Specific Latent Heat (Oxford AQA IGCSE Physics)

Specific Latent Heat of Vaporisation

• The specific latent heat of vaporisation is defined as:

The amount of energy required to change the state of one kilogram of the substance between a liquid and a gas with no change in temperature

• This applies when vaporising a liquid or condensing a gas

• A substance's temperature will remain constant until all of the substance has vaporised

• For a vaporising liquid:

  • A liquid will vaporise at its boiling point

  • Its temperature will remain constant until all of the liquid substance has vaporised

  • Energy is transferred into the system

  • The latent heat of vaporisation is the amount of energy per kg needed for all the particles in the liquid to overcome the intermolecular forces of attraction holding them together in their liquid state

• For a condensing gas:

  • A gas will condense at its boiling point

  • Its temperature will remain constant until all of the gaseous substance has condensed

  • Energy is transferred away from the system

  • The latent heat of vaporisation is the amount of energy per kg transferred away from the gas until all the particles have succumbed to the intermolecular forces of attraction holding them together in their liquid state

Specific latent heat of vaporisation equation

• The amount of energy E required to vaporise or condense a mass of m with latent heat of vaporisation L_v is:

• Where:

  • E = thermal energy required for a change in state, in joules (J)

  • m = mass, in kilograms (kg)

  • L_v = specific latent heat, in joules per kilogram (J / kg)

• The specific latent heat of vaporisation for water = 2.26 MJ/kg

Specific Latent Heat of Fusion

• The specific latent heat of fusion of a substance is defined as:

The amount of energy required to change the state of one kilogram of the substance between a solid and a liquid with no change in temperature

• This applies when melting a solid or freezing a liquid

• When melting a solid:

  • A solid will melt at its melting point

  • Its temperature will remain constant until all of the solid substance has melted

  • Energy is transferred into the system

  • The latent heat of fusion is the amount of energy needed per kg for all the particles in the solid substance to overcome the intermolecular forces of attraction holding them together in their solid state

• When freezing a liquid:

  • A liquid will freeze at its melting point

  • Its temperature will remain constant until all of the liquid substance has solidified

  • Energy is transferred away from the system

  • The latent heat of fusion is the amount of energy per kg transferred away from the liquid substance until all the particles have succumbed to the intermolecular forces of attraction that hold them together in their new solid structure

Specific latent heat of vaporisation equation

• The amount of energy E required to melt or freeze a mass of m with a latent heat of fusion L_f is:

• Where:

  • E = thermal energy required for a change in state, in joules (J)

  • m = mass, in kilograms (kg)

  • L_f = specific latent heat of fusion, in joules per kilogram (J / kg)

The graph of specific latent heat of fusion and vaporisation