Although millions of reactions are known, most school-level reactions fall into a few neat families. Recognising the family helps you predict the products and write the equation quickly.
Combination reactions
In a combination reaction two or more reactants join to form a single product, A + B → AB. Burning of coal, C + O2 → CO2, and the slaking of quicklime, CaO + H2O → Ca(OH)2, are examples. The slaking of lime releases a large amount of heat, so it is also an exothermic reaction — calcium hydroxide solution (slaked lime) is used to whitewash walls.
Decomposition reactions
A decomposition reaction is the opposite: a single compound breaks down into two or more simpler substances, AB → A + B. These reactions need energy, supplied as heat (thermal decomposition), light (photolytic) or electricity (electrolytic). Heating calcium carbonate gives CaCO3 → CaO + CO2; passing electricity through water gives 2H2O → 2H2 + O2; and silver chloride turns grey in sunlight, 2AgCl → 2Ag + Cl2, a reaction used in black-and-white photography. Because they absorb energy, decomposition reactions are usually endothermic.
Displacement reactions
In a displacement reaction a more reactive element pushes out a less reactive one from its compound, A + BC → AC + B. When an iron nail is dipped in blue copper sulphate solution, iron displaces copper: Fe + CuSO4 → FeSO4 + Cu, and the solution slowly turns pale green while a brown coat of copper appears on the nail. Zinc and lead can likewise displace copper, because they are more reactive than copper.
Double displacement reactions
In a double displacement reaction two compounds exchange their ions, AB + CD → AD + CB. A common type is a precipitation reaction, where an insoluble solid (precipitate) forms. Mixing sodium sulphate and barium chloride solutions gives a white precipitate of barium sulphate: Na2SO4 + BaCl2 → BaSO4 + 2NaCl. Neutralisation, such as HCl + NaOH → NaCl + H2O, is also a double displacement reaction.
- Combination: two or more become one (A + B → AB).
- Decomposition: one becomes two or more (AB → A + B).
- Displacement: a reactive element replaces a less reactive one.
- Double displacement: two compounds swap ions, often forming a precipitate.
Classify and balance: CaO + H2O → Ca(OH)2.
Solution- One product is formed from two reactants — a combination reaction.
- Check atoms: Ca = 1, O on left = 1 + 1 = 2, H = 2; right Ca = 1, O = 2, H = 2.
- The equation is already balanced.
- It releases heat, so it is also exothermic (slaking of lime).
Answer: Combination reaction; CaO + H2O → Ca(OH)2 (balanced).
Classify and balance the electrolysis of water: H2O → H2 + O2.
Solution- One compound splits into two simpler substances — a decomposition reaction (electrolytic).
- Oxygen: 1 on left, 2 on right; place 2 before H2O.
- 2H2O → H2 + O2 now has 4 H on the left.
- Place 2 before H2 to balance hydrogen.
Answer: Decomposition reaction; 2H2O → 2H2 + O2.
When an iron nail is left in copper sulphate solution, the blue colour fades and a brown deposit forms. Write and classify the reaction.
Solution- Iron is more reactive than copper, so iron displaces copper from its salt.
- Equation: Fe + CuSO4 → FeSO4 + Cu.
- The blue Cu2+ ions are replaced by pale-green Fe2+ ions, so the colour fades.
- Brown copper metal deposits on the nail.
Answer: A displacement reaction: Fe + CuSO4 → FeSO4 + Cu.
Predict the precipitate and write the balanced equation when solutions of lead nitrate and potassium iodide are mixed.
Solution- The compounds exchange ions — a double displacement (precipitation) reaction.
- Possible products: lead iodide and potassium nitrate.
- Lead iodide, PbI2, is insoluble and forms a yellow precipitate.
- Balance: Pb(NO3)2 + 2KI → PbI2 + 2KNO3.
Answer: A yellow precipitate of PbI2 forms: Pb(NO3)2 + 2KI → PbI2 + 2KNO3.
Classify: AgCl turns grey when left in sunlight, 2AgCl → 2Ag + Cl2. Name the type and the energy source.
Solution- A single compound (AgCl) breaks into two products (Ag and Cl2).
- This is a decomposition reaction.
- The energy is supplied by light, so it is a photolytic (photochemical) decomposition.
- It is used in black-and-white photography.
Answer: Photolytic decomposition reaction (energy from sunlight).
Identify the type of reaction: HCl + NaOH → NaCl + H2O.
Solution- An acid and a base react, exchanging ions to form a salt and water.
- The H+ of the acid pairs with OH- of the base; Na+ pairs with Cl-.
- This ion exchange makes it a double displacement reaction.
- Because an acid and a base neutralise each other, it is specifically a neutralisation reaction.
Answer: A double displacement (neutralisation) reaction.