A-Level Physics October/November 2024 Q3(c): The substance in (b) has a specific latent heat of fusion LF. Suggest and explain wheth…

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  The specific latent heat of fusion, $L_F$, is likely to be less than the specific latent heat of vaporisation, $L_V$, found in (b)(iii).

  Explanation:

  The energy required for a phase change is used to (i) increase the internal potential energy of the molecules and (ii) do work against the surroundings as the substance expands.

  1. During fusion (solid to liquid), there is only a very small change in volume. In contrast, during vaporisation (liquid to gas), there is a very large increase in volume as the substance expands to become a gas.

  2. The work done by the substance is therefore negligible during fusion, whereas significant work is done during vaporisation. Furthermore, the increase in intermolecular potential energy is smaller for fusion, as molecules in a liquid are still closely packed, unlike in a gas where they are far apart.

  3. Since both the work done and the increase in internal energy are much smaller for fusion compared to vaporisation, the total energy required per unit mass, $L_F$, is significantly less than $L_V$.

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  How the marks are awarded

  • B1 — Correctly identifying that fusion involves a much smaller volume change than vaporisation.
  • B1 — Explaining that the smaller change in intermolecular spacing during fusion results in a smaller increase in internal energy.
  • B1 — Concluding that because the work done is negligible and the internal energy change is smaller, LF must be less than LV.

  Common mistakes

  • Simply stating 'LF is less than LV' without any supporting explanation.
  • Only mentioning the work done component (volume change) and failing to discuss the change in internal potential energy.
  • Incorrectly stating that the kinetic energy of molecules changes during a phase change; temperature is constant, so average KE is constant.
  • Confusing the properties of fusion and vaporisation, e.g., claiming fusion involves a large volume increase.

  Examiner tip: Always break down energy changes during phase transitions into the two key components: the change in internal potential energy and the external work done.

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