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))`