@Tawhid Ibn Humayun Addition reaction is defined as a reaction in which two or more reactants react together to form a single compound.
For example: 1) H₂C=CH₂ + HBr ➨ H₃C-CH₂Br
2) H₂C=CH₂ + HCl ➨ H₃C-CH₂Cl

Addition reactions are undergone by all alkenes and alkynes containing either a double bond or a triple bond. Example: When ethene (C2H4) reacts with hydrogen in the presence of a nickel catalyst, ethane is formed.

An addition reaction occurs when two or more reactants combine to form a single product. Addition reactions occur with unsaturated compounds. Hydrohalogenation involves the addition of a hydrogen atom and a halogen atom to an unsaturated compound (containing a carbon-carbon double bond).
CH2=CH2 + Cl2 → CH2Cl−CH2Cl.