Chemical Reactions and Equations

Q 1. Which of the statements about the reaction below are incorrect?

$2 P b O (s) + C (s) \to 2 P b (s) + C O_{2} (g)$

Statements:
(a) Lead is getting reduced.
(b) Carbon dioxide is getting oxidised.
(c) Carbon is getting oxidised.
(d) Lead oxide is getting reduced.

Answer: (i) (a) and (b)

Explanation:

Carbon is oxidised because it gains oxygen to form CO₂.

$C \to C O_{2}$

Lead oxide (PbO) is reduced to Lead because it loses oxygen.

$P b O \to P b$ So,
✔ (c) is correct
✔ (d) is correct
❌ (a) is incorrect — Lead is produced by reduction of lead oxide, not reduced directly
❌ (b) is incorrect — CO₂ is not oxidised; it is a product formed

Q 2. Equation

$F e_{2} O_{3} + 2 A l \to A l_{2} O_{3} + 2 F e$

The above reaction is an example of a:

(a) Combination reaction
(b) Double displacement reaction
(c) Decomposition reaction
(d) Displacement reaction

Answer: (d) Displacement reaction

Explanation:

Aluminium is more reactive than iron, so it displaces iron from its oxide.
This is also called a thermite reaction used for joining railway tracks.

Q 3. What happens when dilute hydrochloric acid is added to iron filings? Tick the correct answer.

(a) Hydrogen gas and iron chloride are produced.
(b) Chlorine gas and iron hydroxide are produced.
(c) No reaction takes place.
(d) Iron salt and water are produced.

Answer: (a) Hydrogen gas and iron chloride are produced.

Explanation:

Iron reacts with dilute HCl to produce hydrogen gas and ferrous chloride.

$F e (s) + 2 H C l (a q) \to F e C l_{2} (a q) + H_{2} (g)$

Bubbles of hydrogen are seen.

Q 4. What is a balanced chemical equation? Why should chemical equations be balanced?

Answer:

A balanced chemical equation is a chemical equation in which the number of atoms of each element is equal on both the reactant and product sides of the equation.
It shows that no atoms are lost or gained during a chemical reaction.

Example:

Unbalanced:

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

Balanced:

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

Why should chemical equations be balanced?

Chemical equations must be balanced to satisfy the Law of Conservation of Mass, which states that:
👉 Mass can neither be created nor destroyed in a chemical reaction.

Therefore, the total mass of reactants must be equal to the total mass of products, meaning the number of atoms of each element must remain the same on both sides of the equation.

In one exam sentence:

A chemical equation should be balanced to obey the Law of Conservation of Mass, ensuring equal number of atoms of each element on both sides.

Q 5. Translate the following statements into chemical equations and balance them.

(a) Hydrogen gas combines with nitrogen to form ammonia.

Word Equation:

Hydrogen + Nitrogen → Ammonia

Chemical Equation (unbalanced):

$H_{2} + N_{2} \to N H_{3}$

Balanced Chemical Equation:

$3 H_{2} + N_{2} \to 2 N H_{3}$

(b) Hydrogen sulphide gas burns in air to give water and sulphur dioxide.

Word Equation:

Hydrogen sulphide + Oxygen → Sulphur dioxide + Water

Chemical Equation (unbalanced):

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

Balanced Chemical Equation:

$2 H_{2} S + 3 O_{2} \to 2 S O_{2} + 2 H_{2} O$

(c) Barium chloride reacts with aluminium sulphate to give aluminium chloride and a precipitate of barium sulphate.

Word Equation:

Barium chloride + Aluminium sulphate → Aluminium chloride + Barium sulphate

Chemical Equation (unbalanced):

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

Balanced Chemical Equation:

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

(d) Potassium metal reacts with water to give potassium hydroxide and hydrogen gas.

Word Equation:

Potassium + Water → Potassium hydroxide + Hydrogen

Chemical Equation (unbalanced):

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

Balanced Chemical Equation:

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

Q 6. Balance the following chemical equations:

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

Unbalanced:

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

Balanced:

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

(b) $N a O H + H_{2} S O_{4} \to N a_{2} S O_{4} + H_{2} O$

Unbalanced:

$N a O H + H_{2} S O_{4} \to N a_{2} S O_{4} + H_{2} O$

Balanced:

$2 N a O H + H_{2} S O_{4} \to N a_{2} S O_{4} + 2 H_{2} O$

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

Balanced:

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

(Already balanced)

(d) $B a C l_{2} + H_{2} S O_{4} \to B a S O_{4} + H C l$

Unbalanced:

$B a C l_{2} + H_{2} S O_{4} \to B a S O_{4} + H C l$

Balanced:

$B a C l_{2} + H_{2} S O_{4} \to B a S O_{4} + 2 H C l$

Q 7. Write the balanced chemical equations for the following reactions.

(a) Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water

Word Equation:
Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water

Balanced Chemical Equation:

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

(b) Zinc + Silver nitrate → Zinc nitrate + Silver

Word Equation:
Zinc + Silver nitrate → Zinc nitrate + Silver

Balanced Chemical Equation:

$Z n + 2 A g N O_{3} \to Z n (N O_{3})_{2} + 2 A g$

(c) Aluminium + Copper chloride → Aluminium chloride + Copper

Word Equation:
Aluminium + Copper chloride → Aluminium chloride + Copper

Balanced Chemical Equation:

$2 A l + 3 C u C l_{2} \to 2 A l C l_{3} + 3 C u$

(d) Barium chloride + Potassium sulphate → Barium sulphate + Potassium chloride

Word Equation:
Barium chloride + Potassium sulphate → Barium sulphate + Potassium chloride

Balanced Chemical Equation:

$B a C l_{2} + K_{2} S O_{4} \to B a S O_{4} + 2 K C l$

Q 8. Write the balanced chemical equation for the following and identify the type of reaction in each case.

(a) Potassium bromide(aq) + Barium iodide(aq) → Potassium iodide(aq) + Barium bromide(s)

Balanced Equation:

$2 K B r (a q) + B a I_{2} (a q) \to 2 K I (a q) + B a B r_{2} (s)$

Type of Reaction:

Double displacement reaction (precipitation reaction)

(b) Zinc carbonate(s) → Zinc oxide(s) + Carbon dioxide(g)

Balanced Equation:

$Z n C O_{3} (s) \to Z n O (s) + C O_{2} (g)$

Type of Reaction:

Decomposition reaction
(thermal decomposition when heated)

(c) Hydrogen(g) + Chlorine(g) → Hydrogen chloride(g)

Balanced Equation:

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

Type of Reaction:

Combination reaction
(two substances combine to form one product)

(d) Magnesium(s) + Hydrochloric acid(aq) → Magnesium chloride(aq) + Hydrogen(g)

Balanced Equation:

$M g (s) + 2 H C l (a q) \to M g C l_{2} (a q) + H_{2} (g)$

Type of Reaction:

Displacement reaction
(more reactive metal Mg displaces hydrogen from acid)

Q 9. What does one mean by exothermic and endothermic reactions? Give examples.

Answer

A reaction in which heat energy is released is called an exothermic reaction.
Such reactions make the surroundings warm.

Example:

$C (s) + O_{2} (g) \to C O_{2} (g) + Heat$

Another common example:

$C H_{4} + 2 O_{2} \to C O_{2} + 2 H_{2} O + Energy$

A reaction in which heat energy is absorbed from the surroundings is called an endothermic reaction.
Such reactions make the surroundings cooler.

Example:

$C a C O_{3} (s) \overset{Δ}{\to} C a O (s) + C O_{2} (g)$

(or)

$N_{2} (g) + O_{2} (g) + Heat \to 2 N O (g)$

Q 10. Why is respiration considered an exothermic reaction? Explain.

Answer

Respiration is considered an exothermic reaction because food (glucose) is broken down in cells with the help of oxygen to release carbon dioxide, water and a large amount of energy.

Chemical Equation:

$C_{6} H_{12} O_{6} + 6 O_{2} \to 6 C O_{2} + 6 H_{2} O + Energy$

This released energy helps in:

maintaining body temperature,
performing physical activities,
internal body functions.

So, respiration releases energy → exothermic reaction.

Q 11. Why are decomposition reactions called the opposite of combination reactions? Write equations for these reactions.

Answer:

In a combination reaction, two or more substances combine to form a single product, whereas in a decomposition reaction, a single compound breaks down into two or more simpler substances.
Therefore, decomposition is called the opposite of combination.

Example of Combination Reaction:

$2 M g (s) + O_{2} (g) \to 2 M g O (s)$

Example of Decomposition Reaction:

$2 P b (N O_{3})_{2} (s) \overset{Δ}{\to} 2 P b O (s) + 4 N O_{2} (g) + O_{2} (g)$

(or)

$Z n C O_{3} (s) \overset{Δ}{\to} Z n O (s) + C O_{2} (g)$

Q 12. Write one equation each for decomposition reactions where energy is supplied in the form of heat, light or electricity.

(a) Decomposition by heat (Thermal decomposition)

$C a C O_{3} (s) \overset{Δ}{\to} C a O (s) + C O_{2} (g)$

(b) Decomposition by light (Photolytic decomposition)

$2 A g C l (s) \overset{Sunlight}{\to} 2 A g (s) + C l_{2} (g)$

(c) Decomposition by electricity (Electrolytic decomposition)

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

Q 13. What is the difference between displacement and double displacement reactions? Write equations for these reactions.

Answer:

Displacement reaction	Double displacement reaction
A more reactive element displaces a less reactive element from its compound	Two compounds exchange ions to form new compounds
General form: A + BC → AC + B	General form: AB + CD → AD + CB

Displacement reaction

Double displacement reaction

A more reactive element displaces a less reactive element from its compound

Two compounds exchange ions to form new compounds

General form: A + BC → AC + B

General form: AB + CD → AD + CB

Examples:

Displacement reaction

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

Double displacement reaction

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

Q 14. In the refining of silver, the recovery of silver from silver nitrate solution involves displacement by copper metal. Write the reaction involved.

$2 A g N O_{3} (a q) + C u (s) \to C u (N O_{3})_{2} (a q) + 2 A g (s)$

(Copper displaces silver because copper is more reactive.)

Q 15. What do you mean by a precipitation reaction? Explain by giving an example.

Answer:

A precipitation reaction is a reaction in which two aqueous solutions react to form an insoluble solid called precipitate.

Example:

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

BaSO₄ forms as a white precipitate.

Q 16. Explain the following in terms of gain or loss of oxygen with two examples each.

(a) Oxidation

Oxidation is the gain of oxygen / loss of hydrogen.

Examples:

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

(b) Reduction

Reduction is the loss of oxygen / gain of hydrogen.

Examples:

$C u O + H_{2} \to C u + H_{2} O$ $F e_{2} O_{3} + 2 A l \to 2 F e + A l_{2} O_{3}$

Q 17. A shiny brown coloured element ‘X’ on heating in air becomes black in colour. Name the element ‘X’ and the black coloured compound formed.

Answer:

Element X = Copper (Cu)
Black compound = Copper(II) oxide (CuO)

Reaction:

$2 C u + O_{2} \to 2 C u O$

Q18. Why do we apply paint on iron articles?

Answer:

Paint prevents iron from coming in contact with air and moisture, thereby preventing rusting (corrosion).

Q 19. Oil and fat containing food items are flushed with nitrogen. Why?

Answer:

Nitrogen prevents oxidation of oils and fats.
Oxidation causes rancidity, which produces a bad smell and taste.
So nitrogen keeps food fresh for longer.

Q 20. Explain the following terms with one example each:

(a) Corrosion

Corrosion is the gradual deterioration of metals due to reaction with air, moisture or chemicals.

Example:

$4 F e + 3 O_{2} + x H_{2} O \to 2 F e_{2} O_{3} \cdot x H_{2} O$

Formation of rust on iron

(b) Rancidity

Rancidity is the spoilage of food containing fats and oils due to oxidation, producing bad smell and taste.

Example:
Chips packets flushed with nitrogen prevent rancidity.

Tag: Chemical Reactions and Equations

Class 10th Science Chapter 1 – Chemical Reaction and Equations – Exercises

Q 1. Which of the statements about the reaction below are incorrect?

Answer: (i) (a) and (b)

Explanation:

Q 2. Equation

Fe2O3+2Al→Al2O3+2Fe

Answer: (d) Displacement reaction

Explanation:

Q 3. What happens when dilute hydrochloric acid is added to iron filings? Tick the correct answer.

Answer: (a) Hydrogen gas and iron chloride are produced.

Explanation:

Q 4. What is a balanced chemical equation? Why should chemical equations be balanced?

Answer:

Example:

Why should chemical equations be balanced?

In one exam sentence:

Q 5. Translate the following statements into chemical equations and balance them.

(a) Hydrogen gas combines with nitrogen to form ammonia.

Word Equation:

Chemical Equation (unbalanced):

Balanced Chemical Equation:

(b) Hydrogen sulphide gas burns in air to give water and sulphur dioxide.

Word Equation:

Chemical Equation (unbalanced):

Balanced Chemical Equation:

(c) Barium chloride reacts with aluminium sulphate to give aluminium chloride and a precipitate of barium sulphate.

Word Equation:

Chemical Equation (unbalanced):

Balanced Chemical Equation:

(d) Potassium metal reacts with water to give potassium hydroxide and hydrogen gas.

Word Equation:

Chemical Equation (unbalanced):

Balanced Chemical Equation:

Q 6. Balance the following chemical equations:

(a) HNO3+Ca(OH)2→Ca(NO3)2+H2O

(b) NaOH+H2SO4→Na2SO4+H2O

(c) NaCl+AgNO3→AgCl+NaNO3

(d) BaCl2+H2SO4→BaSO4+HCl

Q 7. Write the balanced chemical equations for the following reactions.

(a) Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water

(b) Zinc + Silver nitrate → Zinc nitrate + Silver

(c) Aluminium + Copper chloride → Aluminium chloride + Copper

(d) Barium chloride + Potassium sulphate → Barium sulphate + Potassium chloride

Q 8. Write the balanced chemical equation for the following and identify the type of reaction in each case.

(a) Potassium bromide(aq) + Barium iodide(aq) → Potassium iodide(aq) + Barium bromide(s)

Balanced Equation:

Type of Reaction:

(b) Zinc carbonate(s) → Zinc oxide(s) + Carbon dioxide(g)

Balanced Equation:

Type of Reaction:

(c) Hydrogen(g) + Chlorine(g) → Hydrogen chloride(g)

Balanced Equation:

Type of Reaction:

(d) Magnesium(s) + Hydrochloric acid(aq) → Magnesium chloride(aq) + Hydrogen(g)

Balanced Equation:

Type of Reaction:

Q 9. What does one mean by exothermic and endothermic reactions? Give examples.

Answer

Example:

Example:

Q 10. Why is respiration considered an exothermic reaction? Explain.

Answer

Chemical Equation:

Q 11. Why are decomposition reactions called the opposite of combination reactions? Write equations for these reactions.

Answer:

Example of Combination Reaction:

Example of Decomposition Reaction:

Q 12. Write one equation each for decomposition reactions where energy is supplied in the form of heat, light or electricity.

(a) Decomposition by heat (Thermal decomposition)

(b) Decomposition by light (Photolytic decomposition)

(c) Decomposition by electricity (Electrolytic decomposition)

Q 13. What is the difference between displacement and double displacement reactions? Write equations for these reactions.

Answer:

Examples:

Displacement reaction

Double displacement reaction

Q 14. In the refining of silver, the recovery of silver from silver nitrate solution involves displacement by copper metal. Write the reaction involved.

Q 15. What do you mean by a precipitation reaction? Explain by giving an example.

Answer:

$F e_{2} O_{3} + 2 A l \to A l_{2} O_{3} + 2 F e$

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

(b) $N a O H + H_{2} S O_{4} \to N a_{2} S O_{4} + H_{2} O$

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

(d) $B a C l_{2} + H_{2} S O_{4} \to B a S O_{4} + H C l$