Which reagent confirms sulfate ions, and what is the precipitate?

• A. Acidified barium chloride solution — White precipitate
• B. Silver nitrate solution — White precipitate
• C. Sodium hydroxide solution — Blue precipitate
• D. Ammonium chloride solution — White precipitate

Answer: (A)

Sulfate ions are identified by forming an insoluble salt with a barium ion. When acidified barium chloride is added, the sulfate ions meet Ba2+ to give barium sulfate, BaSO4, which precipitates as a white solid. The medium is kept acidic to avoid interference from carbonate ions that would also precipitate with barium. The result—a clear, white precipitate of BaSO4—is a distinctive, reliable indication that sulfates are present.

Other reagents don’t give that same clear signal for sulfates. For example, silver nitrate can also form a precipitate with sulfate, but this test is less specific and can be muddled by other ions; sodium hydroxide would not produce the characteristic sulfate precipitate, and ammonium chloride is just a salt that wouldn’t yield the diagnostic white solid with these conditions.