A student heats a fixed mass of ice at a steady rate. The ice melts and then the water warms up. Explain what happens to the internal energy of the substance during each of these two stages, and why the temperature behaves differently in each stage.

WJEC A-Level Physics (Wales) — 2.3 Capacitance (A-Level only) · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (5 marks)

During the melting stage the supplied heat is used to break the hydrogen bonds between the water molecules; the internal energy of the system increases but this increase is stored as potential energy of the new liquid structure. Because the kinetic energy of the molecules does not change, the temperature remains constant.

When the ice has fully melted the same rate of heating is now applied to liquid water. The heat supplied increases the kinetic energy of the water molecules, so the internal energy continues to rise and the temperature of the water rises.

Examiner tips

  • Use the word ‘potential energy’ when describing the energy stored in bonds during melting
  • Explain that temperature is a measure of kinetic energy, not total internal energy
  • Show the two stages clearly: melting (constant T) then warming (increasing T)
  • Mention that the heat supplied is the same in both stages but is used differently

Common mistakes

  • Confusing internal energy with temperature, saying the temperature rises during melting
  • Saying the heat is used to increase kinetic energy during melting
  • Omitting that the internal energy increase during melting is stored as potential energy

Mark scheme (5 marks)

  1. Internal energy increases during melting
  2. During melting, the energy supplied increases the potential energy of the particles (bonds/intermolecular forces are broken/overcome), not the kinetic energy
  3. Because kinetic energy of the particles does not increase during melting, the temperature does not rise
  4. Internal energy continues to increase as the liquid water warms up
  5. During warming of the liquid, energy supplied increases the kinetic energy of the particles, so temperature rises

Key terms in this question

internal energy · temperature

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