When water is added to compound (A) of calcium, compound B is formed with a hissing sound and evolution of large amount of heat. When `CO_2` , is passed into the solution, it turns milky due to formation of compound C. If excess `CO_2` 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

(i) When water is added to compound (A) of calcium, solution of compound (B) is formed .
`underset((A))CaO+H_2Otounderset((B))Ca(OH)_2`
(ii) On passing `CO_2` into compound B, it forms compound (C )
`Ca(OH)_2+CO_2tounderset("Milky precipitate")CaCO_3+H_2O`
(iii) If excess of `CO_2` is passed to compound D is formed . `underset((C ))CaCO_3+H_2O+CO_2tounderset((D))Ca(HCO_3)_2`
(iv) The milkiness disappears when excess of `CO_2` is passed as soluble calcium bicarbonate is formed.