Explain why a solid has a fixed shape and cannot be compressed, whilst a gas has no fixed shape and can be compressed easily.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
In a solid the particles are arranged in a fixed, regular pattern and are held in place by strong forces of attraction. Because the particles are so close together there are almost no spaces between them, so the solid keeps its shape and cannot be compressed.
In a gas the particles are far apart and the forces of attraction are very weak. The large spaces between the particles mean the gas has no fixed shape and the particles can be pushed closer together, so the gas can be compressed easily.
In a gas the particles are far apart and the forces of attraction are very weak. The large spaces between the particles mean the gas has no fixed shape and the particles can be pushed closer together, so the gas can be compressed easily.
Examiner tips
- Use the words "fixed shape" and "cannot be compressed" for solids, and "no fixed shape" and "can be compressed" for gases.
- Explain the role of particle arrangement and forces of attraction.
- Mention the lack of spaces in solids and the large spaces in gases.
Common mistakes
- Confusing the behaviour of liquids with solids or gases.
- Forgetting to mention the forces of attraction or the particle spacing.
- Using vague terms like "tight" or "loose" instead of "fixed arrangement" and "large spaces".
Mark scheme (4 marks)
- In a solid, particles are held in fixed positions (in a regular arrangement)
- Strong forces of attraction between particles in a solid hold them in place, giving it a fixed shape
- In a solid, there are no/negligible spaces between particles, so it cannot be compressed
- In a gas, particles are far apart with large spaces between them, so the gas has no fixed shape and can be compressed (particles can be pushed closer together)
Key terms in this question
Related
- All Cambridge International IGCSE Chemistry (0620) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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