A student heats a sample of green copper(II) carbonate powder (CuCO3\text{CuCO}_3CuCO3) strongly in an open boiling tube, as shown in the diagram below.
During the reaction, the green powder turns into a black solid residue, and the total mass of the boiling tube and its contents is found to have decreased. Which statement is the best explanation for this observation?
The green CuCO3\text{CuCO}_3CuCO3 melts and evaporates out of the open boiling tube.
Thermal decomposition occurs, forming solid black CuO\text{CuO}CuO and releasing CO2\text{CO}_2CO2 gas.
Copper(II) carbonate reacts with oxygen in the air to produce CO2\text{CO}_2CO2 gas and copper metal.
The heat destroys some of the carbon and oxygen atoms in the compound, reducing the overall mass.