# Magnesium burns in oxygen to form magnesium oxide. The balanced equation for this reaction is: 2Mg + O₂ → 2MgO. A student burns a piece of magnesium ribbon in a sealed container. After the reaction is complete, the student claims that the total mass of the container and its contents has increased. Explain why the student's claim is incorrect, and describe what the law of conservation of mass tells us about the masses of the reactants and products in this reaction.

> Eduqas A-Level Chemistry — 3.1 Introduction to organic chemistry · Explain · 5 marks

## Mark scheme (5 marks)

1. The student's claim is incorrect / the total mass does not increase
2. No atoms are lost or made during a chemical reaction
3. The mass of the products equals the mass of the reactants
4. The oxygen gas is used up / included as a reactant in the sealed container, so no mass is gained from outside
5. The total mass of magnesium and oxygen (reactants) equals the total mass of magnesium oxide (product)

## Key terms

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

## Related

- [Revision notes for Eduqas A-Level 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/magnesium-burns-in-oxygen-to-form-7a9fb481) · Published by Druglandscape Ltd.