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

Appearance of milkiness on passing `CO_(2)` in the solution of compound B indicates that compound B is lime water and compound C is `CaCO_(3)`. Since, compound B is obtained by adding `H_(2)O` to compound A, therefore, compound A is quicklime, `CaO`.
The reactions are as follows
(i) `underset((A))underset("Calcium oxide")(CaO)+H_(2)O rarr underset((B))underset("Lime water")(Ca(OH)_(2))`
(ii) `underset((B))(Ca(OH)_(2))+CO_(2)rarr underset(("Milkiness"))underset("Calcium carbonate")(CaCO_(3))+underset((C))(H_(2)O)`
(iii) When excess of `CO_(2)` is passed, milkiness disappears due to the formation of soluble calcium bicarbonate (D).
`underset((C))underset("Milkiness")(CaCO_(3))+CO_(2)+H_(2)Orarr underset((D))underset(("SOluble in"H_(2)O))underset("Calcium bicarbonate")(Ca(HCO_(3))_(2))`