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Class 10 Science · Chapter 1

1. Chemical Reactions and Equations

Complete solutions to the intext questions and exercises. CBSE Class 10 Science (NCERT).

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Intext Questions (Page 6)

1Why should a magnesium ribbon be cleaned before burning in air?

Magnesium reacts with oxygen and forms a layer of magnesium oxide on the ribbon. This layer stops the ribbon from burning easily.

So the ribbon is rubbed with sandpaper to remove this layer. The clean ribbon burns easily in air.

2Write the balanced equation for the following chemical reactions.

(i) Hydrogen + Chlorine → Hydrogen chloride

H2(g) + Cl2(g) → 2HCl(g)

(ii) Barium chloride + Aluminium sulphate → Barium sulphate + Aluminium chloride

3BaCl2(aq) + Al2(SO4)3(aq) → 3BaSO4(s) + 2AlCl3(aq)

(iii) Sodium + Water → Sodium hydroxide + Hydrogen

2Na(s) + 2H2O(l) → 2NaOH(aq) + H2(g)
3Write a balanced chemical equation with state symbols for the following reactions.

(i) Solutions of barium chloride and sodium sulphate in water react to give insoluble barium sulphate and the solution of sodium chloride.

BaCl2(aq) + Na2SO4(aq) → BaSO4(s) + 2NaCl(aq)

(ii) Sodium hydroxide solution (in water) reacts with hydrochloric acid solution (in water) to produce sodium chloride solution and water.

NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l)

Intext Questions (Page 10)

1A solution of a substance ‘X’ is used for whitewashing. (i) Name the substance ‘X’ and write its formula. (ii) Write the reaction of the substance ‘X’ named in (i) above with water.

(i) Name and formula of ‘X’

‘X’ is calcium oxide (quick lime). Its formula is CaO.

(ii) Reaction with water

CaO(s) + H2O(l) → Ca(OH)2(aq) + Heat

Calcium hydroxide (slaked lime) is formed. A lot of heat is also given out.

2Why is the amount of gas collected in one of the test tubes in Activity 1.7 double of the amount collected in the other? Name this gas.

In Activity 1.7, water is decomposed by electricity.

2H2O(l) → 2H2(g) + O2(g)

Water contains hydrogen and oxygen in the ratio 2 : 1 by volume. So the volume of hydrogen gas formed is double the volume of oxygen.

Answer: Hydrogen gas

Intext Questions (Page 13)

1Why does the colour of copper sulphate solution change when an iron nail is dipped in it?

Iron is more reactive than copper. So iron displaces copper from copper sulphate solution.

Fe(s) + CuSO4(aq) → FeSO4(aq) + Cu(s)

Iron sulphate is formed. So the blue colour of copper sulphate solution changes to light green.

2Give an example of a double displacement reaction other than the one given in Activity 1.10.

Silver nitrate reacts with sodium chloride. A white precipitate of silver chloride is formed.

AgNO3(aq) + NaCl(aq) → AgCl(s) + NaNO3(aq)
3Identify the substances that are oxidised and the substances that are reduced in the following reactions.

(i) 4Na(s) + O2(g) → 2Na2O(s)

Oxidised: sodium (Na), as it gains oxygen. Reduced: oxygen (O2).

(ii) CuO(s) + H2(g) → Cu(s) + H2O(l)

Oxidised: hydrogen (H2), as it gains oxygen. Reduced: copper(II) oxide (CuO), as it loses oxygen.

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Exercises

1Which of the statements about the reaction below are incorrect? 2PbO(s) + C(s) → 2Pb(s) + CO2(g)  (a) Lead is getting reduced. (b) Carbon dioxide is getting oxidised. (c) Carbon is getting oxidised. (d) Lead oxide is getting reduced.  (i) (a) and (b)  (ii) (a) and (c)  (iii) (a), (b) and (c)  (iv) all

Answer: (i) (a) and (b)

Lead oxide is reduced and carbon is oxidised. So statements (a) and (b) are incorrect.

2Fe2O3 + 2Al → Al2O3 + 2Fe. The above reaction is an example of a (a) combination reaction. (b) double displacement reaction. (c) decomposition reaction. (d) displacement reaction.

Answer: (d) displacement reaction.

Aluminium displaces iron from iron oxide.

3What happens when dilute hydrochloric acid is added to iron filings? Tick the correct answer. (a) Hydrogen gas and iron chloride are produced. (b) Chlorine gas and iron hydroxide are produced. (c) No reaction takes place. (d) Iron salt and water are produced.

Answer: (a) Hydrogen gas and iron chloride are produced.

Fe(s) + 2HCl(aq) → FeCl2(aq) + H2(g)
4What is a balanced chemical equation? Why should chemical equations be balanced?

A chemical equation in which the number of atoms of each element is equal on both sides is called a balanced chemical equation.

Chemical equations should be balanced because of the law of conservation of mass. Mass can neither be created nor destroyed in a chemical reaction. So the total mass of the products must be equal to the total mass of the reactants.

5Translate the following statements into chemical equations and then balance them.

(a) Hydrogen gas combines with nitrogen to form ammonia.

3H2(g) + N2(g) → 2NH3(g)

(b) Hydrogen sulphide gas burns in air to give water and sulphur dioxide.

2H2S(g) + 3O2(g) → 2H2O(l) + 2SO2(g)

(c) Barium chloride reacts with aluminium sulphate to give aluminium chloride and a precipitate of barium sulphate.

3BaCl2(aq) + Al2(SO4)3(aq) → 2AlCl3(aq) + 3BaSO4(s)

(d) Potassium metal reacts with water to give potassium hydroxide and hydrogen gas.

2K(s) + 2H2O(l) → 2KOH(aq) + H2(g)
6Balance the following chemical equations.

(a) HNO3 + Ca(OH)2 → Ca(NO3)2 + H2O

2HNO3 + Ca(OH)2 → Ca(NO3)2 + 2H2O

(b) NaOH + H2SO4 → Na2SO4 + H2O

2NaOH + H2SO4 → Na2SO4 + 2H2O

(c) NaCl + AgNO3 → AgCl + NaNO3

NaCl + AgNO3 → AgCl + NaNO3  (already balanced)

(d) BaCl2 + H2SO4 → BaSO4 + HCl

BaCl2 + H2SO4 → BaSO4 + 2HCl
7Write the balanced chemical equations for the following reactions.

(a) Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water

Ca(OH)2 + CO2 → CaCO3 + H2O

(b) Zinc + Silver nitrate → Zinc nitrate + Silver

Zn + 2AgNO3 → Zn(NO3)2 + 2Ag

(c) Aluminium + Copper chloride → Aluminium chloride + Copper

2Al + 3CuCl2 → 2AlCl3 + 3Cu

(d) Barium chloride + Potassium sulphate → Barium sulphate + Potassium chloride

BaCl2 + K2SO4 → BaSO4 + 2KCl
8Write the balanced chemical equation for the following and identify the type of reaction in each case.

(a) Potassium bromide(aq) + Barium iodide(aq) → Potassium iodide(aq) + Barium bromide(s)

2KBr(aq) + BaI2(aq) → 2KI(aq) + BaBr2(s)

Type: double displacement reaction.

(b) Zinc carbonate(s) → Zinc oxide(s) + Carbon dioxide(g)

ZnCO3(s) → ZnO(s) + CO2(g)

Type: decomposition reaction (thermal decomposition).

(c) Hydrogen(g) + Chlorine(g) → Hydrogen chloride(g)

H2(g) + Cl2(g) → 2HCl(g)

Type: combination reaction.

(d) Magnesium(s) + Hydrochloric acid(aq) → Magnesium chloride(aq) + Hydrogen(g)

Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g)

Type: displacement reaction.

9What does one mean by exothermic and endothermic reactions? Give examples.

Exothermic reactions: Reactions in which heat is released along with the formation of products are called exothermic reactions.

CH4(g) + 2O2(g) → CO2(g) + 2H2O(g) + Energy

Other examples: burning of natural gas, respiration, and the reaction of quick lime with water.

Endothermic reactions: Reactions in which energy is absorbed are called endothermic reactions.

CaCO3(s) + Heat → CaO(s) + CO2(g)

Other examples: decomposition of silver chloride in sunlight, and electrolysis of water.

10Why is respiration considered an exothermic reaction? Explain.

During digestion, food is broken down into glucose. In our cells, glucose combines with oxygen. This reaction gives energy to our body.

C6H12O6(aq) + 6O2(aq) → 6CO2(aq) + 6H2O(l) + energy

Since energy is released, respiration is an exothermic reaction.

11Why are decomposition reactions called the opposite of combination reactions? Write equations for these reactions.

In a combination reaction, two or more substances combine to form a single product.

CaO(s) + H2O(l) → Ca(OH)2(aq)

In a decomposition reaction, a single substance breaks down to give two or more simpler substances.

CaCO3(s) → CaO(s) + CO2(g)

Since one joins substances and the other breaks them, they are opposite to each other.

12Write one equation each for decomposition reactions where energy is supplied in the form of heat, light or electricity.

(i) Heat (thermal decomposition)

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

(ii) Light (photolytic decomposition)

2AgCl(s) → 2Ag(s) + Cl2(g)  (in sunlight)

(iii) Electricity (electrolytic decomposition)

2H2O(l) → 2H2(g) + O2(g)  (electric current)
13What is the difference between displacement and double displacement reactions? Write equations for these reactions.

In a displacement reaction, a more reactive element displaces a less reactive element from its compound.

Fe(s) + CuSO4(aq) → FeSO4(aq) + Cu(s)

In a double displacement reaction, two compounds exchange their ions.

Na2SO4(aq) + BaCl2(aq) → BaSO4(s) + 2NaCl(aq)
14In the refining of silver, the recovery of silver from silver nitrate solution involved displacement by copper metal. Write down the reaction involved.
Cu(s) + 2AgNO3(aq) → Cu(NO3)2(aq) + 2Ag(s)

Copper is more reactive than silver. So it displaces silver from silver nitrate solution.

15What do you mean by a precipitation reaction? Explain by giving examples.

A reaction that produces an insoluble substance is called a precipitation reaction. The insoluble substance is called a precipitate.

Na2SO4(aq) + BaCl2(aq) → BaSO4(s) + 2NaCl(aq)

Here, white precipitate of barium sulphate is formed.

AgNO3(aq) + NaCl(aq) → AgCl(s) + NaNO3(aq)

Here, white precipitate of silver chloride is formed.

16Explain the following in terms of gain or loss of oxygen with two examples each. (a) Oxidation (b) Reduction

(a) Oxidation

Gain of oxygen by a substance is called oxidation.

2Cu + O2 → 2CuO  (copper is oxidised)
C + O2 → CO2  (carbon is oxidised)

(b) Reduction

Loss of oxygen by a substance is called reduction.

CuO + H2 → Cu + H2O  (copper oxide is reduced)
ZnO + C → Zn + CO  (zinc oxide is reduced)
17A shiny brown coloured element ‘X’ on heating in air becomes black in colour. Name the element ‘X’ and the black coloured compound formed.

Element ‘X’ is copper (Cu). The black compound is copper(II) oxide (CuO).

2Cu + O2 → 2CuO
18Why do we apply paint on iron articles?

Iron articles get rusted when they are in contact with moist air. Paint forms a layer on the iron. It does not let air and moisture reach the iron. So paint prevents rusting.

19Oil and fat containing food items are flushed with nitrogen. Why?

When fats and oils are oxidised, they become rancid. Their smell and taste change.

Nitrogen is an unreactive gas. Flushing with nitrogen removes oxygen from the packet. So the food does not get oxidised and stays fresh for a long time.

20Explain the following terms with one example each. (a) Corrosion (b) Rancidity

(a) Corrosion

When a metal is attacked by substances around it such as moisture and acids, it is said to corrode. This process is called corrosion.

Example: Rusting of iron. A brown flaky substance forms on iron in moist air. Black coating on silver and green coating on copper are other examples.

(b) Rancidity

When fats and oils are oxidised, their smell and taste change. This is called rancidity.

Example: Chips or butter kept open for a long time taste and smell bad.

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