A perfume bottle is opened at one end of a room. After a short time, a person standing at the other end of the room can smell the perfume. Using kinetic theory, explain why the perfume smell spreads through the room and why it takes some time to reach the other end.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
Perfume molecules are in constant random motion.
They collide with air molecules and spread out in all directions.
This spreading from a region of higher concentration to a region of lower concentration is diffusion.
Because the perfume molecules must travel a long, indirect path, undergoing many collisions with air molecules, they do not move straight across the room.
Hence the net movement of perfume molecules across the room takes time and is slow compared with the speed of the individual molecules.
They collide with air molecules and spread out in all directions.
This spreading from a region of higher concentration to a region of lower concentration is diffusion.
Because the perfume molecules must travel a long, indirect path, undergoing many collisions with air molecules, they do not move straight across the room.
Hence the net movement of perfume molecules across the room takes time and is slow compared with the speed of the individual molecules.
Examiner tips
- Use the term "diffusion" explicitly.
- Explain that collisions cause indirect, long paths.
- Mention that the net movement is slow compared with individual speeds.
- Keep the answer concise and use the exact wording from the mark scheme.
Common mistakes
- Failing to mention diffusion as the mechanism.
- Describing the motion as a straight line rather than random collisions.
- Using vague terms like "spread" without explaining the role of collisions.
Mark scheme (5 marks)
- Perfume particles / molecules are in constant random motion
- Perfume particles collide with air particles / molecules and spread out in all directions
- This spreading of particles from a region of higher concentration to lower concentration is called diffusion
- The perfume particles must travel a long / indirect path because they undergo many collisions with air molecules
- The overall (net) movement of perfume particles across the room therefore takes time / is slow compared to the speed of individual particles
Key terms in this question
Related
- All WJEC A-Level Physics (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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