# A student dissolves a small amount of sodium hydroxide pellets in water inside an open beaker. After the reaction is complete, the student measures the total mass of the beaker and its contents. She notices the mass is slightly greater than the mass of just the sodium hydroxide pellets and water measured separately before mixing. Explain why the total mass appears to have increased, and describe what the law of conservation of mass tells us about what actually happened to the atoms during this process.

> Eduqas GCSE Chemistry — C3 Quantitative chemistry · Explain · 4 marks

> Sodium hydroxide dissolves exothermically in water. The student carried out the experiment on a balance in an open laboratory.

## Mark scheme (4 marks)

1. The law of conservation of mass states that mass (atoms/matter) cannot be created or destroyed in a chemical reaction or physical process
2. The apparent mass increase is because the student did not account for the mass of water vapour / moisture absorbed from the air, OR sodium hydroxide is hygroscopic and absorbs water from the atmosphere
3. No atoms are gained or lost during dissolving — the sodium hydroxide and water particles simply mix / rearrange
4. If the experiment were carried out in a fully closed / sealed system, the total mass before and after would be equal / unchanged

## Key terms

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

## Related

- [Revision notes for Eduqas GCSE Chemistry](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-dissolves-a-small-amount-8e32be97) · Published by Druglandscape Ltd.