When calcium carbonate (CaCO3) is heated, it undergoes a decomposition reaction to form calcium oxide (CaO) and carbon dioxide (CO2):
\[ \text{CaCO}_3 (s) \rightarrow \text{CaO} (s) + \text{CO}_2 (g) \]
In this reaction, the mass of the reactants is equal to the mass of the products, according to the law of conservation of mass.
Given that you start with 30 grams of calcium carbonate (CaCO3), the total mass of the products (calcium oxide and carbon dioxide) will also be 30 grams immediately after the reaction, but the carbon dioxide will be released as a gas.
Therefore, if you are considering only the mass of the solid product (CaO) and the gaseous product (CO2) does not remain with the system, the initial mass will still be 30 grams due to the mass conservation principle.
Thus, the total mass of the products remains 30 grams.