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The compound A on heating gives a colourless gas and a residue that is dissolved in water to obtain B. Excess of CO2 is bubbled through aqueous solution of B, C is formed which is recovered in the solid form. Solid C on gentle heating gives back A. The compound is

1. CaSO4·2H2O
2. CaCO3
3. Na2CO3
4. K2CO3

Correct answer: CaCO3

Solution

The compound A is CaCO3. On heating, it decomposes to give CO2 (a colorless gas) and CaO (residue). CaO dissolves in water to form Ca(OH)2 (B). Bubbling excess CO2 through Ca(OH)2 forms CaCO3 (C), which can be recovered as a solid. Gentle heating of CaCO3 regenerates CaCO3, completing the cycle.

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