# A student carries out a chemical reaction and notices that the total mass of the products is less than the total mass of the reactants she started with. She claims this proves the law of conservation of mass is wrong. Explain why the student's claim is incorrect, and explain how the law of conservation of mass applies to all chemical reactions.

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

> A student heats a metal carbonate in an open crucible. She records the mass before and after heating and finds that the mass has decreased.

## Mark scheme (5 marks)

1. No atoms are lost or made during a chemical reaction
2. The mass of the products equals the mass of the reactants
3. A gas (carbon dioxide) is produced and escapes into the atmosphere / is not collected
4. If the mass of the escaped gas were included / the reaction were carried out in a closed container, the total mass would remain the same
5. The student's claim is incorrect because the decrease in mass is explained by the loss of a gaseous product, not by atoms being destroyed

## 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/a-student-carries-out-a-chemical-c1674c13) · Published by Druglandscape Ltd.