When bubbling gas through limewater, what indicates the presence of carbon dioxide?

When carbon dioxide is bubbled through limewater, which is a solution of calcium hydroxide, the presence of this gas is indicated by a cloudiness that develops in the clear solution. This cloudiness occurs because carbon dioxide reacts with calcium hydroxide to form calcium carbonate, which is insoluble in water. As a result, calcium carbonate precipitates out of the solution, causing it to appear cloudy.

The formation of this precipitate is a classic test for carbon dioxide, making it possible to visually confirm the gas's presence. The other choices do not accurately reflect the chemical reaction that takes place; for instance, if the solution becomes clear, changes to blue, or shows no visible change, none of these responses would correctly indicate the presence of carbon dioxide in the limewater. Hence, the development of a cloudy solution is the definitive evidence of carbon dioxide's presence.