In-Text Questions and Solutions

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

Sol: Magnesium ribbon is a reactive metal but its reactivity is partly inhibited because it reacts with oxygen in the air to form a layer of magnesium oxide on the magnesium ribbon. Since magnesium oxide is a thermal (heat) insulator, the layer of magnesium oxide should be cleaned by rubbing with a sand paper so that we can carry out the reaction using pure magnesium.

2) Write the balanced equation for the following reaction:

(i) Hydrogen +Chlorine → Hydrogen chloride

Ans) H₂ + Cl₂ → 2HCL

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

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

(iii) Sodium +Water → Sodium hydroxide + Hydrogen

Ans) 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 the solution of sodium chloride

Ans) BaCl_2(aq) + Na2SO_4(aq) → BaSO_4(s) + 2NaCl_(aq)

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

Ans) NaOH_(aq) + HCl_(aq) → NaCl_(aq) + H₂ O_(l)

4) A solution of a substance ‘X’ is used for white washing.

(i) Name the substance ‘X’ and write its formula.

Ans) The substance “X’ that is used for white washing is calcium oxide. Its chemical formula is CaO.

(ii) Write the reaction of the substance ‘X’ named in (i) above with water

Ans) Calcium oxide (CaO), commonly known as quicklime. Quick lime reacts vigorously with water to produce calcium hydroxide (slaked lime) releasing a large amount of heat. So the reaction is highly exothermic. Calcium oxide and water react to form only a single product, calcium hydroxide. So the reaction is a combination reaction.

5) Why 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.

Ans) Activity 1.7 is an experiment on electrolysis of water. Electrolysis of water is its decomposition to give hydrogen and oxygen gases due to the passage of an electric current. Water (H2O) is formed when hydrogen and oxygen combine in the ratio of 2:1 by volume. Therefore, the gas with double volume is hydrogen as two atoms of hydrogen combine with one atom of oxygen to form water hence, volume of hydrogen is double.

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

Ans) Copper sulphate solution is blue in colour when an iron nail is dipped in it, its blue colour fades and green color appears. This happens because Iron is more reactive than Cu. Iron displaces copper from copper sulphate solution to form a new product namely iron(II) sulphate (green colour) and copper metal.

Fe_(s) + CuSO_4(aq) → FeSO_4(aq) + Cu_(s)

7) Give an example of a double displacement reaction other than the one given in Activity 1.10 (Barium chloride and Sodium sulphate).

Ans) AgNO_3(aq) + NaCl_(aq) → AgCl_(s) + NaNO_3(aq)

Sodium chloride (NaCl) and silver nitrate (AgNO₃) are both solids that are highly soluble in water. They give a double displacement reaction where the ions switch places and give sodium nitrate (NaNO₃) and a white precipitate of silver chloride (AgCl) as the products.

8) Identify the substances that are oxidized and the substances that are reduced in the following reactions.

(i) 4Na(s) + O2(g) → 2Na2O(s)

Ans) Sodium gains oxygen and gets oxidized to Sodium oxide while oxygen loses oxygen and gets reduced to sodium oxide.

(ii) CuO_(s) + H_2(g) → Cu_(s) + H₂O_(l)

Ans) Copper oxide loses oxygen and gets reduced to copper metal while hydrogen gains oxygen and gets oxidized to water.

9) Which of the statement about the reaction below are incorrect?

2PbO_(s) + C_(s) → 2Pb_(s) +CO_2(g)

a) Lead is getting reduced.
b) Carbon dioxide is getting oxidized.
c) Carbon is getting oxidized.
d) Lead oxide is getting reduced.

(1) (a) and (b)
(2) (a) and (c)
(3) (a) (b) and (c)
(4) all

Ans) oxidation is gain of oxygen and reduction reaction is loss of oxygen.

In the above given chemical equation, the incorrect statements are (a) lead is getting reduced and (b) carbon dioxide is getting oxidized. So, (1) is incorrect.

10) 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

Ans) The given chemical equation is an example of a displacement reaction. As Al is more reactive than Fe, it displaces Fe from Fe₂O₃ to form a new product Al₂O₃ (Aluminium oxide).