Q-} why does milkiness disappear from limewater when excess co2 is passes through it??

When carbon dioxide gas is passed through lime water, it turns milky due to the formation of calcium carbonate which is insoluble in water. 

 Ca(OH)2 + CO2 → CaCO3 + H2O

However, when excess of carbon dioxide is passed through this solution, the milkyness disappears. This is due the formation of calcium bicarbonate, which is soluble in water. Thus, a clear solution is obtained. 

 CaCO3 + H2O + CO2 → Ca(HCO3)2

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when carbon dioxide is passed through lime water, a white precipitate of calcium carbonate is formed.

If excess CO2is added, the following reaction takes place:

CaCO3+ H2O + CO2→Ca (HCO3)2

The product formed Ca (HCO3)2is colorless and is soluble in the solution.

Hope it helps.

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