Aqueous calcium hydroxide reacts with carbon dioxide to form a white precipitate, calcium carbonate, and water. Carbon dioxide dissolves in water to produce carbonic acid (H2CO3), The carbonic acid reacts with OH- to produce the carbonate ion that precipitates with calcium ion. Here's the equation:
`Ca(OH)_2 + CO_2 -> CaCO_3 + H_2O`
Aqueous calcium hydroxide is called lime water. Lime water can be used as an indicator for carbon dioxide because, when CO2 is bubbled through it, it turns cloudy as the insoluble calcium carbonate is produced. Exhaling through a straw into limewater turns it milky-colored as the carbon dioxide from your lungs reacts. This reaction can be used to observe CO2 production in processes such as fermentation.
Continuing to add CO2 to limewater after it's become cloudy will cause it to become clear again. The carbonic acid that forms reacts with the calcium carbonate to form soluble calcium hydrogen carbonate.