Explain what happens to the internal energy of a fixed mass of water as it is heated from 20 °C until it has completely turned to steam at 100 °C.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
The internal energy of a fixed mass of water rises as it is heated from 20 °C to 100 °C because the kinetic energy of the molecules increases with temperature. When the water reaches 100 °C it begins to boil; the temperature remains constant during the phase change, but the internal energy continues to increase as energy is supplied to break the intermolecular bonds. Thus the potential energy of the molecules rises while the kinetic energy stays the same, and the water is converted into steam.
Examiner tips
- Use the term ‘internal energy’ and explain the two contributions (kinetic and potential).
- Show that temperature is constant during boiling but energy is still transferred.
- Mention that the energy supplied is latent heat of vaporisation.
- Use UK spelling (e.g. ‘behaviour’).
Common mistakes
- Confusing temperature change with internal energy change during boiling.
- Saying the internal energy stops increasing once boiling starts.
- Using ‘heat’ instead of ‘internal energy’ in the explanation.
Mark scheme (5 marks)
- Internal energy increases as water is heated from 20 °C to 100 °C
- Because the kinetic energy of the molecules increases (as temperature rises)
- During boiling/change of state, the temperature stays constant / does not change
- During boiling, internal energy still increases / energy is still being transferred to the water
- The potential energy of the molecules increases during boiling as intermolecular bonds/forces are broken / molecules are separated
Key terms in this question
Related
- All AQA A-Level Physics (7408) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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