Explain why a substance in the liquid state has a definite volume but no definite shape, in terms of the arrangement and motion of its particles.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Particles in a liquid are close together, so the volume is fixed and the liquid cannot be easily compressed.
The intermolecular forces are strong enough to keep the particles close, but not strong enough to hold them in fixed positions.
Because the particles can move, slide and flow past one another, the liquid takes the shape of its container.
Thus a liquid has a definite volume but no definite shape.
The intermolecular forces are strong enough to keep the particles close, but not strong enough to hold them in fixed positions.
Because the particles can move, slide and flow past one another, the liquid takes the shape of its container.
Thus a liquid has a definite volume but no definite shape.
Examiner tips
- Use the key phrases ‘definite volume’, ‘no definite shape’, ‘close together’, ‘intermolecular forces’, and ‘particles can move’ to show understanding.
- Show the logical sequence: close packing → fixed volume, weaker forces → no fixed positions, mobility → shape of container.
- Keep the answer concise and directly linked to the mark scheme points.
Common mistakes
- Forgetting to mention the role of intermolecular forces or particle motion.
- Using vague terms like ‘particles are close’ without linking to volume or shape.
- Writing a long paragraph instead of clear, short points.
Mark scheme (4 marks)
- Particles in a liquid are close together / touching, so the volume is fixed / cannot be easily compressed.
- Intermolecular forces in a liquid are strong enough to keep particles close but not strong enough to hold them in fixed positions.
- Particles in a liquid can move / flow / slide past one another.
- Because particles can flow/move past each other, the liquid takes the shape of its container / has no definite shape.
Key terms in this question
definite volume · no definite shape
Related
- All IB DP Chemistry Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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