A student is heating a block of ice. The student notices that, whilst the ice is melting, the temperature of the substance does not rise even though heat energy is continuously being supplied. Explain why the temperature does not rise during melting, and describe what happens to the mass of the substance during this change of state.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A block of ice is placed in a beaker and heated steadily on a hotplate. A thermometer placed in the beaker shows that the temperature stays constant whilst ice and liquid water are both present, even though heating continues.
Model answer (4 marks)
During melting the heat supplied is used to break the intermolecular forces between the water molecules – the energy is latent heat of fusion. Because this energy goes into changing the state rather than increasing the average kinetic energy of the particles, the temperature remains constant. The mass of the substance is conserved; the mass of the liquid water produced equals the mass of the ice that has melted.
Examiner tips
- Use the term "latent heat of fusion" to show understanding of the energy involved.
- Explain that the energy is used to break bonds, not to raise kinetic energy, to justify the constant temperature.
- State that mass is conserved – the mass of water equals the mass of ice that has melted.
- Use clear, concise sentences and correct UK spelling.
Common mistakes
- Confusing latent heat with specific heat capacity.
- Saying the temperature rises during melting.
- Claiming the mass changes during the phase change.
Mark scheme (4 marks)
- During melting, the energy supplied is used to break the intermolecular forces / bonds between particles (rather than raise temperature).
- The energy supplied during melting is called latent heat (specific latent heat of fusion).
- Because the energy goes into changing state rather than increasing the average kinetic energy of the particles, the temperature remains constant.
- Mass is conserved during the change of state, so the mass of the liquid water produced equals the mass of the ice that melted.
Key terms in this question
melting · temperature · mass
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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