Important Chemical Reactions
Every chemical reaction that appears in CBSE Class 10 Science board exams — with balanced equations, observations, and exactly what examiners look for.
Types of Chemical Reactions
Combination Reaction
very high frequencyBalanced Equation
CaO + H₂O → Ca(OH)₂
Type
Combination
Observation: Heat is released (exothermic). Quicklime turns to slaked lime.
Two or more substances combine to form a single product. Always exothermic — write heat release in answer.
Decomposition Reaction (Thermal)
very high frequencyBalanced Equation
CaCO₃ → CaO + CO₂ (heat)
Type
Decomposition
Observation: Calcium carbonate breaks into calcium oxide and carbon dioxide on heating.
One reactant breaks into two or more products. Opposite of combination. Common: 2H₂O₂ → 2H₂O + O₂
Electrolytic Decomposition
very high frequencyBalanced Equation
2H₂O → 2H₂ + O₂ (electric current)
Type
Decomposition
Observation: Hydrogen collects at cathode (double volume), oxygen at anode.
Ratio of H₂ : O₂ = 2:1 by volume. A very common 1-mark question.
Displacement Reaction
very high frequencyBalanced Equation
Fe + CuSO₄ → FeSO₄ + Cu
Type
Displacement
Observation: Blue colour of CuSO₄ fades. Iron nail turns reddish-brown (copper deposits).
More reactive metal displaces less reactive metal. Zn + H₂SO₄ → ZnSO₄ + H₂ is another common example.
Double Displacement (Precipitation)
very high frequencyBalanced Equation
Na₂SO₄ + BaCl₂ → BaSO₄↓ + 2NaCl
Type
Double Displacement
Observation: White precipitate of BaSO₄ forms. Insoluble product separates from solution.
↓ symbol = precipitate. Write state symbols — (aq), (s), (l), (g) — for full marks.
Neutralization Reaction
high frequencyBalanced Equation
NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l)
Type
Double Displacement
Observation: Acid and base react to form salt and water. pH moves towards 7.
Always acid + base → salt + water. The heat released is called heat of neutralization.
Oxidation, Reduction and Redox
Thermite Reaction
very high frequencyBalanced Equation
Fe₂O₃ + 2Al → Al₂O₃ + 2Fe
Type
Redox / Displacement
Observation: Large amount of heat released. Used in welding railway tracks.
Al is oxidised (gains O), Fe₂O₃ is reduced (loses O). Al acts as reducing agent.
Oxidation of Copper
high frequencyBalanced Equation
2Cu + O₂ → 2CuO (heat)
Type
Oxidation
Observation: Copper surface turns black (copper oxide forms).
When H₂ gas is passed over black CuO, it turns back to brown copper — reduction.
Hydrogen Reducing Metal Oxide
high frequencyBalanced Equation
CuO + H₂ → Cu + H₂O
Type
Redox
Observation: Black CuO turns reddish-brown. H₂ is oxidised, Cu²⁺ is reduced.
CuO → reducing agent is H₂. CuO is oxidising agent. Classic oxidation-reduction pair.
Corrosion and Rancidity
Rusting of Iron
very high frequencyBalanced Equation
4Fe + 3O₂ + 6H₂O → 4Fe(OH)₃ (hydrated iron oxide)
Type
Oxidation
Observation: Reddish-brown rust forms on iron in presence of oxygen and moisture.
Both O₂ and H₂O are required — iron rusts faster in salt water (electrolyte speeds reaction).
Rancidity
high frequencyBalanced Equation
Fats/Oils + O₂ → Peroxides (bad smell/taste)
Type
Oxidation
Observation: Smell and taste of food changes — butter, chips go stale.
Prevention: antioxidants (N₂ flushing, vacuum packing). Fridge slows it down.
Acids, Bases and Salts
Acid + Metal
very high frequencyBalanced Equation
Zn + H₂SO₄ → ZnSO₄ + H₂↑
Type
Displacement
Observation: Hydrogen gas is produced (bubbles). Metal dissolves.
H₂ gas test — burning splint produces pop sound.
Acid + Metal Carbonate
very high frequencyBalanced Equation
Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂↑
Type
Double Displacement
Observation: CO₂ gas produced — turns lime water milky.
CO₂ test — lime water (Ca(OH)₂) turns milky due to CaCO₃ precipitate.
Acid + Metal Hydroxide
very high frequencyBalanced Equation
HCl + NaOH → NaCl + H₂O
Type
Neutralization
Observation: pH moves to 7. Heat is released.
This is also called a neutralization reaction. Product is always salt + water.