Heat Transfer & Changes in State (WJEC GCSE Physics)

• A certain amount of energy is required to change the state of a certain mass of a substance
  • This amount of energy is known as the latent heat

• The specific latent heat is defined as:

The amount of energy required to change the state of 1 kg of a substance with no change in temperature

• Latent heat can be calculated using:

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)

• Higher Tier students will need to be able to rearrange this equation

Thermal Energy, Mass and Specific Latent Heat Equation Triangle

Cover up the variable you need to calculate, and the triangle shows you how the equation is arranged

• Changing a liquid to a gas requires more energy than changing a solid to a liquid

• The energy required to change the state of a substance between a solid and a liquid is called the latent heat of fusion
• This includes:
  • A solid melting into a liquid
  • A liquid freezing into a solid

• The energy required to change the state of a substance between a liquid and a gas is called the latent heat of vaporisation
• This includes:
  • A liquid boiling into a gas
  • A gas condensing into a liquid

• For example the latent heat of fusion for water is 336 kJ, whereas the latent heat of vaporisation is 2260 kJ

Higher Tier

Latent Heat of Fusion

• The specific latent heat of fusion is defined as:

The energy required to convert 1 kg of a substance between a solid and a liquid state with no change in temperature

• This applies when melting a solid or freezing a liquid
• Melting and freezing both occur at the melting point of the substance

• When a solid substance is melting, its temperature stays constant until all of the substance has melted
• The latent heat of fusion is the amount of energy needed per kg for all the particles in the substance to break the bonds holding them together in their solid state
• When a solid substance is freezing, its temperature stays constant until all of the substance has frozen
• The latent heat of fusion is the amount of energy released per kg when all the particles in the substance form the bonds that hold them together in their solid state

Latent Heat of Vaporisation

• The specific latent heat of vaporisation is defined as:

The energy required to convert 1 kg between a liquid and a gaseous state with no change in temperature

• This applies when vaporising a liquid or condensing a gas
• Vaporisation and condensing both occur at the boiling point of the substance

• When a liquid is boiling, its temperature stays constant until all of the substance has vapourised
• The latent heat of vaporisation is the amount of energy needed per kg for all the particles in the substance to break the bonds holding them together in their liquid state

• When a liquid is condensing, its temperature stays constant until all of the substance has condensed
• The latent heat of vaporisation is the amount of energy released per kg when all the particles in the substance form the bonds that hold them together in their liquid state

Graph of Phase Changes for a Substance

 The flat areas of the graph show the phase change