What is the test and observation used to identify sulfate ions (SO42-)?

To identify sulfate ions (SO4^2-), the correct method involves reacting them with barium chloride. When a solution containing sulfate ions is mixed with barium chloride, a white precipitate of barium sulfate (BaSO4) forms. This reaction is a classic test for the presence of sulfate ions and is highly reliable due to the distinct appearance of the white precipitate, which does not dissolve in water.

The formation of the barium sulfate precipitate is a clear visual indication that sulfate ions are present in the solution. This method leverages the low solubility of barium sulfate in water, making it an effective test. In contrast, the other proposed methods would not provide the correct identification for sulfate ions.