The reaction in which there is an exchange of ions between the reactant takes place is called double displacement reaction.

For example:

Na₂SO₄ + BaCl₂→ BaSO₄ + 2NaCl

Sodium Barium Barium Sodium sulphate chloride sulphate chloride

In the above reaction, the exchange of ions (SO₄^2- and Ba²⁺) takes place. The white precipitate of BaSO₄ is formed by the reaction of SO₄^2- and Ba²⁺. The other product formed NaCl remains in the solution.