# A student heats calcium carbonate in an open crucible. The student notices that the mass of the crucible and its contents decreases during the reaction. Explain why the mass decreases, even though the law of conservation of mass states that mass is conserved.

> AQA GCSE Chemistry (8462) — 4.3.1 Chemical measurements, conservation of mass and the quantitative interpretation of chemical equations · Explain · 4 marks

> Calcium carbonate undergoes thermal decomposition when heated: calcium carbonate → calcium oxide + carbon dioxide

## Mark scheme (4 marks)

1. Carbon dioxide (gas) is produced as a product
2. The carbon dioxide gas escapes / is released into the atmosphere / surroundings
3. The mass of the gas is not measured / not included in the recorded mass
4. Mass is still conserved because the total mass of calcium oxide AND carbon dioxide equals the original mass of calcium carbonate

## Key terms

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

## Related

- [Revision notes for AQA GCSE Chemistry (8462)](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/a-student-heats-calcium-carbonate-in-50ca87b4) · Published by Druglandscape Ltd.