# Chemical Reactions and Equations : NCERT Intext Questions NCERT Book Page Number - 6

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

Magnesium forms magnesium oxide when reacts with oxygen present in 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, magnesium ribbon is cleaned before burning.

Q.2       Write the balanced equation for the following chemical reactions.
(i) Hydrogen + Chlorine → Hydrogen chloride
(ii) Barium chloride + Aluminium sulphate → Barium sulphate + Aluminium chloride
(iii) Sodium + Water → Sodium hydroxide + Hydrogen
Sol.

(i) ${H_2} + C{L_2} \to 2HCl$
(ii) $3BaC{l_2} + A{l_2}{\left( {S{O_4}} \right)_3} \to 3BaS{O_4} + 2AlC{l_3}$
(iii) $2Na + 2{H_2}O \to 2NaOH + {H_2}$

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 to give insoluble barium sulphate and the solution of sodium chloride.
(ii) Sodium hydroxide solution (in water) reacts with hydrochloric acid solution (in water) to produce sodium chloride solution and water.
Sol.

(i) $BaC{l_2}\left( {aq} \right) + NaS{O_4} \to BaS{O_4}\left( S \right) + 2NaCl\left( {aq} \right)$
(ii) $NaOH\left( {aq} \right) + HCl\left( {aq} \right) \to Nacl\left( {aq} \right) + {H_2}O\left( l \right)$

NCERT Book Page Number - 10

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

(i) The chemical name of substance, which is used for white washing is Calcium oxide. Its common name is lime or quick lime.
The chemical formula of Calcium Oxide is CaO.
(ii) 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.
Sol.

(Hydrolysis of water has been performed in the activity 1.7 in the text book.)
When electric current is passed through water it decomposes into its constituent components, i.e. hydrogen and oxygen gases. During electrolysis of water hydrogen gas collected in one test tube and has double amount than 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.

NCERT Book – Page Number – 13

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

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 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.
Sol.

$Pb{\left( {N{O_3}} \right)_2}\left( {aq} \right) + Kl\left( {aq} \right) \to KN{O_3} + Pb{l_2}$
When lead nitrate reacts with potassium iodide, potassium nitrate and lead iodide is formed. In this reaction the both the reactants exchanges their irons after reaction. This type of reaction is known as double displacement reaction.

Q.3        Identify the substances that are oxidised and the substances that are reduced in the following reactions.
(i) $4Na\left( S \right) + {O_2}\left( g \right) \to 2N{a_2}O\left( S \right)$
(ii) $CuO\left( S \right) + {H_2}\left( g \right) \to Cu\left( S \right) + {H_2}O\left( l \right)$
Sol.

(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, reduced.

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