Zinc carbonate decomposes when heated according to the equation: ZnCO₃ → ZnO + CO₂. When a student heats zinc carbonate in an open crucible, the mass of the solid in the crucible decreases. However, the student claims that the law of conservation of mass has been broken. Explain why the student is incorrect and describe what has happened to the total mass of all substances involved in the reaction.

The law of conservation of mass states that atoms are neither created nor destroyed in a chemical reaction, so the total mass of all products must equal the total mass of the reactants.

In the decomposition of zinc carbonate, ZnCO₃ → ZnO + CO₂, the solid zinc carbonate loses mass because the CO₂ is a gas. The CO₂ escapes from the crucible into the atmosphere and is therefore not included in the mass measured after heating.

If the mass of the released CO₂ were added back to the mass of the remaining solid, the total mass would be the same as the initial mass of ZnCO₃. Thus the student’s claim that mass is lost is incorrect – the mass is conserved, it has simply moved from the solid to the gas phase.