Decomposition reactions are called the opposite of combination reactions because of the following:

• In decomposition reactions, larger molecules break to give simple molecules while in combination reactions, two or more smaller.
• elements compounds combine to form new compounds.
• Decomposition reactions are endothermic while most of the combination reactions are exothermic.

For example:

Combination reaction

2H₂(g) + O₂(g) + Heat ➝ 2H₂O(l)

H₂(g) + Cl₂(g) + Sunlight ➝ 2HCl(g)

Decomposition reaction

CaCO₃(s) ➝ CaO(s) + CO₂(g)

2Pb(NO₃)₂(s) ➝ 2PbO(s) + 4NO₂(g) + O₂(g)