In a decomposition reaction, a single substance splits to form two or more simpler substances whereas in combination reaction, two or more substances combine to form a single substance. Hence, decomposition reactions are called the opposite of combination reactions.

(i) Decomposition reaction:

In this reaction, calcium carbonate decomposes into calcium oxide and carbon dioxide on heating. This is an important decomposition reaction used in various industries.

(ii) Combination reaction: Formation of water

In this reaction, two substances hydrogen and oxygen combine to form single substance water, so this is a combination reaction.

We can see from the above examples that a decomposition reaction is opposite of a combination reaction.