# Magnesium reacts with hydrochloric acid according to the equation: Mg + 2HCl → MgCl₂ + H₂. Explain why the mass of the products always equals the mass of the reactants in this reaction, and state what happens to the total number of atoms during the reaction.

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

> Magnesium ribbon is added to excess hydrochloric acid in a sealed flask. The flask is weighed before and after the reaction. The total mass does not change.

## Mark scheme (5 marks)

1. The law of conservation of mass states that no atoms are lost or made during a chemical reaction
2. The atoms are simply rearranged / the same atoms are present before and after the reaction
3. Because the total number of atoms is the same on both sides of the equation, the total mass of products equals the total mass of reactants
4. The total number of atoms does not change / stays the same during the reaction
5. The equation is balanced, showing equal numbers of each type of atom on both sides (e.g. 1 Mg, 2 H, 2 Cl on each side)

## Key terms

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

## 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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