### GCSE Physics

Energy
Forces
Forces and Motion
Waves in Matter
Electricity
Magnetism and Electromagnetism
Particle Model of Matter
Atomic Structure
Space Physics

# Specific Latent Heat

When the state of a substance is changed, energy is required. This energy is referred to as latent heat. Specific latent heat is the amount of energy required to change the state of 1 kilogram (kg) of a substance without changing its temperature.

There is a boundary between solids and liquids, and another one between liquids and gases. This is why there are two types of latent heat:

1. Specific Latent Heat of Fusion – The amount of energy required to melt or freeze 1 kg of a substance, without a change in temperature. This occurs at the melting point.

• Solid/liquid boundary

2. Specific Latent Heat of Vaporisation – The amount of energy needed to boil or condense 1 kg of a substance without changing its temperature. This process occurs at the boiling point

• Liquid/gas boundary

## Latent Heat of Fusion and Vaporisation

The graph below shows state changes compared to temperature changes.

### Latent heat of fusion

When heating a solid substance, its temperature increases until it reaches its melting point. At this point, a solid-liquid mixture forms and the temperature remains constant until all of the substance has melted.

• The latent heat of fusion is the energy required to break the bonds between the particles of the solid substance.

If a liquid substance is cooled, its temperature decreases until it reaches its freezing point. At this point, a solid-liquid mixture forms and the temperature remains constant until all of the substance has solidified.

This forms bonds between the particles of the substance, and latent heat is transferred to the surroundings. As the bonds get stronger, the particles form a rigid structure.

### Latent heat of vaporisation

When heating a liquid substance, its temperature will increase until it reaches its boiling point. At this point, a liquid-gas mixture forms and the temperature remains constant until all of the substance has boiled and turned into gas.

• The latent heat of vaporisation is the energy required to break the bonds between the particles of the liquid substance, allowing them to form a gas.

If a gas is cooled, its temperature decreases until it reaches the same temperature as its boiling point. At this point, a liquid-gas mixture forms and the temperature remains constant until all of the substance has turned into liquid.

New bonds form between particles and latent heat is transferred to the surroundings.

## Calculating Change in Thermal Energy

We can calculate the change in thermal energy of a system using the equation:

• ΔEt = Change in thermal energy measured in joules (J)
• m = Mass measured in kilograms (kg)
• l = specific latent heat measured in joules per kilogram (J/kg)
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