Reaction with water
Carbon dioxide reacts with water to form the weak acid, carbonic acid, H2CO3:
CO2 + H2O → H2CO3
The acid cannot be isolated from the solution and so is often simply written as CO2(aq). Carbonic acid causes the acidity in fizzy drinks.
Reaction with limewater
Carbon dioxide reacts with limewater (a solution of calcium hydroxide, Ca(OH)2), to form a white precipitate (appears milky) of calcium carbonate, CaCO3. Adding more carbon dioxide results in the precipitate dissolving to form a colourless solution of calcium hydrogencarbonate.
Ca(OH)2(aq) + CO2(g) → CaCO3(s) + H2O(l)
CaCO3(s) + CO2(g) + H2O(l) → Ca(HCO3)2(aq)
This is used as a test for carbon dioxide. When carbon dioxide is bubbled through limewater the limewater changes from colourless to milky.