Why does the formation of barium sulphate on mixing aqueous solutions indicate a precipitation reaction?

Correct answer: It forms an insoluble solid that separates from the solution.

In a precipitation reaction, ions from two aqueous solutions combine to form a solid that is insoluble in water. For example,
\(\mathrm{BaCl_2(aq) + Na_2SO_4(aq) \rightarrow BaSO_4(s) + 2NaCl(aq)}\).
\(\mathrm{BaSO_4}\) separates from the solution as an insoluble white solid, so it is a precipitate. In contrast, the soluble substance described in option B would remain in a clear solution rather than form a precipitate.