# Specific latent heat

Latent heat

Latent heat of a substance is the amount of energy absorbed or released by the substance during a change in its physical state that occurs without changing its temperature.

• SI unit of latent heat is the joule (J).
• The latent heat associated with melting a solid or freezing a liquid is called the latent heat of fusion (Lf); that associated with vapourizing a liquid or a solid or condensing a vapour is called the latent heat of vaporization (Lv).

Specific latent heat of fusion

Specific latent heat of fusion, lf, of a substance is defined as the amount of heat required to change a unit mass of the substance from solid to liquid state, without any change in the temperature.

• SI unit of specific latent heat of fusion is joule per kilogram (Jkg-1)
• $Q = ml_{f}$, where
Q = amount of thermal energy absorbed or released
m = mass of substace
lf = specific latent heat of fusion.

Specific latent heat of vapourization

Specific latent heat of vapourization, lv, of a substance is defined as the amount of heat required to change unit mass of the substance from liquid state to gas state without a temperature change.

• SI unit of specific latent heat of vapourization, lv, of a substance is joule per kilogram (Jkg-1)
• $Q = ml_{v}$ , where
Q = amount of thermal energy absorbed or released
m = mass of substace
lf = specific latent heat of vapourization.

Latent heat in terms of molecular behaviour

Latent heat energy is absorbed or given out while a substance undergoes state change. The average kinetic energy of the molecules does not change so that the temperature remains constant.

• During melting, heat absorbed by the solid is used to break the inter-molecular bonds between the molecules of solid substance.
• During vapourization, heat absorbed by the liquid is used to break the inter-molecular bonds completely between the molecules of liquid substance.

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### What Others Are Saying:

1. Haider Khan scribbled

never mind, both of them have the same SI unit.

2. Haider Khan scribbled

Hello! Just wanted to inform that instead of giving the SI unit of specific latent heat of vapourization, the definition of Lv is given.