Specific Latent Heat

• Latent (meaning hidden) heat describes the energy required to change the state of a substance

  • The specific part again refers to 1 kg of a substance.
• • The amount of energy Q required to melt or vaporise a mass of m with latent heat L is:

  Thermal energy required for a change of state = Mass × Specific latent heat 

  Q = mL
• • Where:
  • Q = thermal energy required for a change in state, in joules (J)
  • m = mass, in kilograms (kg)
  • L = specific latent heat, in joules per kilogram (J/kg)
• • This equation can be rearranged with the help of a formula triangle:

  
• For context, the values of latent heat for water are:

  • Specific latent heat of fusion = 330 kJ/kg
  • Specific latent heat of vaporisation = 2.26 MJ/kg
  • Therefore, evaporating 1 kg of water requires roughly seven times more energy than melting the same amount of ice to form water.