# Iron rusts when it reacts with oxygen and water. A student leaves an iron nail in moist air and notices that the mass of the nail increases over time. Explain why the mass of the nail increases, and describe what would happen to the total mass of the nail and the surrounding air as the reaction takes place.

> Edexcel GCSE Chemistry (1CH0) — 1.8 Calculations involving masses · Explain · 4 marks

> Iron can react with oxygen in moist air to form iron oxide (rust). The chemical formula of rust is Fe₂O₃.

## Mark scheme (4 marks)

1. Oxygen (and water) from the air join with the iron to form iron oxide (rust)
2. The mass of the nail increases because atoms of oxygen (and water) from the air are added to it
3. The total mass of the nail plus the surrounding air stays the same / is conserved
4. Because atoms are neither created nor destroyed during a chemical reaction (conservation of mass)

## Related

- [Revision notes for Edexcel GCSE Chemistry (1CH0)](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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