The word “latent” has come from the Latin word “latere" which means to hide or cover up. This explains the concept of latent heat which is the extra heat required to change the state of a substance, for example, if a substance is in the solid-state, latent heat provides the extra energy to change its state to liquid and then from the liquid state to the gaseous state. This heat is required apart from the heat given to melt or vaporize the substance and it works without raising or decreasing the actual temperature of the substance no matter which state it is currently in.
Latent Heat of Fusion
The latent heat of fusion or melting, of a solid, is the quantity of heat in joules required to transform a solid, at its melting point, to a liquid without any variation in temperature. The latent heat of fusion of ice is 3.34 x 10⁵ joules per kilograms (or, 3.34 x 10⁵J/Kg).
The enthalpy change during melting or fusion is the latent heat that measures the amount of substance dissolved when transforming from the state of solid to the liquid state. The liquid state of a substance requires higher inward energy to convert to the solid-state which means energy must be provided to the solid-state to convert the substance into its fluid state, therefore, when the substance converts back to its solid-state, it releases energy in the form of latent heat.
Therefore, the enthalpy of melting is, mostly, a positive quantity with the exception of Helium-3.
Latent Heat of Vaporization
The term latent heat of vaporization is the amount of heat required for the transformation of a liquid at its boiling point to gas at a constant temperature. The latent heat of vaporization differs for various liquids.
In the figure given above, the liquid is converting into vapour by heating.Particles of water vapour at 100°C (373K) have more energy than liquid water at the same temperature. This is due to the assimilation of extra energy in the form of latent heat of vaporization.
The heat of vaporization is characterized as the amount of energy (heat) required to convert 1g of a liquid into a gas without any change in the temperature of the liquid itself. Therefore, like the heat of fusion, the latent heat of vaporization measures the heat given to a liquid in order to change its state into its gaseous state. In case of water, the latent heat to transform it into vapour is the energy of boiling water at which it changes form without change in temperature. This latent heat amounts to 22.5×105 J/kg.