A compound A on heating gives a colorless 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 :

 $\mathrm{A}\stackrel{∆}{\to } \mathrm{colorless} \mathrm{gas} + \mathrm{residue}$

$\mathrm{residue}+{\mathrm{H}}_2\mathrm{O}\to \mathrm{B} \stackrel{\mathrm{excess} {\mathrm{CO}}_2}{\to }\mathrm{C}\stackrel{∆}{\to } \mathrm{A}$

This is possible only when A is CaCO₃​. The reactions are as follows 

${\mathrm{CaCO}}_3\stackrel{∆}{\to } \underset{\mathrm{colourless} \mathrm{gas}}{{\mathrm{CO}}_2}+ \underset{\mathrm{residue}}{\mathrm{CaO} }$

$\mathrm{CaO}+{\mathrm{H}}_2\mathrm{O} \to \underset{\left(\mathrm{B}\right)}{\mathrm{Ca}{\left(\mathrm{OH}\right)}_2} \underset{\mathrm{excess}}{\overset{{\mathrm{CO}}_2}{\to }} \underset{\left(\mathrm{C}\right)}{\mathrm{Ca}{\left({\mathrm{HCO}}_3\right)}_2} \stackrel{∆}{\to } {\mathrm{CaCO}}_3$

Thus, calcium carbonate on heating gives colorless 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.