ICSE Class 10 Chemistry: Chemical Reactions — Complete Notes 2026

Types of Chemical Reactions

• Combination (Synthesis): A + B → AB; e.g., CaO + H₂O → Ca(OH)₂; exothermic usually
• Decomposition: AB → A + B; e.g., 2H₂O → 2H₂ + O₂ (electrolysis); often endothermic
• Single displacement: A + BC → AC + B; more reactive metal displaces less reactive one

Displacement Reactions

• Single displacement: Zn + CuSO₄ → ZnSO₄ + Cu; zinc displaces copper (Zn more reactive)
• Double displacement (metathesis): AB + CD → AD + CB; e.g., AgNO₃ + NaCl → AgCl↓ + NaNO₃
• Precipitate: when product is insoluble; indicated by ↓; white AgCl precipitate classic example

Oxidation and Reduction

• Oxidation: loss of electrons (OIL — Oxidation Is Loss); or gain of oxygen; or loss of hydrogen
• Reduction: gain of electrons (RIG — Reduction Is Gain); or loss of oxygen; or gain of hydrogen
• Redox: always simultaneous; what one substance loses, another gains; oxidising agent gets reduced

Oxidation Numbers

• Rules: element = 0; monoatomic ion = charge; oxygen = −2 (usually); hydrogen = +1 (usually)
• Sum of oxidation numbers in compound = 0; in ion = charge of ion
• Example: in K₂Cr₂O₇: K = +1 (×2), O = −2 (×7); so Cr: 2(+1) + 2Cr + 7(−2) = 0 → Cr = +6

Exothermic and Endothermic Reactions

• Exothermic: releases heat; combustion, respiration, neutralisation; ΔH negative
• Endothermic: absorbs heat; dissolving NH₄Cl in water, photosynthesis, decomposition of CaCO₃; ΔH positive
• Thermal decomposition: applying heat to decompose compound; CaCO₃ → CaO + CO₂ (lime kiln)

Balancing Chemical Equations

• Law of Conservation of Mass: mass of reactants = mass of products; atoms conserved
• Balancing method: balance atoms one element at a time; hydrogen and oxygen last; use whole number coefficients only
• State symbols: (s) solid, (l) liquid, (g) gas, (aq) aqueous solution; add for completeness

ICSE Exam Tips

• Types of reactions: classify and justify classification; ICSE asks both name and explanation
• Redox: identify oxidising agent and reducing agent; state what is oxidised and what is reduced
• Balanced equations: write balanced equations for 10 common reactions; marks available for correct equation