Combination reaction
Two or more substances join to form one product: A + B → AB.
- CaO(s) + H₂O(l) → Ca(OH)₂(aq) + heat: quicklime becomes slaked lime, used for whitewash. The thin layer of Ca(OH)₂ reacts slowly with CO₂ in air to form shiny CaCO₃ on the wall.
- 2Mg + O₂ → 2MgO: magnesium ribbon burns with a dazzling white flame.
- C + O₂ → CO₂ (burning coal), 2H₂ + O₂ → 2H₂O.
Decomposition reaction
One compound breaks into two or more simpler substances: AB → A + B. It needs energy, so it is usually endothermic. By the kind of energy used:
Thermal decomposition (heat)
CaCO₃ → CaO + CO₂ (making lime from limestone); 2FeSO₄ → Fe₂O₃ + SO₂ + SO₃ (green crystals turn brown and give a burning-sulphur smell); 2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂ (brown fumes).
Electrolytic decomposition (electricity)
2H₂O → 2H₂ + O₂ when current passes through acidified water. Hydrogen collects at twice the volume of oxygen.
Photolytic decomposition (light)
2AgCl → 2Ag + Cl₂ and 2AgBr → 2Ag + Br₂ in sunlight: the white salt turns grey. This is used in black-and-white photography.
Displacement reaction
A more reactive element pushes a less reactive element out of its compound: A + BC → AC + B.
- Fe + CuSO₄ → FeSO₄ + Cu: the blue solution turns pale green and a brown coat of copper forms on the iron nail.
- Zn + CuSO₄ → ZnSO₄ + Cu; Pb + CuCl₂ → PbCl₂ + Cu.
If the metal is less reactive (copper in iron sulphate), nothing happens. Order to remember for now: Zn > Fe > Pb > Cu.
Double displacement and precipitation reactions
Two compounds exchange their ions: AB + CD → AD + CB. When one product is an insoluble solid, it settles as a precipitate, so the reaction is also called a precipitation reaction.
- Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s) + 2NaCl(aq): white precipitate of barium sulphate.
- AgNO₃ + NaCl → AgCl↓ + NaNO₃: white curdy precipitate.
- Pb(NO₃)₂ + 2KI → PbI₂↓ + 2KNO₃: bright yellow precipitate.
Acid + base (neutralisation) is also a double displacement: NaOH + HCl → NaCl + H₂O.
Exothermic and endothermic reactions
Exothermic reactions release heat: burning of fuels, CaO + water, respiration (C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy) and decomposition of vegetable matter into compost.
Endothermic reactions absorb heat, light or electricity: most decompositions (CaCO₃ on heating), photosynthesis, and electrolysis of water.
Exam pattern: identify-the-type questions (1 mark), "why is respiration exothermic?" (2 marks) and observation-based questions on FeSO₄ heating or Pb(NO₃)₂ + KI (3 marks) are common.
Key formulas and definitions
- Combination: A + B → AB
- Decomposition: AB → A + B (by heat, electricity or light)
- Displacement: A + BC → AC + B (A more reactive than B)
- Double displacement: AB + CD → AD + CB
- Precipitate: insoluble solid formed, shown by (s) or ↓
- Exothermic: heat released; Endothermic: heat absorbed
Worked examples
1. Identify the type: CaO + H₂O → Ca(OH)₂ + heat
Two reactants make one product, so it is a combination reaction. Heat is released, so it is also exothermic.
2. Identify the type: 2H₂O → 2H₂ + O₂ (electric current)
One compound breaks into two elements using electricity: electrolytic decomposition. It absorbs energy, so it is endothermic.
3. Why does the blue colour of copper sulphate fade when an iron nail is dipped in it?
Iron is more reactive than copper and displaces it: Fe + CuSO₄ → FeSO₄ + Cu. Blue CuSO₄ is replaced by pale green FeSO₄, and brown copper coats the nail.
4. Lead nitrate solution is mixed with potassium iodide solution. What do you see? Write the equation.
A yellow precipitate of lead iodide forms. Pb(NO₃)₂(aq) + 2KI(aq) → PbI₂(s) + 2KNO₃(aq). It is a double displacement (precipitation) reaction.
5. Ferrous sulphate crystals are heated in a dry test tube. State two observations and the type of reaction.
Green crystals turn brown (Fe₂O₃) and a smell of burning sulphur comes (SO₂, SO₃). 2FeSO₄ → Fe₂O₃ + SO₂ + SO₃ is thermal decomposition.
6. Will copper displace zinc from zinc sulphate solution? Explain.
No. Copper is less reactive than zinc, so it cannot push zinc out. No reaction takes place.
Common mistakes
- Calling every reaction with two products decomposition. Decomposition starts with only ONE reactant.
- Mixing up displacement (one element swaps in) and double displacement (two compounds swap ions).
- Thinking a precipitate is a gas. It is an insoluble solid that settles down.
- Saying all decompositions are exothermic. They need energy, so they are usually endothermic.