Explain why the temperature of boiling water does not rise above 100 °C even when a kettle continues to supply energy to it.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A chef notices that no matter how long a pan of water is left boiling on a high flame, a thermometer placed in the water always reads 100 °C. The chef wonders why the extra energy from the flame does not make the water hotter.
Model answer (4 marks)
During boiling the energy supplied by the flame is used to change the state of the water from liquid to vapour rather than to increase the kinetic energy of the molecules.
The energy required for this phase change is the latent heat of vapourisation (specific latent heat of vapourisation). It is the energy needed to overcome the intermolecular forces that hold the water molecules together in the liquid state.
Because the energy is consumed in breaking these forces, the temperature of the water remains constant at the boiling point (100 °C) until all the liquid has been converted to vapour.
Only when the entire quantity of water has boiled away can further energy increase the temperature of the remaining vapour.
The energy required for this phase change is the latent heat of vapourisation (specific latent heat of vapourisation). It is the energy needed to overcome the intermolecular forces that hold the water molecules together in the liquid state.
Because the energy is consumed in breaking these forces, the temperature of the water remains constant at the boiling point (100 °C) until all the liquid has been converted to vapour.
Only when the entire quantity of water has boiled away can further energy increase the temperature of the remaining vapour.
Examiner tips
- Use the term "latent heat of vapourisation" and explain it is the energy to overcome intermolecular forces.
- State that temperature remains constant during a phase change.
- Show that boiling occurs at the boiling point for the given pressure.
- Mention that energy goes into changing state, not raising temperature.
Common mistakes
- Confusing latent heat with specific heat capacity.
- Saying the temperature rises after boiling starts.
- Forgetting to mention the role of intermolecular forces.
Mark scheme (4 marks)
- The energy supplied during boiling is used to change state (from liquid to vapour/gas), not to raise temperature
- This energy is called latent heat (specific latent heat of vaporisation)
- During a change of state the temperature remains constant
- The intermolecular forces between particles must be overcome/broken for the particles to escape into the vapour state, and this requires energy
Key terms in this question
Related
- All OCR GCSE Physics A: Gateway Science (J249) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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