The enthalpy of fusion, also known as the heat of fusion or specific melting heat, is the change in enthalpy resulting from the addition or removal of heat from 1 mole of a substance to change its state from a solid to a liquid (melting) or the reverse processes of freezing. It is also called the latent heat of fusion, and the temperature at which it occurs is called the melting point.
When thermal energy is withdrawn from a liquid or solid, the temperature falls. When thermal energy is added to a liquid or solid, the temperature rises. However, at the transition point between solid and liquid (the melting point), extra energy is required (the heat of fusion).
In going from liquid to solid (freezing), the molecules of a substance become arranged in a more ordered state. For them to attain the order of a solid, slightly less heat is withdrawn at the point of crystallization. That not withdrawn heat is stored in the form of primarily potential energy to build the solid lattice. In going from solid to liquid (melting), the molecules of a substance become arranged in a less ordered state. To create the relative disorder from the solid crystal to liquid, slightly more heat is added at the point of decrystallization. That energy from heat is utilized to break the solid lattice. This heat does not result in a temperature change, and is called a latent (or hidden) heat.
The heat of fusion can be observed by measuring the temperature of water as it freezes. If a closed container of room temperature water is plunged into a very cold environment (say −20 °C), the temperature will fall steadily until it drops just below the freezing point (0 °C). The temperature then will rebound and hold steady while the water crystallizes. Once the water is completely frozen, its temperature will fall steadily again.