PAGE NO. 6 – In-Text Questions (Solved)

Q1. Why should a magnesium ribbon be cleaned before burning in air?

Answer:

Magnesium ribbon is cleaned before burning to remove the layer of magnesium oxide that forms on its surface due to reaction with air.
This oxide layer prevents magnesium from burning properly because it blocks direct contact with oxygen in the air.
Cleaning exposes fresh shiny magnesium, which burns readily with a bright flame.

Reaction

$2 M g + O_{2} \to 2 M g O$

Q2. Write the balanced equation for the following chemical reactions:

(i) Hydrogen + Chlorine → Hydrogen chloride

Unbalanced:

$H_{2} + C l_{2} \to H C l$

Balanced:

$H_{2} + C l_{2} \to 2 H C l$

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

Unbalanced:

$B a C l_{2} + A l_{2} (S O_{4})_{3} \to B a S O_{4} + A l C l_{3}$

Balanced:

$3 B a C l_{2} + A l_{2} (S O_{4})_{3} \to 3 B a S O_{4} + 2 A l C l_{3}$

(iii) Sodium + Water → Sodium hydroxide + Hydrogen

Unbalanced:

$N a + H_{2} O \to N a O H + H_{2}$

Balanced:

$2 N a + 2 H_{2} O \to 2 N a O H + H_{2}$

Q3. 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 sodium chloride solution.

$B a C l_{2} (a q) + N a_{2} S O_{4} (a q) \to B a S O_{4} (s) + 2 N a C l (a q)$

BaSO₄ forms as a white precipitate, hence (s)

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

$N a O H (a q) + H C l (a q) \to N a C l (a q) + H_{2} O (l)$

PAGE NO. 10 – In-Text Questions (Solved)

Q 1. A solution of a substance ‘X’ is used for whitewashing.

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

Answer:

The substance ‘X’ is Calcium oxide (also called Quicklime).
Formula: CaO

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

Answer:

When calcium oxide reacts with water, it forms calcium hydroxide (slaked lime), which is used for whitewashing.
The reaction is exothermic (produces heat).

$C a O (s) + H_{2} O (l) \to C a (O H)_{2} (a q) + heat$

Explanation (for exams):

Calcium hydroxide forms a milky suspension called lime water, which hardens on walls by reacting with carbon dioxide in air.

$C a (O H)_{2} + C O_{2} \to C a C O_{3} + H_{2} O$

Calcium carbonate gives the white finish in whitewashing.

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.

Answer :

In Activity 1.7, water is decomposed by passing electric current through it.
Water (H₂O) decomposes into hydrogen and oxygen gases.

According to the chemical equation:

$2 H_{2} O (l) \to 2 H_{2} (g) + O_{2} (g)$

The ratio of hydrogen to oxygen gas formed is 2 : 1.
That means twice the volume of hydrogen gas is produced compared to oxygen.
Therefore, the amount of gas collected in one test tube is double.

Name of the gas collected in larger amount:

Hydrogen gas (H₂)

PAGE NO. 13 – In-Text Questions (Solved)

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

Answer :

The colour of copper sulphate solution changes because iron is more reactive than copper and it displaces copper from the copper sulphate solution.
As a result, copper gets deposited on the iron nail and the blue colour of copper sulphate changes to green due to the formation of iron sulphate.

Chemical Reaction:

$F e (s) + C u S O_{4} (a q) \to F e S O_{4} (a q) + C u (s)$

Blue solution of copper sulphate → fades
Green colour appears because FeSO₄ (Iron sulphate) is formed
Brown deposit of copper forms on iron nail

In one examination sentence:

The blue colour changes because iron displaces copper from copper sulphate and forms green iron sulphate solution.

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

Answer:

An example of a double displacement reaction is:

$A g N O_{3} (a q) + N a C l (a q) \to A g C l (s) + N a N O_{3} (a q)$

Explanation:

In this reaction:

Silver nitrate (AgNO₃) and sodium chloride (NaCl) exchange ions.
Silver chloride (AgCl) forms as a white precipitate.
This is a double displacement reaction because ions are exchanged between two compounds to form new products.

In one exam sentence:

A double displacement reaction occurs when two compounds exchange ions to form a precipitate such as in:

$A g N O_{3} + N a C l \to A g C l + N a N O_{3}$

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

(i) $4 N a (s) + O_{2} (g) \to 2 N a_{2} O (s)$

Answer:

Sodium (Na) is oxidised
Oxygen (O₂) is reduced

Explanation:

Sodium gains oxygen and is therefore oxidised.

$4 N a \overset{O_{2}}{\to} 2 N a_{2} O$

Oxygen accepts electrons and forms oxide, so it is reduced.

(ii) $C u O (s) + H_{2} (g) \to C u (s) + H_{2} O (l)$

Answer:

Hydrogen (H₂) is oxidised
Copper oxide (CuO) is reduced

Explanation:

Hydrogen gains oxygen and forms water, so hydrogen is oxidised.

$H_{2} + O \to H_{2} O$

Copper oxide loses oxygen and forms copper, so CuO is reduced.

$C u O \to C u$

Class 10th Science Chapter 1 – Chemical Reactions and Equations – In-text Questions

PAGE NO. 6 – In-Text Questions (Solved)

Q1. Why should a magnesium ribbon be cleaned before burning in air?

Answer:

Reaction

Q2. 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

Q3. 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 sodium chloride solution.

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

PAGE NO. 10 – In-Text Questions (Solved)

Q 1. A solution of a substance ‘X’ is used for whitewashing.

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

Answer:

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

Answer:

Explanation (for exams):

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.

Answer :

Name of the gas collected in larger amount:

PAGE NO. 13 – In-Text Questions (Solved)

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

Answer :

Chemical Reaction:

In one examination sentence:

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

Answer:

Explanation:

In one exam sentence:

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

(i) $4 N a (s) + O_{2} (g) \to 2 N a_{2} O (s)$

Answer:

Explanation:

(ii) $C u O (s) + H_{2} (g) \to C u (s) + H_{2} O (l)$

Answer:

Explanation:

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Class 10th Science Chapter 1 – Chemical Reactions and Equations – In-text Questions

PAGE NO. 6 – In-Text Questions (Solved)

Q1. Why should a magnesium ribbon be cleaned before burning in air?

Answer:

Reaction

Q2. 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

Q3. 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 sodium chloride solution.

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

PAGE NO. 10 – In-Text Questions (Solved)

Q 1. A solution of a substance ‘X’ is used for whitewashing.

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

Answer:

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

Answer:

Explanation (for exams):

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.

Answer :

Name of the gas collected in larger amount:

PAGE NO. 13 – In-Text Questions (Solved)

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

Answer :

Chemical Reaction:

In one examination sentence:

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

Answer:

Explanation:

In one exam sentence:

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)

Answer:

Explanation:

(ii) CuO(s)+H2(g)→Cu(s)+H2O(l)

Answer:

Explanation:

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(i) $4 N a (s) + O_{2} (g) \to 2 N a_{2} O (s)$

(ii) $C u O (s) + H_{2} (g) \to C u (s) + H_{2} O (l)$

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