Class 10 Science Chapter 1 – Chemical Reactions and Equations

Chapter 1: Chemical Reactions and Equations

Exercise Questions with Answers

Question 1:

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

Answer:
Magnesium ribbon should be cleaned before burning to remove the layer of magnesium oxide that forms on its surface due to the reaction with oxygen in the air. This layer prevents proper burning of magnesium. Cleaning ensures the magnesium ribbon reacts efficiently with oxygen.

Question 2:

Write the balanced equation for the following chemical reactions:
(a) Hydrogen + Chlorine → Hydrogen chloride
(b) Barium chloride + Aluminium sulphate → Barium sulphate + Aluminium chloride
(c) Sodium + Water → Sodium hydroxide + Hydrogen

Answer:
(a)

$\text{H}_2 + \text{Cl}_2 → 2\text{HCl}$

(b)

$\text{3BaCl}_2 + \text{Al}_2(\text{SO}_4)_3 → 3\text{BaSO}_4 + 2\text{AlCl}_3$

(c)

$2\text{Na} + 2\text{H}_2\text{O} → 2\text{NaOH} + \text{H}_2$

Question 3:

Write a balanced chemical equation with state symbols for the following:
(a) Sodium hydroxide solution reacts with hydrochloric acid to produce sodium chloride and water.
(b) Iron reacts with steam to form iron(III) oxide and hydrogen gas.

Answer:
(a)

$\text{NaOH (aq)} + \text{HCl (aq)} → \text{NaCl (aq)} + \text{H}_2\text{O (l)}$

(b)

$3\text{Fe (s)} + 4\text{H}_2\text{O (g)} → \text{Fe}_3\text{O}_4\text{ (s)} + 4\text{H}_2\text{ (g)}$

Question 4:

A solution of a substance ‘X’ is used for whitewashing.
(a) Name the substance ‘X’ and write its formula.
(b) Write the reaction of the substance ‘X’ with water.

Answer:
(a) The substance ‘X’ is quicklime, and its chemical formula is

$\text{CaO}$

(calcium oxide).

(b) Reaction with water:

$\text{CaO (s)} + \text{H}_2\text{O (l)} → \text{Ca(OH)}_2\text{ (aq)} \, \text{(Slaked lime)}$

Question 5:

Why is the amount of gas collected in one of the test tubes in Activity 1.7 (electrolysis of water) twice the amount collected in the other? Name the gas.

Answer:
The amount of gas collected in one test tube is twice the other because water (

$\text{H}_2\text{O}$

) contains hydrogen and oxygen in a 2:1 ratio. During electrolysis:

At the cathode, hydrogen gas (H₂) is released.
At the anode, oxygen gas (O₂) is released.

Since hydrogen is present in double the quantity in water, its volume is twice that of oxygen.

Question 6:

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

Answer:
When an iron nail is dipped in copper sulphate (

$\text{CuSO}_4$

) solution, a displacement reaction occurs because iron is more reactive than copper. Iron displaces copper from the solution:

$\text{Fe (s)} + \text{CuSO}_4\text{ (aq)} → \text{FeSO}_4\text{ (aq)} + \text{Cu (s)}$

The blue colour of copper sulphate fades, and the solution turns green due to the formation of ferrous sulphate (

$\text{FeSO}_4$

Question 7:

What do you mean by a precipitation reaction? Explain by giving an example.

Answer:
A precipitation reaction is a chemical reaction in which two solutions react to form an insoluble solid (precipitate).

Example:
When barium chloride reacts with sulphuric acid, a white precipitate of barium sulphate is formed:

$\text{BaCl}_2\text{ (aq)} + \text{H}_2\text{SO}_4\text{ (aq)} → \text{BaSO}_4\text{ (s)} + 2\text{HCl (aq)}$

Question 8:

Explain the following terms with an example:
(a) Corrosion
(b) Rancidity

Answer:
(a) Corrosion:
The gradual destruction of metals when they react with moisture, air, or chemicals.
Example: Rusting of iron:

$\text{4Fe} + \text{3O}_2 + 6\text{H}_2\text{O} → 4\text{Fe(OH)}_3$

(b) Rancidity:
The oxidation of oils or fats in food, resulting in a foul smell and taste.
Example: Spoiling of fried foods when exposed to air for a long time.

Additional Important Questions and Answers

Question 1:

What are the characteristics of a chemical reaction?

Answer:
The main characteristics of a chemical reaction are:

Change in State: A chemical reaction may result in a change in the physical state of substances (e.g., solid to liquid).
Example: Formation of water from hydrogen and oxygen.
Change in Colour: A colour change indicates a chemical reaction.
Example: Blue copper sulphate solution turns green when iron reacts with it.
Evolution of Gas: Some reactions release gas as a product.
Example: Reaction of zinc with dilute sulphuric acid produces hydrogen gas.
Change in Temperature: Exothermic or endothermic reactions can cause temperature changes.
Example: Combustion of fuels is an exothermic reaction.
Formation of a Precipitate: An insoluble solid may form during a reaction.
Example: Reaction of barium chloride with sulphuric acid forms barium sulphate precipitate.

Question 2:

What is a combination reaction? Write an example.

Answer:
A combination reaction is a chemical reaction in which two or more substances combine to form a single product.

Example:

$\text{CaO (s)} + \text{H}_2\text{O (l)} → \text{Ca(OH)}_2\text{ (aq)} \, \text{(Slaked lime)}$

$CaO (s) + H_{2} O (l) \to Ca(OH)_{2} (aq) (Slaked lime)$

Here, calcium oxide and water combine to form calcium hydroxide.

Question 3:

What is a redox reaction? Explain with an example.

Answer:
A redox reaction is a reaction in which one substance is oxidized (loses electrons) and another is reduced (gains electrons).

Example:

$\text{Zn (s)} + \text{CuSO}_4\text{ (aq)} → \text{ZnSO}_4\text{ (aq)} + \text{Cu (s)}$

$Zn (s) + CuSO_{4} (aq) \to ZnSO_{4} (aq) + Cu (s)$

Oxidation: Zinc is oxidized to $\text{Zn}^{2+}$

$Zn^{2+}$ .
Reduction: Copper ions ( $\text{Cu}^{2+}$

$Cu^{2+}$ ) are reduced to copper metal ( $\text{Cu}$

$Cu$ ).

Question 4:

What is the difference between exothermic and endothermic reactions? Give examples.

Answer:

Exothermic Reaction	Endothermic Reaction
Heat is released during the reaction.	Heat is absorbed during the reaction.
Causes a rise in temperature.	Causes a fall in temperature.
Example: Combustion of methane:	Example: Photosynthesis:
$\text{CH}_4 + 2\text{O}_2 → \text{CO}_2 + 2\text{H}_2\text{O} + \text{Heat}$ $CH_{4} + 2 O_{2} \to CO_{2} + 2 H_{2} O + Heat$	$\text{6CO}_2 + 6\text{H}_2\text{O} → \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2$ $6CO_{2} + 6 H_{2} O \to C_{6} H_{12} O_{6} + 6 O_{2}$

Question 5:

What are decomposition reactions? Explain with examples.

Answer:
A decomposition reaction is a reaction where a single compound breaks down into two or more simpler substances.

Thermal Decomposition:
Caused by heat.

$\text{CaCO}_3\text{ (s)} → \text{CaO (s)} + \text{CO}_2\text{ (g)}$ $CaCO_{3} (s) \to CaO (s) + CO_{2} (g)$
Electrolytic Decomposition:
Caused by electricity.

$\text{2H}_2\text{O (l)} → 2\text{H}_2\text{ (g)} + \text{O}_2\text{ (g)}$ $2H_{2} O (l) \to 2 H_{2} (g) + O_{2} (g)$
Photolytic Decomposition:
Caused by light.

$2\text{AgCl (s)} → 2\text{Ag (s)} + \text{Cl}_2\text{ (g)}$ $2 AgCl (s) \to 2 Ag (s) + Cl_{2} (g)$

Question 6:

Why do we apply paint on iron articles?

Answer:
Paint prevents iron articles from rusting. The paint acts as a protective barrier and prevents iron from coming into direct contact with moisture and air, which are essential for rusting.

Question 7:

Identify the type of reaction in the following examples:
(a)

$\text{Pb(NO}_3)_2 + 2\text{KI} → \text{PbI}_2 + 2\text{KNO}_3$

$Pb(NO_{3})_{2} + 2 KI \to PbI_{2} + 2 KNO_{3}$
(b)

$\text{CH}_4 + 2\text{O}_2 → \text{CO}_2 + 2\text{H}_2\text{O}$

$CH_{4} + 2 O_{2} \to CO_{2} + 2 H_{2} O$
(c)

$\text{2H}_2\text{O} → 2\text{H}_2 + \text{O}_2$

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

Answer:
(a) Double Displacement Reaction: Precipitation reaction where insoluble lead iodide is formed.

(b) Combustion Reaction: Methane burns in oxygen to produce carbon dioxide and water.

Question 8:

Explain the process of rusting with the chemical equation.

Answer:
Rusting is the corrosion of iron when it reacts with oxygen and moisture.

Chemical equation:

$4\text{Fe} + 3\text{O}_2 + 6\text{H}_2\text{O} → 4\text{Fe(OH)}_3$

$4 Fe + 3 O_{2} + 6 H_{2} O \to 4 Fe(OH)_{3}$

The hydrated ferric oxide

$(\text{Fe(OH)}_3)$

$(Fe(OH)_{3})$ forms a reddish-brown layer called rust.