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.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Steel has a density of approximately 7800 kg/m³. Expanded polystyrene has a density of approximately 10–30 kg/m³.
Model answer (4 marks)
Density is the mass per unit volume of a substance or object.
Steel has a much greater mass for the same volume compared to expanded polystyrene.
Expanded polystyrene contains a large volume of trapped air, giving it a very large volume for its mass.
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.
Steel has a much greater mass for the same volume compared to expanded polystyrene.
Expanded polystyrene contains a large volume of trapped air, giving it a very large volume for its mass.
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.
Examiner tips
- Define density as mass per unit volume. Show the comparison of mass for equal volumes. Explain the role of trapped air in polystyrene’s volume. Link the mass/volume ratio to the density difference.
Common mistakes
- Using the wrong definition of density (e.g., volume per mass). Failing to mention the role of trapped air or large volume in polystyrene. Not explicitly stating that density = mass ÷ volume.
Mark scheme (4 marks)
- Density is the mass per unit volume (of a substance/object).
- Steel has a much greater mass for the same volume compared to polystyrene.
- Expanded polystyrene contains a large volume of trapped air / has a very large volume for its mass.
- 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 in this question
Related
- All Cambridge International IGCSE Physics (0625) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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