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Chemical Reactions

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Overview

In a chemical reaction substances change into new substances; reactions are classified by how reactants combine, break or exchange parts.

Key points

Combination: two or more substances form one product (CaO + H₂O → Ca(OH)₂, exothermic).
Decomposition: one substance breaks into simpler ones (CaCO₃ → CaO + CO₂ on heating).
Displacement: a more reactive metal displaces a less reactive one (Fe + CuSO₄ → FeSO₄ + Cu).
Double displacement exchanges ions, often forming a precipitate.
Oxidation is gain of oxygen or loss of hydrogen; reduction is the opposite; together they are redox.
Equations are balanced because mass is conserved (Lavoisier).

Explanation

Exothermic reactions release heat; endothermic reactions absorb it.
Corrosion (rusting) and rancidity are oxidation effects.
Signs of a reaction: colour change, gas, precipitate, temperature change.
Respiration is exothermic.
Photosynthesis is endothermic.

Step by step

Identify the concept or reaction.
Recall the definition, equation or example.
Check balance, units and direction.

Examples

Quicklime with water → combination, exothermic.
Heating limestone → decomposition.
Iron nail in copper sulphate turns brown → displacement.

Traps and advanced points

Trap: Unbalanced equations.
Trap: Mixing displacement and double displacement.
Trap: Wrong definition of oxidation.

Quick revision

Five reaction types.
Conservation of mass.
Exothermic vs endothermic.

Practice MCQs

CaO + H₂O → Ca(OH)₂ is a —
1. double displacement reaction
2. decomposition reaction
3. combination reaction
4. displacement reaction
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Answer: (c) combination reaction. Two substances form one product.
Heating limestone (CaCO₃ → CaO + CO₂) is a —
1. displacement reaction
2. neutralisation
3. combination reaction
4. decomposition reaction
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Answer: (d) decomposition reaction. One substance breaks into simpler ones.
An iron nail in copper sulphate solution turns brown because of —
1. sublimation
2. decomposition
3. displacement
4. combination
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Answer: (c) displacement. Iron displaces copper.
Equations are balanced because of the law of —
1. conservation of mass
2. gravitation
3. inertia
4. reflection
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Answer: (a) conservation of mass. Mass is neither created nor destroyed (Lavoisier).
Reactions that release heat are —
1. reversible only
2. nuclear
3. endothermic
4. exothermic
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Answer: (d) exothermic. Exothermic reactions give out heat.
Photosynthesis is —
1. endothermic
2. a displacement reaction
3. combustion
4. exothermic
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Answer: (a) endothermic. It absorbs light energy.
Oxidation is defined as —
1. gain of hydrogen
2. loss of oxygen
3. gain of electrons
4. gain of oxygen or loss of hydrogen
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Answer: (d) gain of oxygen or loss of hydrogen. Oxidation: gain of O or loss of H (or loss of electrons).
Rancidity of oily food is caused by —
1. sublimation
2. oxidation
3. reduction
4. neutralisation
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Answer: (b) oxidation. Fats and oils get oxidised.
Formation of a precipitate when two solutions are mixed usually shows a —
1. physical change only
2. combustion
3. double displacement reaction
4. decomposition reaction
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Answer: (c) double displacement reaction. Ions exchange and an insoluble salt forms.
Rusting of iron requires —
1. only nitrogen
2. carbon dioxide only
3. oxygen and water
4. only heat
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Answer: (c) oxygen and water. Iron rusts in the presence of air and moisture.