La entalpía de fusión o calor de fusión (ΔHfus) es la cantidad de energía necesaria para hacer que un mol de un elemento que se encuentre en su punto de fusión pase del estado sólido al líquido, a presión constante. En otras palabras, es la cantidad de energía que un sistema puede intercambiar con su entorno. Es una magnitud de termodinámica (H), cantidad de energía que se puede intercambiar. La entalpía de fusión es un calor latente ya que durante el proceso de cambio de estado no se da un cambio apreciable de temperatura. El calor es completamente invertido en modificar la estructura del material para dar movilidad a sus unidades moleculares. Cuando para estudiar la energía necesaria para el cambio de sólido a líquido se hace referencia a la unidad de masa el parámetro empleado es el «calor específico de fusión» en cal/g o J/g. Sin embargo cuando se quiere hacer referencia a la unidad absorbida por mol de sustancia en cambio de estado se emplea la «entalpía de fusión» en kJ/mol.

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The enthalpy of fusion of a substance, also known as (latent) heat of fusion, is the change in its enthalpy resulting from providing energy, typically heat, to a specific quantity of the substance to change its state from a solid to a liquid at constant pressure. This energy includes the contribution required to make room for any associated change in volume by displacing its environment against ambient pressure. The temperature at which the phase transition occurs is the melting point. By convention, the pressure is assumed to be 1 atm (101.325 kPa) unless otherwise specified. The 'enthalpy' of fusion is a latent heat, because during melting the introduction of heat cannot be observed as a temperature change, as the temperature remains constant during the process. The latent heat of fusion is the enthalpy change of any amount of substance when it melts. When the heat of fusion is referenced to a unit of mass, it is usually called the specific heat of fusion, while the molar heat of fusion refers to the enthalpy change per amount of substance in moles. The liquid phase has a higher internal energy than the solid phase. This means energy must be supplied to a solid in order to melt it and energy is released from a liquid when it freezes, because the molecules in the liquid experience weaker intermolecular forces and so have a higher potential energy (a kind of bond-dissociation energy for intermolecular forces). When liquid water is cooled, its temperature falls steadily until it drops just below the line of freezing point at 0 °C. The temperature then remains constant at the freezing point while the water crystallizes. Once the water is completely frozen, its temperature continues to fall. The enthalpy of fusion is almost always a positive quantity; helium is the only known exception. Helium-3 has a negative enthalpy of fusion at temperatures below 0.3 K. Helium-4 also has a very slightly negative enthalpy of fusion below 0.77 K (−272.380 °C). This means that, at appropriate constant pressures, these substances freeze with the addition of heat. In the case of 4He, this pressure range is between 24.992 and 25.00 atm (2,533 kPa). These values are mostly from the CRC Handbook of Chemistry and Physics, 62nd edition. The conversion between cal/g and J/g in the above table uses the thermochemical calorie (calth) = 4.184 joules rather than the International Steam Table calorie (calINT) = 4.1868 joules.

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