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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.