Copper carbonate decomposes when heated according to the balanced equation: CuCO₃ → CuO + CO₂. A student heats a sample of copper carbonate and notices that the total mass of the reaction vessel and its contents decreases during the reaction. Using the law of conservation of mass, explain why the mass appears to decrease, and describe what happens to the number of atoms of each element during this reaction.

The law of conservation of mass states that the total number of atoms of each element is unchanged in a chemical reaction. In the decomposition of copper carbonate, CuCO₃ → CuO + CO₂, the carbon and oxygen atoms that were in the carbonate ion are now distributed between the solid CuO and the gaseous CO₂. The copper atoms remain in the solid CuO. Because CO₂ is a gas, it leaves the reaction vessel and is not included in the measured mass. Therefore the mass of the contents of the vessel decreases, even though the total mass of the system (reactants + products) is conserved. The number of copper, carbon and oxygen atoms is the same on both sides of the equation, so no atoms are lost or created.

Examiner tips: 1) State the law of conservation of mass. 2) Identify CO₂ as a gas that escapes. 3) Explain that the apparent mass loss is due to the gas leaving the vessel. 4) Show that atom counts are unchanged.

Common mistakes: 1) Forgetting that CO₂ is a gas. 2) Claiming atoms are lost. 3) Not linking the mass loss to the escape of CO₂.