Question

Why are decomposition reactions called the opposite to combination reactions?

Answer 1

decomposition reactions are called the opposite to combination reaction as.
decomposition reaction is a reaction in which a compound decomposes to form two or more products.

but in combination reaction to or more react and react to produce a one compound.

so this is the opposite reaction of combination reaction

Answer 2

In a combination reaction two or more substances are combined to form new substance while in the decomposition reaction one substance decomposes into two or more substances. Hence, the decomposition reactions are called opposite of combination reactions.