A 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

The reactions can be summarized as follows : A→∆ colorless gas + residueresidue+H2O→B →excess CO2​C→∆ AThis is possible only when A is CaCO3​. The reactions are as follows CaCO3→∆ CO2​colourless gas+ CaO residue​CaO+H2O →Ca(OH)2(B) →excessCO2 Ca(HCO3)2(C) →∆  CaCO3​Thus, calcium carbonate on heating gives colourless gas carbon dioxide and residue calcium oxide. Calcium oxide reacts with water to form calcium hydroxide. Calcium hydroxide reacts with excess carbon dioxide to form calcium hydrogen carbonate. This on heating gives calcium carbonate.