CBSE Class 10 - Chemical Reactions & Equations Notes & MCQ

1. Chemical Reaction

A chemical reaction is a process in which one or more substances (reactants) are converted into one or more new substances (products) with different physical and chemical properties.

Characteristics/Indicators:

• Change in state (solid → liquid/gas)
• Change in colour
• Evolution of gas (bubbles/odor)
• Change in temperature (heat released/absorbed)
• Formation of precipitate (insoluble solid)
• Evolution of light

Example: Burning of magnesium ribbon → bright light + white ash (magnesium oxide)

2. Chemical Equation

A symbolic representation of a chemical reaction using formulas of reactants and products.

Path: Word equation → Skeleton equation → Balanced equation

Example:
Word: Magnesium + Oxygen → Magnesium oxide
Skeleton: Mg + O₂ → MgO
Balanced: 2Mg + O₂ → 2MgO

Physical States & Symbols:

• (s) solid, (l) liquid, (g) gas, (aq) aqueous
• ↓ precipitate, ↑ gas evolved, Δ heat

3. Balanced Chemical Equation

Why balance? Per the Law of Conservation of Mass, mass is neither created nor destroyed. Number of atoms must be equal on both sides (LHS = RHS).

Steps to Balance (Hit & Trial):

1. Write correct formulas for reactants and products.
2. Write the skeleton equation.
3. Count atoms of each element on both sides.
4. Balance complex molecules first, then single elements (H & O last).
5. Verify with small whole number coefficients.

Example:
Unbalanced: Fe + H₂O → Fe₃O₄ + H₂
Balanced: 3Fe + 4H₂O → Fe₃O₄ + 4H₂

4. Types of Chemical Reactions

Type	General Form	Example	Notes
Combination	A + B → AB	CaO + H₂O → Ca(OH)₂	Exothermic
Decomposition	AB → A + B	2H₂O → 2H₂ + O₂	Endothermic
Displacement	A + BC → AC + B	Fe + CuSO₄ → FeSO₄ + Cu	Reactivity based
Double Displacement	AB + CD → AD + CB	Na₂SO₄ + BaCl₂ → BaSO₄↓	Precipitate
Redox	—	CuO + H₂ → Cu + H₂O	O gain / H loss

5. Effects of Oxidation

Corrosion

Slow eating away of metals. Ex: Rusting of iron.
Prevention: Painting, galvanisation, oiling/greasing.

Rancidity

Oxidation of fats & oils in food leading to bad odor.
Prevention: Nitrogen flushing, antioxidants, airtight containers.

Quick Exam Tips:

• Always include physical states (s, l, g, aq).
• Respiration is exothermic; Photosynthesis is endothermic.
• Memorize: 2Mg + O₂ → 2MgO.

Practice MCQs

Chemical Reactions and Equations - Class 10 Chemistry

1

Which of the following is not a physical change?

A) Melting of ice

B) Evaporation of water

C) Burning of magnesium ribbon

D) Dissolving sugar in water

Correct Answer: C

2

The reaction 2H₂ + O₂ → 2H₂O is an example of:

A) Decomposition reaction

B) Displacement reaction

C) Combination reaction

D) Double displacement

Correct Answer: C

3

Which type of reaction usually requires heat, light or electricity?

A) Combination reaction

B) Decomposition reaction

C) Displacement reaction

D) Redox reaction

Correct Answer: B

4

In the reaction Zn + CuSO₄ → ZnSO₄ + Cu, the type of reaction is:

A) Combination

B) Decomposition

C) Displacement

D) Double displacement

Correct Answer: C

5

The reaction Na₂SO₄ + BaCl₂ → BaSO₄↓ + 2NaCl is:

A) Combination reaction

B) Decomposition reaction

C) Displacement reaction

D) Double displacement

Correct Answer: D

6

The addition of oxygen to a substance is called:

A) Reduction

B) Oxidation

C) Redox

D) Precipitation

Correct Answer: B

7

Which of the following is an example of a redox reaction?

A) CaCO₃ → CaO + CO₂

B) CuO + H₂ → Cu + H₂O

C) NaCl + AgNO₃ → AgCl

D) 2H₂O → 2H₂ + O₂

Correct Answer: B

8

The brown gas evolved when lead nitrate is heated is:

A) SO₂

B) NO₂

C) CO₂

D) O₂

Correct Answer: B

9

Rusting of iron is an example of:

A) Combination reaction

B) Slow oxidation

C) Fast decomposition

D) Displacement reaction

Correct Answer: B

10

Which method helps prevent rancidity in food items?

A) Adding antioxidants

B) Heating strongly

C) Exposing to sunlight

D) Keeping in open air

Correct Answer: A

11

Balanced equation for reaction of iron with steam:

A) Fe + H₂O → Fe₃O₄ + H₂

B) 3Fe + 4H₂O → Fe₃O₄ + 4H₂

C) 2Fe + 3H₂O → Fe₂O₃

D) Fe + 2H₂O → Fe(OH)₂

Correct Answer: B

12

In which reaction is a precipitate formed?

A) Zn + H₂SO₄ → ZnSO₄ + H₂

B) Pb(NO₃)₂ + 2KI → PbI₂↓

C) CaO + H₂O → Ca(OH)₂

D) 2Mg + O₂ → 2MgO

Correct Answer: B

13

The reaction 2AgCl → 2Ag + Cl₂ (in sunlight) is:

A) Thermal decomposition

B) Electrolytic decomposition

C) Photolytic decomposition

D) Displacement reaction

Correct Answer: C

14

Oxidising agent in: CuO + H₂ → Cu + H₂O?

A) H₂

B) Cu

C) CuO

D) H₂O

Correct Answer: C

15

White precipitate from BaCl₂ + Na₂SO₄:

A) Barium chloride

B) Sodium sulphate

C) Barium sulphate

D) Sodium chloride

Correct Answer: C

16

Which of the following is an exothermic reaction?

A) Photosynthesis

B) Decomposition of CaCO₃

C) Respiration

D) Electrolysis of water

Correct Answer: C

17

Process responsible for grey coating on silver:

A) Corrosion

B) Rancidity

C) Reduction

D) Precipitation

Correct Answer: A

18

Law satisfied when equation is balanced:

A) Definite proportions

B) Conservation of mass

C) Multiple proportions

D) Combining volumes

Correct Answer: B

19

2FeSO₄ → Fe₂O₃ + SO₂ + SO₃ (on heating) is:

A) Combination reaction

B) Displacement reaction

C) Thermal decomposition

D) Double displacement

Correct Answer: C

20

In chips packets, nitrogen gas is flushed to:

A) Increase taste

B) Prevent rancidity

C) Make it heavier

D) Change colour

Correct Answer: B