Which test confirms the presence of carbonate ions in solution?

• A. Add barium chloride solution; white precipitate BaSO4 forms (in acidic solution).
• B. Add sodium hydroxide and warm; ammonia produced turns damp red litmus blue.
• C. Add acid to release CO2; CO2 turns limewater cloudy.
• D. Add AgNO3 in acidic solution; white/cream/yellow precipitates.

Answer: (C)

Carbonate ions react with acids to release carbon dioxide. When acid is added to a carbonate-containing solution, fizzing occurs as CO2 escapes. If that CO2 is bubbled into limewater (a solution of calcium hydroxide), it turns the limewater cloudy due to the formation of insoluble calcium carbonate. This combination—gas released on adding acid and limewater turning milky—confirms carbonate ions are present. The other tests target sulfate, ammonium, or halide ions, not carbonates.