On mixing the clear solutions of two ionic compounds, a substance which is insoluble in water, is formed. This insoluble substance formed is known as precipitate. Any reaction that produces a precipitate is called a precipitation reaction.
When sodium sulphate solution is mixed with barium chloride solution, a white substance BaSO4 is formed.
Na2SO4(aq) + BaCl2(aq) --------> BaSO4(s)↓ +2NaCl(aq)
The white precipitate of BaSO4 is formed by the reaction of SO4-2and Ba2+. The other product formed is sodium chloride which remains in the solution.