When water is heated from room temperature until it boils, its particles gain energy and the state changes from liquid to gas. Explain, in terms of particles and intermolecular forces, why water has a fixed volume as a liquid but no fixed volume as a gas.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Water exists as a liquid at room temperature and pressure. When heated to 100 °C it becomes a gas (steam).
Model answer (4 marks)
In a liquid the particles are close together and are in contact with each other. Intermolecular forces (hydrogen bonding and dipole–dipole) hold the particles within a fixed space, so the liquid has a fixed volume. In a gas the particles have much more kinetic energy and move rapidly in all directions, so they are widely and randomly spread out. The intermolecular forces are effectively overcome or negligible, so the particles are not held together and the gas expands to fill the container, giving no fixed volume.
Examiner tips
- Use the words ‘close together’ and ‘intermolecular forces’ for the liquid; mention ‘fixed volume’.
- For the gas, state ‘high kinetic energy’, ‘widely spread out’ and ‘forces negligible’, leading to ‘no fixed volume’.
- Keep the answer concise – 4 marks, one point each.
Common mistakes
- Confusing the role of temperature with volume; saying the liquid expands when heated.
- Forgetting to mention that the gas has no fixed volume because the forces are negligible.
- Using vague terms like ‘bigger’ or ‘more space’ instead of ‘fixed volume’ or ‘no fixed volume’.
Mark scheme (4 marks)
- In a liquid, particles are close together / touching / in contact with each other
- In a liquid, intermolecular forces hold the particles together / keep them within a fixed space, giving a fixed volume
- In a gas, particles have much more energy and move rapidly in all directions / are widely/randomly spread out
- In a gas, intermolecular forces are (effectively) overcome / negligible / absent, so particles are not held together and expand to fill any container, giving no fixed volume
Key terms in this question
intermolecular forces · liquid · gas · particles · fixed volume
Related
- All Pearson Edexcel International GCSE Chemistry (4CH1) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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