Combination

Combination also called synthesis is the combining of two substances to form a third substance. A simple representative of a combination reaction can be shown below:

A + B --> AB

Here A is a substance and B is another substance. After reaction, the two substances yield another substance representated as AB.

The production of water is also a combination reaction in which hydrogen gas combine with oxygen gas to form water as shown below:

Reactant + Reactant ---> Product

Basically combination and decomposition reactions are reverse reactions of each other.