Sulfate ions in solution are identified with barium chloride solution. First the sample is acidified with dilute hydrochloric acid, then barium chloride solution is added. If sulfate ions are present, a white precipitate of barium sulfate forms.
The ionic equation is Ba2+(aq) + SO42-(aq) → BaSO4(s). For sodium sulfate the full equation is Na2SO4 + BaCl2 → BaSO4 + 2NaCl, so the solution left behind contains sodium chloride.
The dilute hydrochloric acid is added first to remove carbonate ions. Carbonate ions would also give a white precipitate with barium ions, which would be mistaken for sulfate.
In required practical 7 you use the tests for flame colours, metal hydroxides, carbonates, halides and sulfates to identify the ions in unknown single ionic compounds. Do one test at a time, record each result, and use all the results together to decide the compound.