Question

Why decomposition reaction is known as reverse of combination reaction?

Answer 1

In combination reaction..
2 or more substances combine to form a single substance..

Whereas

Im decomposition reaction..
a compound splits up into 2 or more simpler substances..

EX. of combination reaction:
2Mg + O2 --> 2MgO

EX. of Decomposition reaction:
CaCO3 --> CaO + CO2

hope it helped ...