# The law of conservation of mass states that no atoms are lost or made during a chemical reaction. A student burns magnesium ribbon in air and notices that the solid product has a greater mass than the original magnesium ribbon. Explain why the mass of the product is greater than the mass of the magnesium, and explain why this observation does not break the law of conservation of mass.

> WJEC A-Level Chemistry (Wales) — 3.5 Aromatic chemistry (A-Level only) · Explain · 5 marks

## Mark scheme (5 marks)

1. The magnesium reacts with oxygen (from the air) during burning
2. The product (magnesium oxide) contains atoms of both magnesium and oxygen
3. The mass increases because the oxygen atoms from the air have been added to the magnesium
4. No atoms are lost or made — the total mass of magnesium plus oxygen used equals the mass of magnesium oxide formed
5. The law is not broken because if the mass of oxygen used is included alongside the magnesium, the total mass of reactants equals the total mass of the product

## 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)
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Source: [GradeNine](https://www.gradenine.co.uk/q/the-law-of-conservation-of-mass-e01a1a2c) · Published by Druglandscape Ltd.