(a) Combination reaction: A reaction in which two or more substances combine to form a single substance is called a combination reaction.

A + B → AB

e.g (i) When iron and sulphur are heated together, they combine to form iron sulphide.

Fe + S + heat → FeS

(ii) When carbon bums in oxygen to form a gaseous compound called carbon dioxide.

C + O₂ + heat → CO₂ + heat

(b) Decomposition reaction: A reaction in which a compound breaks up due to the application of heat into two or more simple substances is called decomposition reaction.

AB + heat → A + B

e.g. (i) Mercuric oxide when heated, decomposes to form two elements mercury and oxygen

2HgO(s) + heat → 2Hg(s) + O₂(g)

(ii) CaCO₃, when heated, decomposes to calcium oxide and carbon dioxide.

CaCO₃ + heat → CaO + CO₂

(c) Displacement reaction: A reaction in which a more active element displaces a less active element from a compound is called displacement reaction.

AB + C → CB + A

e.g. (i) Zinc, displaces copper from copper sulphate solution.

Zn + CuSO₄ (aq) → ZnSO₄ (aq) + Cu

(ii) Iron piece when added to copper sulphate solution, copper is displaced.

Fe + CuSO₄ → FeSO₄ + Cu.

(d) Double decomposition reaction: A chemical reaction in which two compounds in their aqueous state exchange their ions to form new compounds is called a double decomposition reaction.

AB + CD → CB + AD

e.g. (i) AgNO₃ + HCl AgCl + HNO₃(aq)

(ii) NaOH (aq) + HCl (aq) NaCl (aq) + H₂O.