# Explain what is meant by the law of conservation of mass and explain why chemical equations must be balanced.

> WJEC A-Level Chemistry (Wales) — 3.7 Spectroscopy and chromatography · Explain · 5 marks

> When magnesium burns in oxygen, the reaction can be represented by the equation: 2Mg + O₂ → 2MgO

## 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 balanced equation has the same number of atoms of each element on both sides
4. This is because atoms are rearranged but not created or destroyed during a reaction (linking conservation of mass to the need to balance)
5. Correct application to the given equation: there are 2 Mg atoms and 2 O atoms on each side, showing the equation is balanced

## 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/explain-what-is-meant-by-the-5f923f9a) · Published by Druglandscape Ltd.