Question

why are decomposition reaction is called opposite of combination reaction . write equation for these reaction

Answer 1

decomposition reaction - h2o->h2+o2
combination reaction - c+o2->co2

Answer 2

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.

Example:-

Decomposition reaction:- CaCO3 -----( heat )------ CaO + CO2

Combination reaction:- 2Mg + O2 ------- 2MgO

Hence, Decomposition reactions are often called opposite of combination reactions.