# Magnesium reacts with hydrochloric acid to form magnesium chloride and hydrogen gas. The balanced symbol equation for this reaction is: Mg + 2HCl → MgCl₂ + H₂. Describe what the balanced symbol equation shows about the atoms and formula units involved in this reaction, and explain why the equation must be balanced.

> Pearson Edexcel International GCSE Chemistry (4CH1) — 1.5 Chemical formulae, equations and calculations · Describe · 4 marks

> Magnesium is a reactive metal that fizzes vigorously when added to dilute hydrochloric acid. The balanced symbol equation for this reaction is: Mg + 2HCl → MgCl₂ + H₂.

## Mark scheme (4 marks)

1. One atom/formula unit of magnesium reacts with two molecules/formula units of hydrochloric acid (HCl)
2. One formula unit of magnesium chloride (MgCl₂) and one molecule of hydrogen (H₂) are produced
3. The total number of each type of atom is the same on both sides of the equation / atoms are conserved
4. Atoms cannot be created or destroyed in a chemical reaction, so the equation must be balanced to reflect this / law of conservation of mass

## Key terms

- [balanced symbol equation](https://www.gradenine.co.uk/glossary/balanced-symbol-equation)
- [atom](https://www.gradenine.co.uk/glossary/atom)
- [formula unit](https://www.gradenine.co.uk/glossary/formula-unit)

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