A ice cube is placed in a glass of warm lemonade. After a few minutes, the lemonade cools down and the ice cube melts. Explain the energy transfers involved as the ice cube melts and the lemonade cools to reach the same temperature.

WJEC A-Level Physics (Wales) — 3.4 Thermal physics · Explain · 5 marks · View as Markdown

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

An ice cube at 0 °C is dropped into a glass of warm lemonade at 20 °C. After several minutes, both the ice and lemonade reach the same temperature.

Model answer (5 marks)

The warm lemonade transfers thermal energy to the cooler ice cube. The energy flows until both reach the same temperature – thermal equilibrium. The energy absorbed by the ice causes it to change state from solid to liquid (melt). While melting, the ice’s temperature remains at 0 °C; it does not rise. Consequently the lemonade loses thermal energy and its temperature decreases.

Examiner tips

  • Use the word ‘transfer’ and ‘absorbed’ to show direction of energy flow.
  • Mention that the ice stays at 0 °C during melting to show understanding of latent heat.
  • Show that the lemonade cools as it gives up energy.

Mark scheme (5 marks)

  1. Thermal energy transfers from the warmer lemonade to the cooler ice cube
  2. Energy is transferred until both reach the same temperature (thermal equilibrium)
  3. The thermal energy absorbed by the ice causes it to change state from solid to liquid (melt)
  4. The temperature of the ice does not rise while it is melting (temperature stays at 0 °C during the change of state)
  5. The lemonade loses thermal energy so its temperature decreases

Key terms in this question

temperature

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