When copper carbonate is heated, it undergoes a decomposition reaction, breaking down into copper oxide and carbon dioxide gas. The carbon dioxide gas is released into the atmosphere, which results in a loss of mass from the sample. This process can be represented by the chemical equation:
\[ \text{CuCO}_3 \text{ (s)} \rightarrow \text{CuO (s)} + \text{CO}_2 \text{ (g)} \]
Since the gas escapes and cannot be measured, the total mass of the remaining solid decreases.