A student is investigating how the temperature of water changes as it is heated on a hot plate. She measures the temperature every two minutes and records her results. She notices that the temperature stops rising for a period of time even though the hot plate is still switched on. Explain why the temperature of the water remains constant during this period, and describe what is happening to the water at this point.

OCR A-Level Physics B: Advancing Physics (H557) — 1.1 Practical skills assessed · Explain · 5 marks · View as Markdown

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

A student heats a beaker of water on a hot plate. She records the temperature every two minutes. After some time, the temperature stops rising even though the hot plate remains switched on at the same power.

Model answer (5 marks)

The water is undergoing a phase change – it is boiling/evaporating. The energy supplied by the hot plate is used as latent heat of vaporisation, not to increase the kinetic energy of the water molecules. Because temperature is a measure of the average kinetic energy, no increase in kinetic energy means the temperature remains constant. During this period the mass of water that turns into vapour equals the mass of liquid that has evaporated, so mass is conserved.

Examiner tips

  • Mention the phase change and latent heat explicitly
  • Explain that temperature is linked to kinetic energy
  • Show that energy goes into breaking intermolecular bonds, not raising temperature
  • Include the conservation of mass point

Common mistakes

  • Confusing latent heat with sensible heat
  • Saying the water simply 'stops heating' without mentioning phase change
  • Ignoring that mass is conserved during evaporation

Mark scheme (5 marks)

  1. The water is changing state (from liquid to gas / boiling / evaporating)
  2. The energy supplied by the hot plate is being used as latent heat
  3. No energy is available to increase the kinetic energy of the particles
  4. Temperature is a measure of the average kinetic energy of the particles, so if kinetic energy does not increase, temperature does not rise
  5. Mass is conserved during the change of state (the mass of vapour produced equals the mass of liquid that evaporated)

Key terms in this question

temperature

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