Which observation would indicate sulfate ions when BaCl2 is used?

• A. White precipitate forms
• B. Blue solution forms
• C. No change
• D. Color change to pink

Answer: (A)

The idea being tested is the formation of an insoluble salt when a specific ion is present. When sulfate ions are in the solution, adding BaCl2 provides Ba2+ ions that combine with SO4^2− to form barium sulfate, BaSO4, which is highly insoluble. Because BaSO4 has almost no solubility, it precipitates out as a white solid. That white precipitate is the clear sign that sulfate ions are present. If there are no sulfate ions, no BaSO4 precipitate forms and the solution stays basically unchanged. In practice, the test is often done under slightly acidic conditions to avoid interference from carbonate ions, which could also give a white precipitate with barium. The other visual changes described—blue solution or pink color—do not result from this reaction, so they don’t indicate sulfate presence.