Heat of fusion is the thermal energy that needs to be added in order to turn a solid into liquid (melting) or the thermal energy that needs to be subtracted in order to turn a liquid into solid (solidification). The transformation from solid to liquid and liquid to solid occurs at a constant temperature.
One of the most common examples of heat of fusion is when it is subtracted from water to form ice. Water in its liquid form can be placed in a freezer where one can observe its temperature steadily lowering . When it reaches the point where it is turning into solid, the temperature will be at a constant until before steadily lowering again. The same can be said about its exact opposite which is melting, wherein ice’s temperature will continually rise, until such time that it is converted to liquid wherein it will have a constant temperature.
Heat of fusion is measure in the unit calories/gram or Joules/mole. Heat of fusion’s counterparts are heat of vaporization, or the heat energy required to turn liquid into vapor, and heat of sublimation, or the heat energy required to turn solid directly to gas.