# Magnesium reacts with hydrochloric acid according to the equation: Mg + 2HCl → MgCl₂ + H₂. Explain why the law of conservation of mass means that this equation must be balanced, and describe what the balancing numbers in this equation tell us about the amounts, in moles, of each substance involved.

> Eduqas A-Level Chemistry — 3.6 Amines and polymers (A-Level only) · Explain · 5 marks

> Magnesium is a reactive metal that fizzes vigorously when added to hydrochloric acid, producing a colourless gas.

## Mark scheme (5 marks)

1. The law of conservation of mass states that no atoms are lost or made during a chemical reaction
2. Therefore the mass of the products equals the mass of the reactants
3. 1 mole of Mg reacts with 2 moles of HCl
4. 1 mole of MgCl₂ is produced
5. 1 mole of H₂ gas is produced

## Key terms

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

## 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-reacts-with-hydrochloric-acid-according-e1f0f5fd) · Published by Druglandscape Ltd.