Latent Heat (AQA GCSE Physics)

• The energy needed for a substance to change state is called the latent heat

• When a change of state occurs, the energy transferred to or away from a substance changes the internal energy of the substance but not its temperature
• This means that whilst the substance is changing state (e.g. from liquid to gas), the temperature of the substance remains constant, despite the fact that energy is still being transferred to the substance
  • This is because the energy is being used to overcome intermolecular forces of attraction 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 free to flow over one another
  • Therefore, to change the state from solid to a liquid, the molecules need to gain enough energy to overcome intermolecular forces of attraction holding them in their rigid solid structure 

• Molecules in a liquid are less tightly bound together, whereas, in a gas, they are free to move completely apart from one another  
  • Therefore, to change the state from a liquid to a gas, the molecules need to gain enough energy to overcome the intermolecular forces of attraction holding them close together in their liquid structure 

• This is done by latent heat

• The temperature remains constant when melting and boiling, despite energy being transferred to the substance, as shown in the graph below:

When a substance changes state, its temperature remains constant, even though energy is being transferred to or away from the substance

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