A student cools a sample of liquid wax until it becomes solid. Explain what happens to the internal energy of the wax during this process, referring to both kinetic energy and potential energy of the particles.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
The internal energy of the wax decreases as it cools and solidifies.
During the cooling phase the kinetic energy of the wax molecules falls because the temperature drops.
When the wax freezes the temperature remains constant, so the kinetic energy of the molecules does not change.
However, during the phase change the potential energy of the molecules decreases as they arrange into a more ordered, fixed structure.
During the cooling phase the kinetic energy of the wax molecules falls because the temperature drops.
When the wax freezes the temperature remains constant, so the kinetic energy of the molecules does not change.
However, during the phase change the potential energy of the molecules decreases as they arrange into a more ordered, fixed structure.
Examiner tips
- Use the word ‘decreases’ for internal energy and kinetic energy; mention ‘remains constant’ for temperature during freezing; link potential energy to ordering of particles.
- Include both cooling and phase‑change stages to show full understanding.
Common mistakes
- Saying the kinetic energy increases during freezing; confusing temperature change with kinetic energy change.
- Omitting the potential energy decrease during the phase change; treating the whole process as a single cooling step.
Mark scheme (4 marks)
- The internal energy of the wax decreases (as it cools and solidifies).
- The kinetic energy of the particles decreases as the temperature falls.
- During the change of state (freezing), the temperature stays constant so the kinetic energy of the particles does not change.
- During the change of state, the potential energy of the particles decreases as the particles move into more ordered/fixed positions.
Key terms in this question
internal energy · kinetic energy · potential energy
Related
- All AQA GCSE Physics (8463) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Internal energy and energy transfers questions
- Explain why the temperature of a substance does not change whilst it is melting,…
- Explain what happens to the internal energy of steam as it condenses into liquid…
- Explain what happens to the internal energy of a substance when it is heated and…
- Describe what happens to the internal energy of a block of ice as it is heated f…