Specific Latent Heat (Edexcel GCSE Physics)

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Specific Latent Heat

  • 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:

Specific Latent Heat Triangle IGCSE, downloadable IGCSE & GCSE Physics revision notes

  • 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

Worked example

Calculate the energy transferred to the surroundings as 0.60 kg of stearic acid changed state from liquid to solid.The specific latent heat of fusion of stearic acid is 199 000 J/kg.

Step 1: List the known quantities

    • Mass, m = 0.60 kg
    • Specific latent heat of fusion, L = 199 000 J/kg

Step 2: Write down the relevant equation

Q = mL

Step 3: Substitute in the values

Q = 0.60 × 199 000 = 119 400 J

Examiner Tip

Remember that L is used as the symbol of specific latent heat of fusion or vaporisation. This equation will be given on your equation sheet, however, it is important you know how to use it!

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Ashika

Author: Ashika

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Ashika graduated with a first-class Physics degree from Manchester University and, having worked as a software engineer, focused on Physics education, creating engaging content to help students across all levels. Now an experienced GCSE and A Level Physics and Maths tutor, Ashika helps to grow and improve our Physics resources.