Specific Latent Heat (SQA National 5 Physics): Revision Note

Specific latent heat

• 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 is represented by the symbol with units joules per kilogram (J kg^-1)

Change of state

• The state or phase of a substance is whether it is a solid, liquid or gas

• A change of state, sometimes called a phase change, is when a substance changes from one state to another

• For example, when:

  • a solid turns to a liquid (melting)

  • a liquid turns to a gas (evaporation)

• State changes occur at the melting and boiling point of a substance

  • Melting and freezing occur at the melting point

  • Evaporation and condensing occur at the boiling point

Latent heat of fusion & vaporisation

• There are two types of specific latent heat:

• Specific latent heat of fusion

  • Changing the state between a solid and liquid 

    • Solid to liquid, or liquid to solid

• Specific latent heat of vaporisation

  • Changing the state between a liquid and gas

  • Liquid to gas, or gas to liquid 

• It is important to note that there is no change in temperature whilst a substance is undergoing a change of state

• State changes can be identified on a graph by the areas of constant temperature

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

• When a solid substance is melted, 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 overcome the intermolecular forces of attraction holding them together in their solid state

• If a substance in its liquid state is frozen, the substance will solidify at the same temperature as its melting point

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

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

• When a liquid substance is vaporised, its temperature will stay constant until all of the substance has vaporised

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

• If a substance in a gas and is condensed, it will condense at the same temperature as its boiling point

• In this case, the latent heat of vaporisation is the amount of energy per kg transferred away from the substance until all the particles in the substance have succumbed to the intermolecular forces of attraction that hold them together in their liquid state