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 disppears in the last step.

Explanation

When `CO_2` gas is passed through solution of compound B, it turns milky, therefore , solution of compound B is `Ca(OH)_2`.
The milkiness is due to the formation of `CaCO_3(C)`. Since compound B is obtained by dissolving compound (A) in water , compound A must be quick lime , CaO. The reactions are :
`underset("Calcium oxide (A)")(CaO) + H_2O to underset("Lime water(B) ")(Ca(OH)_2)`
`Ca(OH)_2 + CO_2 to underset("Calcium carbonate (C) (Milkiness)")(CaCO_3) + H_2O`
Milkiness disappears on passing excess `CO_2` because of the formation of soluble calcium bicarbonate.
`CaCO_3 + CO_2 + H_2O to underset("Soluble (D) Milkiness disappears")(Ca(HCO_3)_2)`