Which qualitative test demonstrates the production of carbon dioxide in a reaction?

• A. Bubble the gas through limewater; limewater turns cloudy.
• B. Heat the gas to observe a color change.
• C. Bubble the gas through distilled water; the water remains clear.
• D. Pass the gas over a substance to form fumes.

Answer: (A)

When testing a gas for carbon dioxide, the key clue is a visible chemical change that CO2 can cause. Bubbling the gas through limewater (a solution of calcium hydroxide) produces a milky, cloudy suspension. This happens because CO2 reacts with Ca(OH)2 to form calcium carbonate, which is insoluble and appears as a white precipitate: CO2 + Ca(OH)2 → CaCO3(s) + H2O. The cloudiness confirms the gas is CO2.

The other ideas don’t give that same distinct sign. Heating the gas doesn’t produce a characteristic color change for CO2. Bubbling through distilled water or simply passing the gas to form fumes doesn’t generate the milky appearance that Ca(OH)2 gives with CO2.