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.
For Example:
Decomposition Reaction:
\(CaCO_{3(s)}\rightarrow CaO_{(s)}+CO_{2(g)}\)
Combination Reaction:
\(CaO _{(s)} + H_2O _{(l)} → Ca(OH)_{2 (aq)}\)
