If limewater turns cloudy, what gas is likely present?

When limewater, which is a dilute solution of calcium hydroxide, turns cloudy, it indicates the presence of carbon dioxide. The reaction that occurs involves carbon dioxide dissolving in the limewater to form calcium carbonate, which is insoluble in water. This results in the formation of a white precipitate, causing the solution to appear cloudy.

The chemical equation for this reaction is as follows:

[ \text{Ca(OH)}_2 (aq) + \text{CO}_2 (g) \rightarrow \text{CaCO}_3 (s) + \text{H}_2\text{O} (l) ]

This evidence of cloudiness is a classic test for carbon dioxide. Other gases listed, such as oxygen, hydrogen, and nitrogen, do not undergo a similar reaction with limewater and would not cause the solution to turn cloudy. Therefore, the correct identification of the gas responsible for the change in the appearance of limewater is carbon dioxide.