Home › Free notes › Class 10 Science › Chemical Reactions and Equations

Chemical Reactions and Equations – Class 10 Science notes, important questions and MCQs

NCERT Class 10 Science, chapter 1. Notes, key terms, important questions with answers and MCQs.

Chapter overview

A chemical reaction forms new substances. We write it as a balanced chemical equation and classify it as combination, decomposition, displacement, double displacement or redox.

Signs and equations

Signs of a reaction: change of colour or state, gas given off, change in temperature, precipitate formed.
A balanced equation has equal atoms of each element on both sides (law of conservation of mass).
State symbols: (s), (l), (g), (aq).

Types of reactions

Combination: A + B → AB (e.g. CaO + H₂O → Ca(OH)₂, exothermic).
Decomposition: AB → A + B by heat, light or electricity (e.g. 2AgCl → 2Ag + Cl₂ in sunlight).
Displacement: a more reactive metal displaces a less reactive one (Fe + CuSO₄ → FeSO₄ + Cu).
Double displacement: ions are exchanged, often forming a precipitate (Na₂SO₄ + BaCl₂ → BaSO₄↓ + 2NaCl).

Oxidation and reduction

Oxidation: gain of oxygen or loss of hydrogen; reduction: loss of oxygen or gain of hydrogen.
Redox: both happen together (CuO + H₂ → Cu + H₂O).
Corrosion (rusting of iron) and rancidity (oxidation of fats) are everyday oxidation effects.

Key terms

Exothermic reaction: a reaction that gives out heat
Endothermic reaction: a reaction that absorbs heat
Precipitate: an insoluble solid formed in a solution
Rancidity: spoiling of oily food by oxidation

Formulas

2H₂ + O₂ → 2H₂O
CaCO₃ → CaO + CO₂ (on heating)
Rust: Fe₂O₃·xH₂O

Important questions with answers

Why should a magnesium ribbon be cleaned before burning in air?2 marks

Magnesium reacts with air to form a layer of magnesium oxide on its surface. This layer stops the ribbon from burning properly, so it is rubbed with sandpaper to expose fresh metal.

Write the balanced equation for the reaction of iron with steam and identify the type of reaction.3 marks

3Fe(s) + 4H₂O(g) → Fe₃O₄(s) + 4H₂(g). It is a displacement (redox) reaction: iron displaces hydrogen from water and gets oxidised.

What are oxidation and reduction? Identify the substance oxidised and the substance reduced in CuO + H₂ → Cu + H₂O, and explain corrosion and rancidity with one way to prevent each.5 marks

Oxidation is gain of oxygen or loss of hydrogen; reduction is loss of oxygen or gain of hydrogen. In the reaction, H₂ is oxidised to H₂O and CuO is reduced to Cu. Corrosion is the slow eating away of metals by air and moisture (rusting of iron); it is prevented by painting, oiling or galvanising. Rancidity is the oxidation of fats and oils in food, giving a bad smell and taste; it is prevented by adding antioxidants, airtight packing or flushing packets with nitrogen.

Quick revision

Balanced equation = mass is conserved.
Four types: combination, decomposition, displacement, double displacement.
Oxidation adds O / removes H; reduction does the opposite.
Prevent rusting by painting, oiling or galvanising.

Exam tips

Balance by changing coefficients, never the subscripts in a formula.
Respiration is exothermic; decomposition reactions are usually endothermic.

Practice MCQs

CaO + H₂O → Ca(OH)₂ is a
1. combination reaction
2. decomposition reaction
3. displacement reaction
4. double displacement reaction
Show answer
Answer: (a) combination reaction. Two substances join to form one.
Rusting of iron is an example of
1. reduction
2. oxidation
3. neutralisation
4. sublimation
Show answer
Answer: (b) oxidation. Iron combines with oxygen and moisture.
A balanced equation follows the law of
1. conservation of mass
2. conservation of energy
3. multiple proportions
4. gravitation
Show answer
Answer: (a) conservation of mass. Atoms are neither created nor destroyed.
Fe + CuSO₄ → FeSO₄ + Cu is a
1. combination reaction
2. decomposition reaction
3. displacement reaction
4. redox only
Show answer
Answer: (c) displacement reaction. Iron, being more reactive, displaces copper.
White silver chloride turns grey in sunlight because
1. it melts
2. it absorbs water
3. it decomposes into silver and chlorine
4. it gains oxygen
Show answer
Answer: (c) it decomposes into silver and chlorine. Photolytic decomposition: 2AgCl → 2Ag + Cl₂.
In CuO + H₂ → Cu + H₂O, the substance oxidised is
1. CuO
2. H₂
3. Cu
4. H₂O
Show answer
Answer: (b) H₂. H₂ gains oxygen to form H₂O.
Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl is a
1. displacement and combination reaction
2. double displacement and precipitation reaction
3. decomposition reaction
4. redox reaction
Show answer
Answer: (b) double displacement and precipitation reaction. Ions exchange and insoluble BaSO₄ precipitates.
Chips packets are filled with nitrogen to prevent
1. corrosion
2. neutralisation
3. precipitation
4. rancidity
Show answer
Answer: (d) rancidity. Nitrogen keeps out oxygen that oxidises the fats.

All chapters of Class 10 Science

Written by Kundali IQ from standard, well-established facts to help with revision. This is not an official NCERT publication; always read your textbook too.