Limestone, \(\mathrm{CaCO_3}\), is heated in a sealed container and decomposes into calcium oxide and carbon dioxide. If the carbon dioxide does not escape, what is the correct conclusion about the total mass of substances before and after the reaction?

Correct answer: The total mass will remain unchanged.

The reaction is \(\mathrm{CaCO_3 \rightarrow CaO + CO_2}\). In a sealed container, the \(\mathrm{CO_2}\) produced remains inside, so the total mass of the products includes both \(\mathrm{CaO}\) and \(\mathrm{CO_2}\). By the law of conservation of mass, this total equals the mass of the original \(\mathrm{CaCO_3}\). Formation of a gas does not reduce total mass; it would decrease only if the gas escaped from the container.