# A steel paper clip and a block of expanded polystyrene have very different densities, yet both are made mostly of empty space at the atomic level. State what density measures, and explain why steel has a much greater density than expanded polystyrene, referring to mass and volume in your answer.

> Cambridge International IGCSE Physics (0625) — 1.4 Density · Explain · 4 marks

> Steel has a density of approximately 7800 kg/m³. Expanded polystyrene has a density of approximately 10–30 kg/m³.

## Mark scheme (4 marks)

1. Density is the mass per unit volume (of a substance/object).
2. Steel has a much greater mass for the same volume compared to polystyrene.
3. Expanded polystyrene contains a large volume of trapped air / has a very large volume for its mass.
4. Because density = mass ÷ volume, a greater mass in the same (or smaller) volume gives a higher density, so steel's density is much greater than that of polystyrene.

## Key terms

- [density](https://www.gradenine.co.uk/glossary/density)
- [mass](https://www.gradenine.co.uk/glossary/mass)
- [volume](https://www.gradenine.co.uk/glossary/volume)

## Related

- [Revision notes for Cambridge International IGCSE Physics (0625)](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/a-steel-paper-clip-and-a-28720c8e) · Published by Druglandscape Ltd.