water, it reacts violently with water.

During this exothermic reaction, hydrogen gas is evolved.

To detect this gas, it is passed through soap solution. The bubbles formed rise up and

burn with a popping sound if a burning matchstick is brought near it.

5. Balance the following chemical equations [3]

(a) HNO3 + Ca(OH)2 Ca(NO3)2 + H2O

(b) FeSO4Fe2O3 + SO2 + SO3

(c) C6H12O6 + O2 CO2 + H2O + Heat

Solution: (a) 3 3 22 2WaterNitric acid Calcium hydroxide Calciumnitrate

2HNO Ca OH Ca NO 2H O

(b) 4 2 3 2 3Iron(II)sulphate SulphurIron(III)Oxide Sulphur

dioxide tioxide

2FeSO Fe O SO SO

(c) 6 12 6 2 2 2Oxygen Carbondioxide WaterGlucose

C H O 6O 6CO 6H O Heat

6. A solution of sodium sulphate is added to barium chloride solution. Write a balanced

chemical equation for the reaction and the type of reaction. [2]



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Solution: 2 4 2 4sodium

sodium Barium Barium chloridesulphate chloride suphate

Na SO aq BaCl aq BaSO s 2NaCl

It is a double displacement reaction because in this reaction, two different ions in the

reactant molecules are displaced by each other.

7. Write balanced equations for the following, mentioning the type of reaction involved.

(a) Aluminium + BromineAluminium bromide

(b) Calcium carbonateCalcium oxide +Carbon dioxide

(c) Silver chlorideSilver + Chlorine [3]

Solution: (a) 2Al(s) + 3Br2 (l)2AlBr3 (s)

It is combination reaction because in this reaction, two reactants, Aland Br2 react

together to form a single product (AlBr3)

(b) CaCO3 (s) CaO(S)+CO2(g)

It is thermal decomposition reaction because this reaction involves the use of energy in

the form of heat for decomposition of the reactant (CaCO3).

(c) 2AgCl(s) Sunlight2Ag(S)+Cl2(g)

It is photochemical decomposition or photolysis because this reaction involves the use of

light energy for the purpose of decomposition of the reactant (AgCl).

8. What is rancidity? How is it prevented? Why do we fill chips packets with nitrogen gas?

How does it help? [4]

Solution: Rancidity is the change in smell and taste of oil or fat product due to oxidation.

It is prevented by using antioxidants or cutting the supply of oxygen.

Chips packets are flushed with nitrogen gas to shield the chips from oxygen supply.

Nitrogen does not cause rancidity of food. This helps in preserving the taste and smell of

food. Manufacturers and shopkeepers are helped as shelf life for the chips is prolonged.

9. Define rusting. How does it occur? Write the reaction involved. What are its effects?

Suggest two ways of preventing rusting. [4]

Solution:

Rusting is corrosion of iron due to the action of oxygen and moisture on its surface.



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It occurs in the presence of moist air due to the following reaction

2 2 2 3 2Iron water Oxygen HydrogenIron III oxide

2Fe s H O O g Fe O s H g

Rusting weakens iron and damages structures such as doors, bridges, cars, etc.

Rusting can be prevented by painting the iron surface or galvanising it (with zinc).

9. Why CuSO4 solution can't be stored in iron container? Give chemical equation also. [2]

Solution: CuSO4 solution can't be stored in iron container because, copper is less

reactive than iron. The more reactive metal (iron) displaces the less reactive metal

(copper) from its solution (copper sulphate solution).

4 4Ferrous sulphateIron Copper sulphate Copper Light green colourBlue

Fe s CuSO aq Cu s FeSO

10. Complete the reactions and state their types.

[2]

(a) 2KClO3 (s)

(b)N2(g) + 3H2(g)

How are the two reactions different?

Solution: (a) 3 2Potassium Potassium Oxygenchloride chloride

2KClO s 2KCl s 3O g

(b) 2 2 3N g 3H g 2NH g

2 2calcium water Calciumchloride hydroxide

CaO s H O l Ca OH aq

It is combination reaction.

The decomposition reaction involves breaking down of a compound into two or more

substances. In combination reaction, two or more substances combine with each other

to form one compound.



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Acids, Bases and Salts

1. How is plaster of Paris chemically different from gypsum? How can they be inter

converted? Write one use of plaster of Paris.

Sol:

Plaster of Paris is chemically different from gypsum, in terms of water of crystallisation.

Gypsum has two moles of water per mole of calcium sulphate

(CaSO4.2H2O). Plaster of Paris has half a mole of water per mole of calcium sulphate (or

two moles of calcium sulphate share one water molecule, i.e.,

CaSO4.1

2H2O or 2CaSO4.H2O

Gypsum, on heating at around 120oC, gets converted to Plaster of Paris (POP) and POP,

upon hydration, gets converted to gypsum.

o120 C4 2 4 2 2

GypsumPlaster of paris

1 1CaSO .2H O CaSO . H O 1 H O

2 2

4 2 2 4 2Gypsum

POP

1 1CaSO . H O 1 H O CaSO .2H O

2 2

Use of Plaster of Paris It is used as plaster for supporting fractured bones in the right

position.

2. Given below are the pH values of four different liquids

7.0,14.0,4.0,2.0

Which of these could be that of

(a) lemon juice (b) distilled water

(c) 1 M, sodium hydroxide (NaOH) solution (d) tomato juice

Sol:

(i) The pH value of lemon juice = 2.0

(ii) The pH value of distilled water = 7

(iii) The pH value of 1 M, sodium hydroxide (NaOH) solution =14.0



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(iv) The pH value of tomato juice = 4.0

3. State Arrhenius concept of acids and bases. Select a strong acid and a weak base from

amongst the following substances H2CO3, HNO3, NaOH, NH4OH.

Sol:

According to Arrhenius theory, acids are substances which provide H+ ions when

dissolved in water, whereas bases are substances which when dissolved in water give OH–

ions, (hydroxide ions). Among the given substances, HNO3 is a strong acid and NH4 OH is a

weak base.

i.e., Strong acid = HNO3

Weak base = NH4 OH

4. A solution turns red litmus blue, its pH is likely to be

(a) 1 (b) 4 (c) 5 (d) 10

Sol:

(d) Its pH is likely to be 10 [Bases turn red litmus blue]. pH range of bases is from 8 to 14.

5. What effect does the concentration of H+(aq) have on the acidic nature of the solution?

Sol:

Acidity of a solution increases with increase in the concentration of hydronium ion (H3

O+).

6. Given two test-tubes 'A' and 'B'. Test-tube 'A' contains blue litmus solution and test-tube

'B' contains red litmus solution. A solution with pH 2 is added in both the test-tubes, in

which of the two would you observe a colour change?

Sol:

The colour of solution in test-tube 'A' will change, i.e., blue litmus solution will turn to red.

[The substance with pH 2 is acidic in nature and acids turn blue litmus red]. In test-tube

'B', no colour change is observed.

7. Write the chemical reaction involved when zinc is added to warm dilute sodium



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hydroxide solution. Can all metals react with caustic soda solution?

Sol:

When zinc is added to warm dilute sodium hydroxide solution, the following reaction

takes place

2 2 2Zinc Sodium hydroxide Sodium zincale Hydrogen

2NaOH Zn Na ZnO H

No, all metals do not react with caustic soda.

8. To an aqueous solution of sodium hydroxide, a few drops of phenolphthalein were added.

What do you observe? To this solution small amount of dilute HCl was added. What do you

observe now? Explain your answer.

Sol:

On adding phenolphthalein to sodium hydroxide solution, the solution turns pink but

when acid is added to it, the solution again turns colourless. This is because

phenolphthalein gives pink colour in alkaline medium. When acid is added to the pink

solution, the acid neutralises the base and the solution ultimately turns colourless.



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Acids , Bases and salts {Chapter Test}

M:M: 30 M: Time:

40Mins.

1. What is the chemical name for baking soda? How is it produced? Why is it used in

baking? Write the reactions involved. [4]

Solution:

Baking soda is sodium hydrogen carbonate (NaHCO3).

Production method It is obtained when ammonia is passed over brine solution in an

environment of CO2.

2 2 3 4 3sodium water carbon ammoniumammonia sodiumchloride dixide chloride bicarbonate

NaCl H O CO NH NH Cl NaHCO

On heating, it decomposes to give CO2. It is thus used for baking as it makes the cake

light and spongy.

3 2 3 2 2water carbonsodium sodium

dioxidebicarbonate carbonate

2NaHCO Na CO H O CO

2. (a) Out of HCl and CH3COOH which solution has higher value of pH?

(b) A student tests a sample of drinking water and reports its pH as 6 at room

temperature, which compound do you think may have been added to water? [2]

Solution: (a) CH3COOH is a weak acid. It ionises partially and therefore it has a higher

pH value.

(b) The compound added to water may be bleaching powder. In water bleaching

powder liberates chlorine which makes its pH6 (i.e., the solution becomes

acidic).

3. Give reasons for the following

(a) Iron develops brown layer on its surface in air.

(b) Copper develops green colour appearance in air.

(c) Iron articles are galvanised. [3]

Solution: (a) A layer of oxide (Fe2O3) develops on iron surface due to its reaction with



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oxygen and moisture in air. This is called rusting.

(b) In air, a layer of green coloured basic copper carbonate is formed on the

surface of copper.

(c) To prevent the iron articles from corrosion (rusting), they are galvanised.

4. What is water of crystallisation? Describe an activity to demonstrate that crystals

contain water of crystallisation. [3]

Solution: Fixed number of water molecules present in one formula unit of a salt is

called water of crystallisation. It is responsible for the shape and colour of the crystal.

Aim To show that crystals contain water of crystallisation.

Materials Required Copper sulphate crystals, burner, boiling tube and holder.

Procedure

(i) Take a few crystals of CuSO4 and heat them in a boiling tube.

(ii) The blue colour of crystals vanishes. Small water droplets appear on the walls of

boiling tube on the upper sides.

(iii) Stop heating and add 4-5 drops of water in the test tube.

(iv) Crystals again turn blue.

Thus, hydrated copper sulphate crystals are blue in colour and have formula

CuSO4 .5H2 O



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5. Define chlor-alkali process. What are the uses of its products? [3]

The process of decomposing the aqueous solution of common salt to form NaOH by

passing electricity is called chlor-alkali process.

Electric2 2 2Current

sodium Chlorine Hydrogenhydroxide At anode Atcathode

2NaCl aq 2H O l 2NaOH aq Cl g H g

Chlorine is used for the treatment of water in wells and pools, in making pesticides,

CFCs and PVC manufacture.

Hydrogen is used in fuels, manufacture of ammonia and margarine.

Sodium hydroxide is used for de-greasing metals in paper making, for soap, detergents

and artificial fibres making.

6. Write the chemical equations for the reaction of

(a) potassium

(b) calcium, with air and water.

(c) Give two examples of metals which don't react with air and water. [3]

Solution:

(a) Potassium Reaction with air

4K+O2 2K2O

Reaction with water

2K+2H2O2KOH+H2

(b) Calcium Reaction with air

2Ca + O2 2CaO

Reaction with water

Ca+ 2H2OCa(OH)2 +H2

Silver and gold do not react with water.

7. Distinguish between strong acid and weak acid with examples. [2]



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Solution: Differences between strong acid and weak acid are as follows

S. No. Strong Acid Weak Acid

(i) Ionise almost completely in

water.

Ionise only partially or to lesser extent in

water.

(ii) Give more number H+ ions in

water.

Give less number of H+ ions in water.

(iii) pH value is less pH value is more.

(iv) e.g., cone sulphuric

acid (H2SO4).

e.g., Acetic acid(CH3COOH)

8. What is aqua regia? How is it obtained? State its properties. [2]

Solution: Aqua regia is a reagent, which can dissolve inactive metals like gold,

platinum, etc.

Aqua regia is obtained by adding cone. HCl (3 parts) and cone. HNO3 (1 part).

Properties It has highly corrosive and fuming nature.

9. (a) For making cake, baking powder is taken. If at home your mother uses baking soda

instead of baking powder in cake,

(i) How will it affect the taste of the cake and why?

(ii) How can baking soda be converted into baking powder?

(iii) What is the role of tartaric acid added to baking soda?

(b) Name the acid present in ant sting and give its chemical formula. Also give the

common method to get relief from the discomfort caused by the ant sting. [4]

Solution: (a) (i) Baking powder has tasty tartaric acid which reacts with Na2 CO3

produced during decomposition of NaHCO3 and neutralises it, so if tartaric acid is not

present, the cake will taste bitter due to the presence of sodium carbonate (Na2 CO3).

(ii) By adding tartaric acid to baking soda we can form baking powder.

(iii) Tartaric acid neutralises the sodium carbonate formed during decomposition of

NaHCO3 hence, it makes the cake tasty and not bitter in taste.

(b) The acid is formic acid (HCOOH). If baking soda (a base) is applied on the sting



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(biting area), it gives relief.

10. (a) A metal carbonate X on reacting with an acid gives a gas which when passed

through a solution Y gives the carbonate back. On the other hand, a gas G that is

obtained at anode during electrolysis of brine is passed on dry Y, it gives a compound Z,

used for disinfecting drinking water. Identify XXG and Z.

(b) What happens when nitric acid is added to egg shell? [4]

Solution:

(a) X is calcium carbonate.

CaCO3 + dil. 2HClCaCl2 + H2O + CO2

X

Solution Y is lime water Ca(OH)2.

Ca(OH)2 + CO2 CaCO3 +H2O

Y

During electrolysis of brine, Cl2 gas is released at anode. So, gas G is Cl2 gas (chlorine

gas).

Ca(OH)2 +Cl2 CaOCl2 + H2O

Y G Z

Hence, Z is calcium oxychloride (bleaching powder).

(ii) Egg shells are made up of calcium carbonate, CaCO3.

So, brisk effervescence due to the liberation of CO2 gas is observed. The reaction is

CaCO3 +2HNO3 Ca(NO3)2 + H2O + CO2

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