Which two tests are described for confirming carbon dioxide?

• A. Limewater turning milky and extinguishing a glowing splint
• B. Only limewater turning milky
• C. Only extinguishing a glowing splint
• D. None

Answer: (A)

The key idea is that carbon dioxide has two reliable signs you can test for: it reacts with limewater to make the solution cloudy, and it does not support combustion, so a glowing splint is extinguished when placed in it. The limewater test works because CO2 reacts with calcium hydroxide to form calcium carbonate, which appears as a milky precipitate. The glowing splint test works because CO2 doesn’t provide enough oxygen to keep a flame going, so the glowing splint goes out. When you use both tests together, you get strong confirmation that the gas is carbon dioxide. Relying on only one test isn’t as conclusive, and a test that uses neither doesn’t identify CO2 at all.