A student is heating a block of ice. At first, the temperature of the ice rises. Then, for a period of time, the temperature stops rising even though the student continues to supply heat energy. Explain why the temperature stops rising during this period, using the idea of latent heat.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A student places a block of ice in a beaker and heats it steadily using a Bunsen burner. The student notices that the temperature of the contents remains constant for several minutes before it begins to rise again.
Model answer (4 marks)
The ice is melting from solid to liquid. The heat supplied is used to break the intermolecular bonds between the water molecules, not to increase their kinetic energy. Because temperature reflects the average kinetic energy, the kinetic energy – and therefore the temperature – remains constant. The energy required to change the state at constant temperature is the latent heat of fusion.
Examiner tips
- Use the term ‘latent heat of fusion’ explicitly.
- Explain that heat is used to break bonds, not to raise kinetic energy.
- Mention that temperature is a measure of kinetic energy.
- Keep the answer concise and to the point.
Common mistakes
- Confusing latent heat with specific heat capacity.
- Saying the temperature rises during melting.
- Using vague phrases like ‘energy is wasted’ instead of ‘used to break bonds’.
Mark scheme (4 marks)
- The ice is changing state (melting) from solid to liquid during this period.
- The energy supplied is used to overcome the intermolecular forces between particles / break bonds between particles, rather than increasing their kinetic energy.
- Because temperature is a measure of the average kinetic energy of the particles, and kinetic energy is not increasing, the temperature does not rise.
- The energy required to change state at constant temperature is called latent heat (specific latent heat of fusion for melting).
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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