Decomposition reactions are those in which a compound breaks down to form two or more substances. These reactions require a source of energy to proceed. Thus, they are the exact opposite of combination reactions in which two or more substances combine to give a new substance with the release of energy.

Decomposition reaction : $AB +$ Energy $\longrightarrow A + B$

$2{{H}_{2}}{{O}_{(l)}}\xrightarrow{\text{ Electroly}sis}2{{H}_{2(g)}}+{{O}_{2(g)}}$

Combination reaction : $A+B \longrightarrow A B+$ Energy

$2{{H}_{2(g)}}+{{O}_{2(g)}}\to 2{{H}_{2}}{{O}_{(l)}}\,+$ Energy