# Explain how the law of conservation of mass applies to a chemical reaction carried out in a closed container, and explain why the measured mass appears to decrease when a reaction produces a gas in an open container.

> AQA A-Level Chemistry (7405) — 3.1.2 Amount of substance · Explain · 5 marks

## Mark scheme (5 marks)

1. In a closed container, mass is conserved / total mass of reactants equals total mass of products
2. Atoms are rearranged / no atoms are created or destroyed during a chemical reaction
3. In an open container, when a gas is produced it escapes / is released into the surroundings
4. The mass of the gas that escapes is no longer on the balance / is not measured
5. Mass is still conserved overall / if the gas were collected the total mass would be unchanged

## Key terms

- [law of conservation of mass](https://www.gradenine.co.uk/glossary/law-of-conservation-of-mass)
- [closed container](https://www.gradenine.co.uk/glossary/closed-container)
- [open container](https://www.gradenine.co.uk/glossary/open-container)
- [gas](https://www.gradenine.co.uk/glossary/gas)

## Related

- [Revision notes for AQA A-Level Chemistry (7405)](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/explain-how-the-law-of-conservation-cd762af7) · Published by Druglandscape Ltd.