# 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.

> WJEC A-Level Chemistry (Wales) — 3.6 Amines and polymers (A-Level only) · Explain · 5 marks

## Mark scheme (5 marks)

1. The law of conservation of mass states that no atoms are lost or made during a chemical reaction
2. So the mass of the products equals the mass of the reactants
3. Carbon dioxide gas is released / escapes into the surroundings
4. The mass of the solid decreases because the CO₂ gas has left the crucible / is no longer being measured
5. If the mass of CO₂ released were included, the total mass would remain the same / equal the mass of ZnCO₃ at the start

## Key terms

- [conservation of mass](https://www.gradenine.co.uk/glossary/conservation-of-mass)

## Related

- [Revision notes for WJEC A-Level Chemistry (Wales)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/zinc-carbonate-decomposes-when-heated-according-6db6f4e0) · Published by Druglandscape Ltd.