NCERT Class 10 Science Chapter 1 – Chemical Reactions and Equations

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

Solution:

Magnesium ribbon should be cleaned before burning in air because Magnesium metal reacts with the atmospheric oxygen and forms Magnesium Oxide (MgO) layer which is a very stable compound. In order to prevent further reactions with Oxygen, it is therefore necessary to clean the ribbon by to remove the layer of MgO.

2. Write a balanced equation for the following chemical reactions.

i) Hydrogen + Chloride —-> Hydrogen chloride

ii) Barium chloride + Aluminium sulphate —-> Barium sulphate + Aluminium chloride

iii) Sodium + Water —-> Sodium hydroxide + Hydrogen

Solution:

i) H₂ + Cl₂ → 2HCl

ii) 3BaCl₂ + Al₂(SO₄)₃ →3BaSO₄ + 2AlCl₃ 

iii) 2Na + 2H₂O → 2NaOH + H₂

3. Write 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 solution of Sodium chloride.

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

Solution:

i) BaCl₂ + Na₂SO₄ → BaSO₄ + 2NaCl

ii) NaOH + HCl → NaCl + H₂O

In-text questions set 2 Page number – 10

1. A 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.

Solution:

i) The substance ‘X’ which is used in whitewashing is quick lime or Calcium Oxide and its formula is CaO.

ii) CaO + H₂O → Ca(OH)₂

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

Solution:

In activity 1.7, gas collected in one of the test tubes is double of the amount collected in the other because water gets hydrolysed to release H₂ and O₂ gas. Here, after electrolysis two molecules of Hydrogen and one molecule of oxygen gas are released; hence, the amount of Hydrogen collected would be double than that of oxygen.

In-text questions set 3 Page number – 13

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

Solution:

When an iron nail is dipped in the copper sulphate solution, iron displaces copper from the copper sulphate because iron is more reactive than copper. Therefore, the colour of the copper sulphate solution changes. The reaction is

Fe + CuSO₄ → FeSO₄ + Cu

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

 Solution:

Reaction Between silver nitrate (AgNO₃) and Sodium chloride (NaCl) is an example of a double displacement reaction. During the reaction, negative and positive ions trade positions as a result of the formation of white silver chloride precipitate. The chemical reaction is given below.

Ag⁺ + NO₃^– + Na⁺ + Cl^– → AgCl + Na⁺ + NO₃^–

3. Identify the substances that are oxidised and that are reduced in the following equation.

i) 4Na(s) + O₂(g) → 2Na₂O(s)

ii) CuO(s) + H₂(g) → Cu(s) + H₂O(l)

Solution:

The Sodium (Na) in the first equation is getting oxidized with the addition of Oxygen (O₂), and the Copper (Cu) in the second equation is reduced due to the addition of Hydrogen (H₂).

Exercise Questions Page number – 14-16

1. Which 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 the above

Solution:

(i) (a) and (b)

Explanation: (a) Because Oxygen is being removed and (b) Because the removed oxygen from Lead is added to the elemental Carbon.

2.  Fe₂O₃ + 2Al → Al₂O₃ + 2Fe

The above reaction is an example of a

1. Combination reaction
2. Double displacement reaction
3. Decomposition reaction
4. Displacement reaction

Solution:

The answer is 4. Displacement reaction.

Explanation: The Oxygen from the Ferrous oxide is getting displaced to the Aluminium metal to form Aluminium Oxide. In this reaction, Aluminum is a more reactive metal than Fe. Therefore, Al will displace Fe from its oxide. This type of chemical reaction, called displacement reaction, is in which one of the elements displaces another. Here, less reactive metal is displaced by more reactive metal. Since one-time displacement is occurring, it is called a single displacement reaction.

3. What happens when dilute hydrochloric acid is added to iron fillings? Tick the correct answer.

1. Hydrogen gas and Iron chloride are produced.
2. Chlorine gas and Iron hydroxide are produced.
3. No reaction takes place.
4. Iron salt and water are produced.

Solution:

1. Hydrogen gas and Iron chloride are produced.

Explanation: The Chlorine from Hydrogen chloride is displaced by the Iron fillings to undergo the following reaction.

2HCl + Fe → FeCl₂ + H₂

4. What is a balanced chemical equation? Why should a chemical equation be balanced?

Solution:

A balanced equation is the one in which number of different atoms on both the reactant and product sides are equal. Balancing chemical equations is necessary for the reaction should obey The Law of Conservation of mass. Balancing the chemical equation has no defined method and is purely a trial-and-error attempt.

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

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

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

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

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

Solution:

(a) Unbalanced: H₂ + N₂ → NH₃

Balanced: 3H₂ + N₂ → 2NH₃

(b) Unbalanced: H₂S + O₂ → H₂O + SO₂

Balanced: 2H₂S + 3O₂ → 2H₂O + 2SO₂

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(c)  Unbalanced:

BaCl₂ + Al₂(SO₄)₃ → AlCl₃ + BaSO₄

Balanced: 3BaCl₂ + Al₂(SO₄)₃ → 2AlCl₃ + 3BaSO₄

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(d) Unbalanced: K + H₂O → KOH + H₂

Balanced:  2K + 2H₂O → 2KOH + H₂

6. Balance the following chemical equations.

(a) HNO₃ + Ca(OH)₂ → Ca(NO₃)₂ + H₂O

(b) NaOH + H₂SO₄ → Na₂SO₄ + H₂O

(c) NaCl + AgNO₃ → AgCl + NaNO₃

(d) BaCl₂ + H₂SO₄ → BaSO₄ + HCl

Solution:

(a) 2HNO₃ + Ca(OH)₂ → Ca(NO₃)₂ + 2H₂O

(b) 2NaOH + H₂SO₄ → Na₂SO₄ + 2H₂O

(c) NaCl + AgNO₃ → AgCl + NaNO₃

(d) BaCl₂ + H₂SO₄ → BaSO₄ + 2HCl

7. Write the balanced chemical equation for the following reactions.

Calcium hydroxide + Carbon dioxide —-> Calcium carbonate + Water

Zinc + Silver nitrate —-> Zinc nitrate + Silver

Aluminium + Copper chloride —-> Aluminium chloride + Copper

Barium chloride + Potassium sulphate —-> Barium sulphate + Potassium chloride

Solution:

2Ca(OH)₂ + 2CO₂ → 2CaCO₃ + 2H₂O

Zn + 2AgNO₃ → Zn(NO₃)₂ + 2Ag

2Al + 3CuCl₂ → 2AlCl₃ + 3Cu

BaCl₂ + K₂SO₄ → BaSO₄ + 2KCl

8. Write a balanced chemical equation for the following and identify the type of reaction of each case.

KBr + BaI₂ → KI + BaBr₂

ZnCO₃ → ZnO + CO₂

H₂ + Cl → HCl

Mg + HCl → MgCl₂ + H₂

Solution:

2KBr + BaI₂ → 2KI + BaBr₂ (Double Displacement Reaction)

ZnCO₃ → ZnO + CO₂ (Decomposition Reaction)

H₂ + Cl → 2HCl (Combination Reaction)

Mg + 2HCl → MgCl₂ + H₂ (Displacement Reaction)

9. What is meant by exothermic and endothermic reactions? Give examples.

Solution:

An endothermic reaction occurs when energy is absorbed from the surroundings in the form of heat (For example, Photosynthesis, melting of ice, evaporation). Conversely, an exothermic reaction is one in which energy is released from the system into the surroundings (Examples: Explosions, concrete setting, nuclear fission and fusion).

10. Why is respiration considered to be an exothermic reaction?

Solution:

For the survival of life, we require energy. We obtain this energy from the food we eat. The food molecules, through the process of digestion, is broken down into simpler molecule like glucose. These substances come in contact with the Oxygen present in our body cells to form Carbon dioxide and water along with a certain amount of energy (Respiration process). Since the energy is in the form of heat (that maintains our body temperature), respiration is considered to be an exothermic reaction. The reaction taking place is:

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy