Latent Heat in Physics:
The amount of heat extracted or applied to change the state of a unit mass of a substance at a constant temperature is called the latent heat of the substance for that change of state.
Hence corresponding to different changes of state latent heat are of four types –
i. Latent heat of fusion
ii. Latent heat of solidification
iii. Latent heat of vaporisation
iv. Latent heat of condensation
Explanation of Latent Heat:
The molecules of a solid substance are arranged in a fixed crystalline structure and therefore the solid has a definite shape. On the other hand, the molecules of a liquid are not so arranged and so they have no fixed shape. Hence, the change of a solid to its liquid state, or melting. It means breaking up the crystalline structure. Latent heat supplies the necessary energy for breaking this crystalline arrangement of molecules and hence there is no change in temperature.
Similarly, at atmospheric pressure, the intermolecular attraction in gases is very small in comparison to that in solids and in liquids. The intermolecular separations in gases are much higher. Hence the change of a liquid to its gaseous state. Vaporization means the total separation of the molecules from their mutual attraction. The energy required for this purpose is supplied by the latent heat.
Since the energy required to increase the intermolecular separation in changing a liquid to gas is much higher than that in changing a solid to liquid latent heat of vaporization is higher than the latent heat of fusion for a substance.