Latent Heat Defined
Let us consider some mass of water being heated in the vessel. Slowly the temperature of water starts increasing and reaches till around 100 degree Celsius, which is the boiling point temperature of water. At this temperature water will not start boiling instantly; rather it keeps on absorbing heat at the constant temperature and thereafter it starts boiling. The amount of heat absorbed by water during its change of phase from liquid to gaseous without increase in its temperature is called as the latent heat of vaporization.
In general the amount of heat required by unit mass of the substance to change its phase at constant temperature and pressure is called as latent heat of the substance. It is denoted by symbol ‘l’ and its unit of measurement is J/kg.
Types of Latent Heat
There are three phases of the substance: solid, liquid and gaseous. Accordingly there are three types of latent heats, which have been described below:
1) Latent heat of fusion: Let us suppose we have one kg of ice and we have to convert it into water, the amount of heat required to do this is called as latent heat of fusion. In general amount of heat required to melt the substance of unit mass from solid to liquid or the amount of heat required to be removed from the unit mass of substance to freeze it from liquid to solid is called as latent heat of fusion denoted by ‘lfu’.
2) Latent heat of vaporization: We have seen the example for boiling water. The amount of heat required by the unit mass of substance to vaporize from liquid to gaseous state or the amount of heat required to be removed from the unit mass of substance to condense from the gaseous to liquid phase is called as latent of vaporization denoted by ‘lfu’.
3) Latent heat of sublimation: Some of the substances like naphthalene get directly converted from solid to gaseous state when kept in the open atmosphere. The amount of heat required by the unit mass of substance to convert directly from solid to gaseous phase or the amount of heat required to be removed from the gaseous substance to change it to the solid phase is called as latent heat of sublimation denoted by ‘lsub’.
Book: Enginering Thermodynamics by P K Nag