A student is investigating the specific latent heat of fusion of ice. They place crushed ice into an insulated container and record the temperature every 30 seconds as the ice melts. They notice that the temperature stays at 0 °C for several minutes before it begins to rise. Explain what the student should do to make their results more reliable, and explain why the temperature remains constant at 0 °C even though heat energy is continuously being supplied to the ice.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A student is investigating the specific latent heat of fusion of ice. They place crushed ice into an insulated container and record the temperature every 30 seconds as the ice melts.
Model answer (5 marks)
1. Repeat the experiment several times and calculate a mean value of the time (or temperature) at which the ice remains at 0 °C.
2. Use a thermometer with a fine scale (e.g. 0.1 °C) so small temperature changes can be detected.
3. Stir the mixture regularly or continuously so the temperature reading represents the whole sample.
4. The supplied heat is used to break the intermolecular forces between water molecules, i.e. to change the state from solid to liquid.
5. Because temperature reflects the average kinetic energy of the particles, and this kinetic energy does not increase while the phase change is occurring, the temperature stays at 0 °C.
2. Use a thermometer with a fine scale (e.g. 0.1 °C) so small temperature changes can be detected.
3. Stir the mixture regularly or continuously so the temperature reading represents the whole sample.
4. The supplied heat is used to break the intermolecular forces between water molecules, i.e. to change the state from solid to liquid.
5. Because temperature reflects the average kinetic energy of the particles, and this kinetic energy does not increase while the phase change is occurring, the temperature stays at 0 °C.
Examiner tips
- Use the word ‘repeat’ and ‘mean’ to show statistical reliability. Mention the thermometer resolution and stirring as ways to minimise systematic error. Explain that latent heat is used for breaking bonds, not raising kinetic energy, to justify the constant temperature.
Common mistakes
- Assuming the temperature rises immediately after ice is added. Using a thermometer with too large a scale (e.g. 1 °C). Neglecting to stir, leading to temperature gradients in the mixture.
Mark scheme (5 marks)
- The student should repeat the experiment (at least twice/multiple times) and calculate a mean/average temperature reading (or mean time at 0 °C)
- The student should use a thermometer with a suitable resolution / small scale divisions (e.g. 0.1 °C) to detect small temperature changes accurately
- The student should stir the contents of the container regularly / continuously to ensure the temperature reading is representative of the whole mixture
- The energy supplied is being used to break the intermolecular forces / bonds between particles (to change state) rather than to increase the kinetic energy of the particles
- Because temperature is a measure of the average kinetic energy of the particles, and the kinetic energy of the particles does not increase during the change of state, the temperature does not rise
Key terms in this question
specific latent heat of fusion · temperature
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