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 defined as the amount of heat required to change unit mass of the substance from liquid state to gas state without a temperature change.
  • $Q = ml_{v}$ , 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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