From the graph above, it is seen that line AB represents conversion of ice into water at a constant temperature. Ice when heated melts at 0oC and then is converted into water maintaining constant temperature of 0oC. Melting point is this constant temperature, at which the ice converts into water. During this transition, the ice also absorbs heat energy. This heat energy is used 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. After all the ice is transformed into water, the temperature of water starts to rise. It increases to 100oC. Line BC in the graph represents rise in temperature of water (0oC to 100oC). After this point, even if heat energy is supplied to water, its temperature remains constant. The heat energy is absorbed by water at this temperature 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, even if heat energy is absorbed by the liquid, its temperature does not change. The constant temperature, when the liquid transforms into a gaseous state is 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.
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