Explain why a gas has a much lower density than a solid made from the same substance.

Edexcel GCSE Physics (1PH0) — 14.1 Particle model · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

In a solid the particles are tightly packed together, so the spacing between them is very small.
In a gas the particles are spread far apart, so the spacing between them is much greater.
Because the particles in a gas are so far apart, the same mass occupies a much larger volume.
Since density is mass per unit volume, a larger volume for the same mass gives a much lower density.

Examiner tips

Use the term ‘spacing between particles’ to show understanding of structure.
Explain that density = mass/volume and link the larger volume to lower density.
Keep the answer concise and directly address the comparison.

Common mistakes

Confusing density with mass or volume alone.
Using vague terms like ‘loose’ instead of ‘spaced far apart’. Failing to mention that the mass is the same in both cases.

Mark scheme (4 marks)

In a solid, particles are tightly packed together / very close together
In a gas, particles are spread far apart / the spacing between particles is much greater
The much greater spacing means the volume occupied by the same mass of gas is far larger
Since density is mass per unit volume, a larger volume for the same mass means a much lower density

Key terms in this question

density · gas · solid