Why are Decomposition Reactions called the opposite of Combination Reaction? Write equations for these reactions.

Answer: Two or more compounds are mixed to produce a new substance in a combination reaction, whereas one material decomposes into two or more components in a decomposition reaction. As a result, decomposition reactions are referred to as the polar opposites of combination reactions.

Reaction of decomposition 

CaCO₃ (s) -> CaO (s) + CO₂ (g) (decomposition reaction)

H₂ (g) + Cl₂ -> 2HCl (g) (combination reaction)