s-Block Elements

A solid compound X on heating gives CO2 gas and a residue. The residue mixed with water forms Y. On passing an excess of CO2 through Y in water, a clear solution Z is obtained. On boiling Z, compound X is reformed. The compound X is

1. Ca(HCO3)2
2. CaCO3
3. Na2CO3
4. K2CO3

Answer

(2) CaCO3

Solution

Heating X: CaCO3 CaO + CO2 (residue = CaO)

Residue + water: CaO + H2O Ca(OH)2 (Y)

Excess CO2: Ca(OH)2 + CO2 CaCO3 + H2O

CaCO3 + H2O + CO2 Ca(HCO3)2 (Z, clear solution)

Boiling Z: Ca(HCO3)2 CaCO3 + H2O + CO2 (X reformed)

∵ Na2CO3 and K2CO3 do not decompose on heating, and Ca(HCO3)2 does not exist as a solid.

∴ X is CaCO3.

Answer: (2)