CIE A Level Physics (9702) 2019-2021

Revision Notes

18.1.3 Specific Latent Heat Capacity

Defining Latent Heat Capacity

  • Energy is required to change the state of substance
  • Examples of changes of state are:
    • Melting = solid to liquid
    • Evaporation/vaporisation/boiling = liquid to gas
    • Sublimation = solid to gas
    • Freezing = liquid to solid
    • Condensation = gas to liquid
  • When a substance changes state, there is no temperature change
  • The energy supplied to change the state is called the latent heat and is defined as:

The thermal energy required to change the state of 1 kg of mass of a substance without any change of temperature

  • There are two types of latent heat:
    • Specific latent heat of fusion (melting)
    • Specific latent heat of vaporisation (boiling)
  • The specific latent heat of fusion is defined as:

 The thermal energy required to convert 1 kg of solid to liquid with no change in temperature

    • This is used when melting a solid or freezing a liquid
  • The specific latent heat of vaporisation is defined as:

 The thermal energy required to convert 1 kg of liquid to gas with no change in temperature

    • This is used when vaporising a liquid or condensing a gas

Calculating Specific Latent Heat

  • The amount of energy Q required to melt or vaporise a mass of m with latent heat L is:

Q = mL

  • Where:
    • Q = amount of thermal energy to change the state (J)
    • m = mass of the substance changing state (kg)
    • L = latent heat of fusion or vaporisation (J kg-1)
  • The values of latent heat for water are:
    • Specific latent heat of fusion = 330 kJ kg-1
    • Specific latent heat of vaporisation = 2.26 MJ kg-1
  • Therefore, evaporating 1 kg of water requires roughly seven times more energy than melting the same amount of ice to form water
  • The reason for this is to do with intermolecular forces:
    • When ice melts: energy is required to just increase the molecule separation until they can flow freely over each other
    • When water boils: energy is required to completely separate the molecules until there are no longer forces of attraction between the molecules, hence this requires much more energy

Worked example: Specific latent heat

Defining_Latent_Heat_Capacity_Worked_Example_-_Defining_Latent_Heat_Capacity_Question, downloadable AS & A Level Physics revision notes

Step 1:            Write the thermal energy required to change state equation

Q = mL

Step 2:            Rearrange for latent heat

Defining Latent Heat Capacity Worked Example equation 1

Step 3:            Substitute in values

m = 530 g = 530 × 10-3 kg

Q = 0.6 MJ = 0.6 × 106 J

Defining Latent Heat Capacity Worked Example equation 2

L is the latent heat of vaporisation because the change in state is from liquid to gas (boiling)

Exam Tip

Use these reminders to help you remember which type of latent heat is being referred to:

  • Latent heat of fusion = imagine ‘fusing’ the liquid molecules together to become a solid
  • Latent heat of vaporisation = “water vapour” is steam, so imagine vaporising the liquid molecules into a gas

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