F.4 Chemistry, 10points!

When carbon dioxide gas is passed into lime water, there are two stages of observation because there are two stages of reaction.

(1) Firstly, lime water turns milky.
Lime water is calcium hydroxide solution. When carbon dioxide gas is passed into lime water, the carbon dioxide gas reacts with calcium hydroxide solution to give calcium carbonate and water. Calcium carbonate is insoluble in water, and it exists as a white precipitate. This makes the solution turns milky.
Ca(OH)­2(aq) + CO2(g) → CaCO3­(s) + H2O(l)

(2) When excess carbon dioxide gas is passed into the mixture, the mixture turn clear again.
When excess carbon dioxide gas is passed into the mixture, the calcium carbonate reacts with the excess carbon dioxide gas and water to give calcium hydrogencarbonate. Calcium hydrogencarbonate is soluble in water, and it exists as a colourless solution. This makes the mixture turns clear again.
CaCO3(s) + H2O(l) + CO2(g) → Ca(HCO3)2(aq)