When water is added to compound (A) of calcium, solution of compound (B) is formed. When carbon dioxide is passed into the solution, it turns milky due to the formation of compound (C). If excess of carbon dioxide is passed into the solution milkiness disappears due to the formation of compound (D). Identify the compounds A, B, C and D. Explain why the milkiness disappears in the last step.

Explanation

The compound A is calcium oxide , CaO and B is calcium hydroxide , `Ca(OH)_2`.
The compound C is calcium carbonate , `CaCO_3` and the compound D is calcium bicarbonate , `Ca(HCO_3)_2`. Calcium bicarbonate `Ca(HCO_3)_2` is soluble in water and therefore , milkiness of solution disappears on passing excess of carbon dioxide into the solution .
`Ca(OH)_2 + CO_2 to underset("(Milkiness)")(CaCO_3) + H_2O`
`CaCO_3 + CO_2 + H_2O to underset("(Milkiness disappears)")(underset("soluble")(Ca(HCO_3)_2))`