# Magnesium burns in oxygen according to the balanced equation: 2Mg + O₂ → 2MgO. A student burns magnesium in a limited supply of oxygen and finds that the reaction stops before all the magnesium is used up. Explain why the reaction stops, identify which reactant is the limiting reactant, and explain what the law of conservation of mass means for this reaction.

> WJEC A-Level Chemistry (Wales) — 3.1 Introduction to organic chemistry · Explain · 5 marks

> Magnesium is a reactive metal that burns brightly in oxygen to form magnesium oxide. The balanced equation for this reaction is: 2Mg + O₂ → 2MgO.

## Mark scheme (5 marks)

1. The reaction stops because the oxygen is completely used up / runs out
2. Oxygen is the limiting reactant because it is completely used up and limits the amount of product formed
3. The magnesium is in excess / not all the magnesium reacts
4. The law of conservation of mass states that no atoms are lost or made during a chemical reaction
5. The mass of the products (magnesium oxide) equals the mass of the reactants that actually react (the magnesium and oxygen that are used up)

## Key terms

- [limiting reactant](https://www.gradenine.co.uk/glossary/limiting-reactant)
- [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/magnesium-burns-in-oxygen-according-to-141e28d5) · Published by Druglandscape Ltd.