AQA GCSE Physics

Revision Notes

3.2.5 Heating & Cooling Graphs

Heating & Cooling Graphs

  • Heating and cooling graphs are used to summarise:
    • How the temperature of a substance changes with increased energy added to it (or removed from it)
    • Where changes of state occur
  • Heating and cooling graphs tend to be the same
    • Heating is when the heat is added and the kinetic energy of the molecules increases (red arrows to the right)
    • Cooling is when heat is removed (or goes to the surroundings) and the kinetic energy of the molecules decreases (blue arrows to the left)

Heating

  • When heat is added to a solid, the temperature starts to increase as the particles vibrate and gain kinetic energy
  • At a substance’s melting point, the heat energy added goes into breaking the intermolecular bonds and there no more increase in kinetic energy or temperature
    • This is melting and the substance is now a liquid
  • As heat is continually added, the temperature of the liquid continues to increase as the particles gain more kinetic energy
  • At a substance’s boiling point, the heat energy added goes into breaking the intermolecular bonds further and there is no more increase in kinetic energy or temperature
    • This is evaporation or vaporisation and the substance is now a gas

Cooling

  • The process is repeated backwards for cooling as heat is taken away
    • A gas turns back into liquid through condensation
    • A liquid turns back into a solid through freezing

Heating cooling curve for pure substance, downloadable IGCSE & GCSE Physics revision notes

Heating/cooling curve of a substance showing the energy changes as temperature is increased/decreased

The different sections of the graph show:

  • ORIGIN to A: Added heat energy is being used to increase the kinetic energy of the particles while it is a solid
  • A to B: Added heat energy is being used to break the bonds between the solid molecules, increasing the potential energy and melting the substance
  • B to C: Added heat energy is being used to further increase the kinetic energy of the particles while the substance is a liquid
  • C to D: Added heat energy is being used to break the bonds between the liquid molecules, further increasing the potential energy and boiling the substance
  • D to E: Added heat energy is being used to further increase the kinetic energy of the particles while the substance is a gas

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