Why is the reaction in which barium sulphate forms on mixing aqueous barium chloride and aqueous sodium sulphate called a precipitation reaction?

Correct answer: Because an insoluble solid forms and separates from the aqueous solution

In a precipitation reaction, mixing two aqueous solutions produces an insoluble solid. Here,
\(\mathrm{BaCl_2(aq)+Na_2SO_4(aq)\rightarrow BaSO_4(s)+2NaCl(aq)}\).
\(\mathrm{BaSO_4}\) is an insoluble white solid in water, so it separates from the solution as a precipitate. Evolution of a gas is not the defining feature of a precipitation reaction.