Carbon dioxide gas turns lime water milky. Lime water is actually a solution of calcium hydroxide with a chemical formula of Ca(OH)2. When carbon dioxide gas is added to lime water, a white precipitate of calcium carbonate is formed.
Calcium hydroxide reacts with carbon dioxide to form calcium carbonate. The balanced equation for the chemical reaction is Ca(OH)2 + CO2 ? CaCO3 + H2O since the chemical formula of calcium carbonate is CaCO3. Because calcium carbonate is insoluble in water, a precipitate is formed instead. Since calcium carbonate is white, the precipitate formed is white. This white precipitate gives the lime water its milky appearance.