Which observation confirms carbon dioxide is present when bubbled through limewater?

• A. Limewater turns milky.
• B. Limewater turns blue.
• C. Limewater remains clear.
• D. Limewater turns red.

Answer: (A)

Carbon dioxide reacts with limewater (calcium hydroxide) to form calcium carbonate, which is only slightly soluble and appears as a white cloudy precipitate. That milky cloud is the telltale sign: tiny particles scatter light and make the solution look opaque, confirming CO2 is present. The other options don’t fit because CO2 in limewater doesn’t cause a blue or red color change, and a clear limewater means no precipitation has formed.