Explain why the temperature of a substance does not change while it is melting, even though it is being heated.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A student heats a block of ice and notices that the temperature stays at 0 °C for a period of time, even though the heater is switched on.
Model answer (5 marks)
Energy is supplied to the ice as heat.
The heat is used to break the hydrogen bonds between the water molecules, i.e. to increase the potential energy of the system.
Because this energy goes into changing the state rather than increasing kinetic energy, the average kinetic energy of the molecules does not rise.
Temperature is a measure of average kinetic energy, so the temperature remains at 0 °C.
Thus the internal energy rises while the temperature stays constant during the phase change.
The heat is used to break the hydrogen bonds between the water molecules, i.e. to increase the potential energy of the system.
Because this energy goes into changing the state rather than increasing kinetic energy, the average kinetic energy of the molecules does not rise.
Temperature is a measure of average kinetic energy, so the temperature remains at 0 °C.
Thus the internal energy rises while the temperature stays constant during the phase change.
Examiner tips
- State that heat is used to break bonds, not to increase kinetic energy. Explain that temperature is linked to kinetic energy, not potential energy. Use the phrase ‘phase change’ to show understanding of melting.”]
- common_mistakes
- :
- Saying the temperature rises during melting. Claiming the heat is wasted or that the ice simply ‘absorbs’ heat. Using vague terms like ‘energy is stored’ without linking to bonds or potential energy.
Mark scheme (5 marks)
- Energy is supplied to the substance during melting
- The energy is used to break the bonds / intermolecular forces between the particles
- The energy does not increase the kinetic energy of the particles
- Temperature is related to the average kinetic energy of the particles, so temperature does not rise
- The energy is stored as potential energy (of the particles) / increases the internal energy of the substance without raising temperature
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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