Double displacement reactions may be defined as the chemical reactions in which one component each of both the reacting molecules get exchanged to form the products. During this reaction, the cations and anions of two different compounds switch places, forming two entirely different compounds. Double displacement reactions generally take place in aqueous solutions in which the ions precipitate and there is an exchange of ions. For example, on mixing a solution of barium chloride with sodium sulphate, a white precipitate of barium sulphate is immediately formed. These reactions are ionic in nature. The reactants changes into ions when dissolved in water and there is an exchange of ions in solution. This results in the formation of product molecule.
This video explains how to perform a double displacement reaction using sodium sulphate and barium chloride solutions.