Explain addition reaction with one example.

An addition reaction is an organic reaction where two or more molecules combine to form a larger one.

Addition reactions are limited to chemical compounds that have multiple bonds, such as molecules with carbon-carbon double bonds (alkenes), with triple bonds (alkynes) or with carbonyl (C=O) groups can undergo addition as they too have double bond character. For example,
${\text{CH}}_2={\text{CH}}_2$ + ${\text{Cl}}_2$ $\to$ ${\text{CH}}_2\text{Cl}-{\text{CH}}_2\text{Cl}$.