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) overset("Heat")(rarr) CaO(s)+CO_(2)(g)`
Combination Reaction :
`CaO(s)+H_(2)O(l) rarr Ca(OH)_(2)(aq)`.
