AQA GCSE Physics

Revision Notes

3.2.3 Latent Heat

Latent Heat

  • A change of state is when a substance changes from one form to another
    • For example, gas to liquid or solid to liquid
  • The energy needed for a substance to change state is called the latent heat
  • When a change of state occurs, the energy supplied changes the amount of energy stored, or the internal energy, of the substance but not its temperature
  • This means that only when changing state (e.g. from liquid to gas), the temperature of the substance remains constant, despite the fact that energy is still being added to the substance
    • This is because the energy is being used to break the bonds between the molecules instead of increasing the kinetic energy of the molecules (and hence the temperature)
  • Molecules in a solid are tightly bound together, whereas, in a liquid, they are freer to move
    • Therefore, to change the state from solid to a liquid, the bonds between the molecules need to weaken
    • This is done by latent heat
  • The temperature remains constant when melting and boiling, despite energy being added, as shown in the graph below:

heat-graph, IGCSE & GCSE Physics revision notes

When a substance changes state, its temperature remains constant, even though it is still gaining (or losing) thermal energy

  • The latent heat comes in two types depending on the change of state:
    • Latent heat of fusion
    • Latent heat of vaporisation

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