10 min readConcept sheetPart I (1 mark: name the type), Part II (2 marks), Part III (4 marks: types of decomposition) and Part IV (7 marks: classify reactions)

Combination, decomposition, displacement, double displacement and combustion reactions

Reactions are grouped by how atoms rearrange: joining, splitting, swapping one partner, swapping two partners, or burning in oxygen.

CombinationA + B → AB, e.g. 2Mg + O₂ → 2MgO

DecompositionAB → A + B, by heat, electricity or light

Single displacementA + BC → AC + B; a more reactive element displaces a less reactive one

Double displacement and combustionAB + CD → AD + CB. Combustion: burning in oxygen, giving heat and light

Five types at a glance

Type	General form	Example (balanced)
Combination	A + B → AB	2Mg(s) + O₂(g) → 2MgO(s); CaO(s) + H₂O(l) → Ca(OH)₂(aq)
Decomposition	AB → A + B	CaCO₃(s) → CaO(s) + CO₂(g) (on heating)
Single displacement	A + BC → AC + B	Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s)
Double displacement	AB + CD → AD + CB	AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq)
Combustion	Substance + O₂ → oxides + heat and light	CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g) + heat

Three kinds of decomposition

1
Thermal decomposition (by heat): 2HgO(s) → 2Hg(l) + O₂(g)
Also CaCO₃ → CaO + CO₂ when limestone is heated.
2
Electrolytic decomposition (by electricity): 2H₂O(l) → 2H₂(g) + O₂(g)
Electric current through acidified water gives two volumes of hydrogen for one of oxygen.
3
Photodecomposition (by light): 2AgBr(s) → 2Ag(s) + Br₂(g)
Pale yellow silver bromide turns grey in sunlight; used in black and white photography.

The activity series and displacement

Metals in order of reactivity: K > Na > Ca > Mg > Al > Zn > Fe > Pb > (H) > Cu > Hg > Ag > Au.
A metal higher in the series displaces a metal lower down from its salt solution: Zn displaces Cu from CuSO₄.
A metal lower down cannot displace one above it: Cu + ZnSO₄ → no reaction.
Metals above hydrogen displace it from dilute acids: Zn(s) + 2HCl(aq) → ZnCl₂(aq) + H₂(g).
Non-metals can displace too: Cl₂(g) + 2KBr(aq) → 2KCl(aq) + Br₂(aq).

Two kinds of double displacement

PrecipitationIons swap partners and an insoluble solid (precipitate) forms. Pb(NO₃)₂(aq) + 2KI(aq) → PbI₂(s) + 2KNO₃(aq): a yellow precipitate of lead iodide.

NeutralisationAn acid and a base swap partners to give a salt and water. NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l).

Activity: iron nail in copper sulphate solution

1
Pour blue copper sulphate solution into a test tube.
2
Rub an iron nail with sandpaper to clean it, then dip it in the solution for about 20 minutes.
3
Observe: a reddish-brown coat of copper forms on the nail, and the blue solution turns pale green.
4★
Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s)
5
Conclusion: iron is more reactive than copper, so it displaces copper. This is a single displacement reaction.

★ key step

In real life

The blue flame of LPG in your kitchen is combustion. Mixing quicklime with water to prepare whitewash is a combination reaction that gets hot. Old black and white photographs relied on light breaking down silver bromide.

Exam angle

2 marks: “What is a single displacement reaction? Give an example.” A reaction in which a more reactive element displaces a less reactive element from its compound, e.g. Zn + CuSO₄ → ZnSO₄ + Cu.

4 marks: “Explain the three types of decomposition reactions with one example each.” Thermal: 2HgO → 2Hg + O₂. Electrolytic: 2H₂O → 2H₂ + O₂. Photo: 2AgBr → 2Ag + Br₂. Name the energy source for each.

7 marks: “Identify the type of each reaction and explain it: (i) 2Mg + O₂ → 2MgO (ii) CaCO₃ → CaO + CO₂ (iii) Zn + CuSO₄ → ZnSO₄ + Cu (iv) AgNO₃ + NaCl → AgCl + NaNO₃” (i) Combination (ii) Thermal decomposition (iii) Single displacement (iv) Double displacement (precipitation). Define each and say what joins, splits or swaps.

Remember it

Join, Split, Swap one, Swap two, Burn. Combination, decomposition, single displacement, double displacement, combustion.