Why are decomposition reactions called the opposite of Combination reactions? Write equations for decomposition reactions.

Combination reaction is said to be the reaction between two or more molecules to form a larger molecule; whereas the decomposition reaction is defined as the splitting of larger molecules into two or more smaller molecules. This essentially explains that the decomposition reaction is the opposite of the combination reaction.

In most cases, the decomposition reaction is endothermic since the heat from the surrounding or induced heat is used to break the bonds of the larger molecule. A few examples of decomposition reactions are

ZnCO3 → ZnO + CO2

CaCO3 + Energy → CaO + CO2

2HgO → 2Hg + O2