A solid compound $$X$$ on heating gives $$C{O_2}$$  gas and a residue. The residue mixed with water forms $$Y.$$ On passing an excess of $$C{O_2}$$  through $$Y$$ in water, a clear solution $$Z$$  is obtained. On boiling $$Z,$$ compound $$X$$ is reformed. The compound $$X$$ is

Solution :
$$\eqalign{ & {\text{Compound }}X{\text{ is}}\,\,CaC{O_3} \cr & \mathop {CaC{O_3}}\limits_X \mathop \to \limits^\vartriangle \mathop {CaO}\limits_{{\text{Residue}}} + C{O_2} \uparrow \cr & \mathop {CaO}\limits_{{\text{Residue}}} + {H_2}O \to \mathop {Ca{{\left( {OH} \right)}_2}}\limits_Y \cr & \mathop {Ca{{\left( {OH} \right)}_2}}\limits_Y + \mathop {C{O_2}}\limits_{{\text{Excess}}} + {H_2}O \to \mathop {Ca{{\left( {HC{O_3}} \right)}_2}}\limits_Z \cr & \mathop {Ca{{\left( {HC{O_3}} \right)}_2}}\limits_Z \mathop \to \limits^\vartriangle \mathop {CaC{O_3}}\limits_X + {H_2}O + C{O_2} \uparrow \cr} $$