Which gas is most likely denser than air, does not support combustion, and turns limewater cloudy?

• A. Ammonia
• B. Hydrogen
• C. Carbon Dioxide
• D. Methane

Answer: (C)

The gas that fits all three clues is carbon dioxide. It is denser than air, so it tends to linger in the lower parts of a container. It does not support combustion; flames are extinguished in CO2-rich environments. The limewater test is a classic confirmatory reaction: CO2 dissolves in water to form carbonic acid, which reacts with calcium hydroxide to produce calcium carbonate, a solid that makes the limewater appear cloudy or milky. The other gases don’t meet all the criteria: ammonia is lighter than air and does not cloud limewater, while hydrogen and methane are flammable and do not cause the limewater to cloud in the same way.