Explain why a solid has a higher density than the same substance in the gaseous state, in terms of particles and their arrangement.

IB DP Chemistry Standard Level (2023 syllabus) — S1.1 Introduction to the particulate nature of matter · 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 closely packed together, with negligible space between them. In a gas the particles are very far apart, with large distances between them. The same mass of a substance therefore occupies a much larger volume in the gaseous state than in the solid state. Density is mass per unit volume, so a greater volume for the same mass gives a lower density in the gas.

Examiner tips

Use the term ‘density = mass/volume’ to show understanding of the definition.
Show the comparison of particle spacing in solids vs gases.
Explain that the mass is the same in both states.
Keep the answer concise and to the point.

Common mistakes

Confusing mass with volume – forgetting that density depends on both.
Using vague terms like ‘more spread out’ without mentioning ‘large distances’ or ‘negligible space’.
Failing to state that the mass is unchanged between states.

Mark scheme (4 marks)

In a solid, particles are closely/tightly packed together (with negligible space between them).
In a gas, particles are very far apart / widely spaced with large distances/spaces between them.
The same mass/number of particles therefore occupies a much larger volume in the gaseous state than in the solid state.
Density is mass per unit volume, so a greater volume for the same mass gives a lower density in the gas.

Key terms in this question

density · solid · gas

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