Calcium carbonate (CaCO3) on reaction with hydrochloric acid ( HCl) produces, calcium chloride (CaCl2), water (H2O) and carbon dioxide (CO2). CaCO3+2‌HCl→CaCl2+H2O+CO2 When carbon dioxide (CO2) is passed in slaked lime [Ca(OH)2], calcium carbonate (CaCO3) is formed. So, X is (CaCO3). Ca(OH)2+CO2→CaCO3+H2O