Which ion is detected by a qualitative test that forms a white precipitate when acidified aqueous barium nitrate is added?

• A. Sulfate
• B. Nitrate
• C. Chloride
• D. Carbonate

Answer: (A)

The test is looking for sulfate ions. When acidified aqueous barium nitrate is added, the sulfate ions in the solution react with the barium ions to form barium sulfate, BaSO4, which is highly insoluble and appears as a white solid. The acid helps prevent interference from carbonate by converting it to CO2, so only sulfate gives this white precipitate under these conditions. Nitrate ions don’t form an insoluble barium salt, so no precipitate appears. Chloride ions also stay in solution with barium, so no precipitate forms. Carbonate ions would normally precipitate as BaCO3, but the acidic medium dissolves carbonate as CO2, so no precipitate is observed.