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:
heat
CaCO3(s) --------> CaO(s) + CO2(g)
Combination Reaction:
CaO (s) + H2O (l) → Ca(OH)2 (aq)
