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.
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