In the given graph, line AB represents conversion of ice into water at constant temperature. When ice is heated, it melts at 0oC and converts into water maintaining constant temperature of 0oC. This constant temperature, at which the ice converts into water is called the melting point of ice. Also, during this transition, the ice absorbs heat energy. This heat energy is utilised for weakening the bonds between the atoms or molecules in the ice to transform itself into liquid. This heat energy absorbed by ice, at constant temperature, to convert it into liquid is called the latent heat of fusion.
Once all the ice is transformed into water, the temperature of water starts rising. It increases upto 100oC. Line BC in the graph represents rise in temperature of water from 0oC to 100oC. Thereafter, even though heat energy is supplied to water, its temperature does not rise. The heat energy is absorbed by water at this temperature and is used to break the bonds between molecules of the liquid and convert the liquid into gaseous state. Thus, during transformation from liquid phase to gas phase, heat energy is absorbed by the liquid, but its temperature does not change. The constant temperature at which the liquid transforms into gaseous state is called the boiling point of the liquid. The heat energy absorbed at constant temperature during transformation of liquid into gas is called the latent heat of vaporization.
