A nurse removes a sealed bag of saline solution from a freezer. The bag contains frozen saline at −5 °C. The nurse places the bag in a warm water bath. Explain what happens to the energy supplied to the saline and its temperature during the entire process of warming from −5 °C until it has fully melted.

OCR GCSE Physics A: Gateway Science (J249) — P1.2 Changes of state · Explain · 4 marks · View as Markdown

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

Saline solution behaves similarly to pure water and has a definite melting point. The warm water bath supplies energy to the bag at a constant rate throughout.

Model answer (4 marks)

The energy supplied by the water bath increases the kinetic energy of the particles, so the temperature of the frozen saline rises from −5 °C until it reaches its melting point. At the melting point the temperature remains constant even though energy continues to be supplied, because the energy is used to break the intermolecular forces between the ice crystals. During the phase change the supplied energy is the latent heat of fusion, which is absorbed without a rise in temperature. Once all the ice has melted the temperature of the saline then rises again as further energy increases the kinetic energy of the liquid particles.

Examiner tips

  • Use the term ‘latent heat of fusion’ to show you understand the energy used in the phase change.
  • Show the temperature rise before the melting point and the constant temperature during melting.
  • Mention that the energy supplied is first used to increase kinetic energy, then to break bonds.
  • Keep the answer concise and use the exact wording from the mark scheme.

Common mistakes

  • Confusing latent heat with specific heat capacity.
  • Saying the temperature rises during the phase change.
  • Using the wrong phase change term (e.g., ‘latent heat of vaporisation’).

Mark scheme (4 marks)

  1. The temperature of the frozen saline rises as energy is supplied, because the energy increases the kinetic energy of the particles.
  2. At the melting point, the temperature remains constant / does not rise even though energy continues to be supplied.
  3. During melting, the energy supplied is used to break the intermolecular forces / bonds between particles rather than raising temperature.
  4. The energy supplied during the change of state is the latent heat (of fusion) / specific latent heat of fusion.

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