Latent heat is the energy in the form of heat that is either absorbed or released when a substance changes its state from one phase to another. The phase change can be either from solid to liquid state, the liquid state to the gaseous state, gaseous to liquid, or liquid to solid. It can be observed during a constant-temperature process, which generally comes under the first-order phase transition. Latent heat is related to a specific property called enthalpy. When latent heat for a material having a unit mass of 1 kg is expressed, then it is termed specific latent heat.

Latent Heat

Latent heat is the energy required by a substance to change its state from one phase to another. It can be considered as a form of energy that is present in a hidden manner and can be supplied or released to change the phase of a substance in an environment without any change in the temperature. 

Latent heat is usually expressed in two of its variations, which are latent heat of fusion and latent heat of vaporisation. Thus, phenomena like melting, evaporation and condensation play an extremely important role. 

Example of Latent Heat of Water

History of Latent Heat

1. Joseph Black, an English scientist, was the pioneer of the idea of latent heat. He introduced it around 1750 and 1762. He introduced latent heat in the context of calorimetry, in which heat transfer occurs due to the change in volume of a material while the temperature remains constant.
2. James Prescott Joule, an English physicist, introduced latent heat as a kind of potential energy. Thus, it was the energy formed due to the interaction of particles in a given configuration.

Specific Latent Heat 

Specific latent heat is the energy that is required to change the phase of a 1 kilogram substance at a steady temperature (thus without causing any change in the temperature).

Mathematically, the specific latent heat formula is:

L=Q ⁄ M 

Where, L = Specific Latent Heat, Q = Heat, M = Mass

The unit of specific latent heat is kJ/kg or J/g

From the specific latent heat formula, it can be understood that the specific latent heat is a quantity that is measured per unit mass, hence it is considered to be an intensive property of matter. Intensive properties are independent of the size and extent of the sample.

Types of Specific Latent Heat

Specific latent heat shows two phenomena called fusion and evaporation or vaporisation. Fusion is the change in the state of a substance from solid to liquid or vice-versa, while vaporisation is the change in the state of a substance from liquid to gases. 

Specific latent heat is expressed in two forms:

1. Specific latent heat of fusion
2. Specific latent heat of vaporisation

Specific latent heat of fusion: 

It can be defined as the amount of heat that is required by a material to convert a unit mass of itself from solid state to liquid state at a particular temperature point called the melting point.

Mathematically, the specific latent heat formula for fusion can be expressed as: 

         Lf=Q ⁄ M

Where Lf = Specific Latent Heat of fusion, Q = Heat energy absorbed, M = Mass of a substance

Example of Specific latent heat of fusion:

1. Water – 334 kJ/kg
2. Oxygen – 22.4 kJ/kg

Water has a high value of specific latent heat of fusion. Therefore, snow takes a long period to melt, and boiling water can be used for cooking.

Specific latent heat of vaporisation:

The specific latent heat of vaporisation can be defined as an amount of energy in the form of heat that is needed by a substance to convert a unit mass of a substance from the liquid phase to vapour state at a specific temperature called its boiling point.

Mathematically, the specific latent heat formula for vaporisation is: 

Lv=Q ⁄ M 

Where Lv = Specific Latent Heat of vaporisation, Q = Heat energy absorbed, M = Mass of a substance

Example:

1. Specific latent heat of vaporisation of water: 2,260 kJ/kg
2. Specific latent heat of vaporisation of oxygen: 213 kJ/kg

The specific latent heat of vaporisation has a higher value compared to that of fusion because of the greater intermolecular space between the gaseous molecules, and hence, the force of attraction between the molecules becomes negligible. Therefore, more energy is required for the conversion from the liquid to gaseous phase.

Conclusion

Latent heat is the heat energy required for the conversion of the state of a substance at a steady temperature. The heat energy required by a unit mass of a substance for its conversion from one phase to another at a constant temperature is defined as the specific latent heat. It is an intensive property and can be expressed as heat of fusion and vaporisation. Both latent heat of fusion and vaporisation are extremely important for the survival of living organisms, since they indirectly affect the climate of the earth.