Latent heat

When heat energy is added to a substance, its temperature generally rises, and when heat energy is removed, its temperature generally falls. This is referred to as ‘sensible heat’.

However, when a substance changes state, such as from a solid to a liquid, a liquid to a gas or vice versa, this change requires the addition or removal of heat energy, but does not result in a change in the temperature of the substance. This is referred to as latent heat.

There are three categories of latent heat:

• Latent heat of fusion: When a substance changes from solid to liquid (or back).
• Latent heat of vaporisation: When a substance changes from liquid to gas (or back).
• Latent heat of sublimation: When a substance changes solid to gas (or back).

Specific latent heat refers to the heat required to change the state of 1 kg of a substance without a change in temperature, expressed in joules per kilogram.

NB this should not be confused with the term ‘specific heat’ which refers to the heat energy per unit mass required to raise the temperature of a substance by one degree Celsius.

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• Phase change.
• Refrigeration.
• Sensible heat.
• Specific heat.
• Temperature
• Thermal comfort.
• Thermal indices.
• Thermal mass.
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• U value.