Explain why the smell of a perfume sprayed in one corner of a room can eventually be detected throughout the entire room, whilst the glass bottle containing the perfume keeps its own shape on the shelf.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A person sprays perfume in one corner of a room. After a short time, the smell can be detected across the whole room. The glass bottle the perfume came from remains unchanged on the shelf.
Model answer (4 marks)
Perfume molecules are released into the air and enter the gas phase. In a gas the particles move randomly in all directions and are widely spaced, so they spread through the whole room until the concentration is uniform. The glass bottle, however, is a solid. Its particles are arranged in a fixed, regular lattice and have very little kinetic energy, so they cannot move from their positions. Consequently the bottle keeps its shape while the perfume molecules disperse.
Examiner tips
- Use the word "gas" for perfume vapour and "solid" for the bottle
- Mention random motion and large gaps for the gas
- Explain fixed lattice and low energy for the solid
- Keep the answer concise and use correct terminology
Common mistakes
- Confusing the perfume with the bottle as a solid, or saying the bottle evaporates
- Using vague terms like "spread out" without linking to gas phase
- Omitting the fixed lattice explanation for the bottle
Mark scheme (4 marks)
- Perfume particles in the gas (vapour) phase move randomly in all directions
- Gas particles are relatively spread out / have large gaps between them, allowing them to fill the room
- The particles in a solid (glass bottle) hold a regular arrangement
- Solid particles have the least amount of energy and cannot move from their fixed positions, so the solid keeps its shape
Related
- All OCR GCSE Chemistry A: Gateway Science (J248) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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