Hi,
On heating calcium carbonate it gives calcium oxide and carbon dioxide.
CaCO3 → CaO + CO2
Mass of Calcium carbonate = 100g
Mass of CaO = 56 g
Mass of carbon dioxide = 44g
According to law of mass of conservation
Mass of reactant ( CaCO3 ) = Mass of product (CaO + CO2)
100g = 56 + 44
100g = 100 g
Mass of reactant and product is constant.
Regards