When copper carbonate (CuCO₃) is heated, it undergoes a thermal decomposition reaction. Upon heating, copper carbonate breaks down into copper oxide (CuO), carbon dioxide (CO₂), and water (H₂O) in the case of hydrated copper carbonate.
The chemical reaction can be represented as:
\[ \text{CuCO}_3 (s) \rightarrow \text{CuO} (s) + \text{CO}_2 (g) \]
Since carbon dioxide is a gas, it escapes into the atmosphere during the heating process. This loss of gas results in a decrease in the mass of the sample when measured after heating.
Therefore, the reason the copper carbonate sample lost mass upon heating is due to the release of carbon dioxide gas formed during the decomposition of copper carbonate.