Thermal Properties of Matter

Specific Latent Heat

Latent Heat

• When a substance cools and changes state there is a period of time when the temperature remains constant…
• even although the substance is still emitting heat.

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Latent Heat

• When a substance changes from solid-liquid energy is taken in to break the bonds. During freezing this energy is released as these bonds reform. The energy released is known as the Latent Heat.
• Solid ↔ Liquid – Latent Heat of Fusion
• Liquid ↔ Gas – Latent Heat of Vaporisation

Specific Latent Heat

• The specific Latent Heat of a substance is defined as the amount of energy per unit mass absorbed or released during a change of phase.

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Energy (J)

Mass (kg)

Specific Latent Heat (J kg^-1)

N.B. You need to choose the correct value of L according to the phase change taking place.

Latent Heat Example 1

• A 5kg mass of ammonia at its boiling point is vaporised when 6.5MJ of heat is supplied to it.
• Calculate the Specific Latent Heat of Vaporisation of ammonia.

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Latent Heat Example 2

• How much energy is released when 1.5kg of water vapour at 100^oC is placed in a freezer and converted to ice at -7^oC?
• Water: L_v=2.26X10⁶ Jkg^-1 L_f=3.34x10⁵ Jkg^-1
• C_water=4180 Jkg^-1oC^-1 C_ice=2100Jkg^-1oC^-1

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