CBSE Class 10 Science: Chemical Reactions — Complete Notes 2026

What is a Chemical Reaction?

• Reactants converted to products; new substances with different properties formed
• Evidence of chemical reaction: colour change, gas evolution, precipitate, temperature change, light emission
• Word equation → skeletal equation → balanced equation with state symbols

Balancing Chemical Equations

• Law of conservation of mass: total mass of reactants = total mass of products
• Balancing by inspection: adjust coefficients (not subscripts) to equate atoms of each element
• Example: Fe + O₂ → Fe₂O₃; balanced as 4Fe + 3O₂ → 2Fe₂O₃

Types of Chemical Reactions

• Combination: A + B → AB (2H₂ + O₂ → 2H₂O)
• Decomposition: AB → A + B (2HgO → 2Hg + O₂); thermal, photo, electrolytic
• Displacement: CuSO₄ + Fe → FeSO₄ + Cu (single); double displacement: AgNO₃ + NaCl → AgCl↓ + NaNO₃

Oxidation and Reduction

• Oxidation: gain of oxygen/loss of hydrogen/loss of electrons; reduction: opposite
• Redox reactions: oxidation and reduction occur simultaneously
• OIL RIG mnemonic: Oxidation Is Loss; Reduction Is Gain (of electrons)

Exothermic and Endothermic Reactions

• Exothermic: energy released (combustion, respiration, neutralisation); ΔH < 0
• Endothermic: energy absorbed (decomposition of CaCO₃, photosynthesis, dissolving NH₄Cl)
• CBSE: identify endo/exothermic from given equation with ΔH value

Corrosion and Rancidity

• Corrosion: metals oxidised by moisture and air; rusting of iron: 4Fe + 3O₂ + xH₂O → Fe₂O₃·xH₂O
• Prevention of corrosion: painting, galvanisation, alloying, cathodic protection
• Rancidity: oxidation of fats/oils in food; prevented by antioxidants, nitrogen flushing, refrigeration

CBSE Board Exam Tips

• CBSE Chapter 1 carries significant marks; practise balancing all reaction types
• Define: thermal decomposition, photodecomposition, precipitation reaction
• Application: state which type of reaction and give one example from daily life