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Class ten Science Chemical Reactions NCERT Solutions

Chapter 1: Chemical Reactions and Equations

Intext Questions Page No. 16

Q.1. Why should a magnesium ribbon be cleaned before burning in the air? Ans. Magnesium forms magnesium oxide when it reacts with the oxygen present in the air. On the magnesium ribbon, a stable layer of magnesium oxide is formed when it is stored, which prevents the further reaction of magnesium ribbon. Hence, to clean up the layer of magnesium oxide, the magnesium ribbon is cleaned before burning

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

(1) Hydrogen + Chlorine → Hydrogen chloride

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

(iii) Sodium + Water → Sodium hydroxide + Hydrogen

 Ans. (i) H2 +Cl2 → 2HCI 

(ii)3BaCl2 + Al2(SO4)3 ——-> 3BaSO4+2AlCI3

(iii)2Na + 2H20 → 2NaOH + H2

Q.3. Write a balanced chemical equation with state symbols for the following reactions. 

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

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

Ans. (i) BaCl2 (aq) + Na2SO4 → BaSO4 (s) + 2NaCl(aq) 

(ii)NaOH(aq) + HCl(aq) → NaCl(aq) + H20(l)


Q.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. Ans. (i) The chemical name of the substance, ‘X’ which is used for whitewashing is Calcium oxide. Its common name is lime or quick lime. 

The chemical formula of calcium oxide is CaO.

(ii) CaO + H2O ——–> Ca(OH)2

When calcium oxide reacts with water it forms calcium hydroxide.

Calcium hydroxide is also known as slaked lime.

Q. 2. 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. (Hydrolysis of water has been performed in the activity 1.7 in the textbook.) 

When an electric current passes through water, it decomposes into its constituent components, i.e. hydrogen and oxygen gases. During electrolysis of water, hydrogen gas is collected in one test tube and has double the amount of oxygen collected in another test tube. This is because a water molecule is formed by the combination of hydrogen and oxygen in the ratio of 2:1. 

Intext Questions Page No. 13

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

Ans. The colour of copper sulphate solution is blue. When an iron nail is dipped in the solution of copper sulphate, it forms ferrous sulphate solution and copper metal by the displacement of copper from the solution. The colour of the ferrous sulphate is green. That’s why the solution becomes green.

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

Ans. Pb (NO3), (aq) + Kl (aq) —> KNO3 + PbI, 

When lead nitrate reacts with potassium iodide, potassium nitrate and lead iodide are formed. In this reaction both the reactants exchange their ions after the reaction. This type of reaction is known as a double displacement reaction.

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

 (i) 4Na(s) + O2(g) —> 2Na2O(s)

(ii) CuO(s)+H2 (g) —> Cu(s) +H2O(l)

Ans. (i) In this reaction, sodium is combined with oxygen, i.e. gains oxygen and hence oxidized. Consequently, oxygen is reduced. (ii) In this reaction hydrogen gains oxygen and forms water, hence it is oxidised. On the other hand, copper loses oxygen and hence, is reduced.