Latent Heat of Fusion

What is Latent Heat of Fusion?

Latent heat of fusion, also known as enthalpy of fusion, is the amount of energy that must be supplied to a solid substance (typically in the form of heat) in order to trigger a change in its physical state and convert it into a liquid (when the pressure of the environment is kept constant). For example, the latent heat of fusion of one kilogram of water, which is the amount of heat energy that must be supplied to convert 1 kg of ice without changing the temperature of the environment (which is kept at zero degrees celsius) is 333.55 kilojoules.

It can be noted that the opposite of latent heat of fusion is the heat of solidification, which is the amount of energy that must be supplied to a liquid in order to facilitate a phase change and the conversion of the liquid into a solid. It can also be noted that the value of the heat of solidification of a substance is always equal in magnitude to the latent heat of fusion but always bears an opposite sign. For example, the amount of energy absorbed by ice to become water is equal to the amount of energy liberated by water to become ice.

The latent heat of fusion of a substance also accounts for the energy required to accommodate any increase in the volume of the substance post the change of its physical state. The temperature at which the substance undergoes the phase transition is called the melting point of the substance. This temperature point can also be referred to as the freezing point of the substance when the heat of solidification is being considered. Unless specified to be otherwise, the pressure of the environment (when expressing the latent heat of fusion of a substance) is always assumed to be 1 atmosphere of pressure (which is roughly equal to 101.325 kilopascals).

Specific Heat of Fusion and Molar Heat of Fusion

Enthalpy of fusion is considered synonymous with latent heat of fusion because the melting of a solid under normal atmospheric pressure usually requires energy in the form of heat. Therefore, the latent heat of fusion of a substance can be defined as the change in the enthalpy of a substance when it undergoes a phase transition from the solid phase to the liquid phase.

If unit mass of the substance is considered, the energy required to convert it into a liquid under constant pressure is called the specific heat of fusion for the substance. If the change in enthalpy is calculated on a per-mole basis, the latent heat of fusion is referred to as the molar heat of fusion of the substance.

Since the liquid phase has a higher internal energy associated with it than the solid state, some positive amount of energy must be supplied to a given solid to facilitate the melting of the solid. Similarly, some positive magnitude of energy is released by almost all liquids when they become solids. This can be explained by the fact that the particles or molecules that constitute liquids have very high potential energies due to the fact that they are held together by relatively weak intermolecular forces. Therefore, the energy required to dissociate the intermolecular forces of attraction between the liquid particles is also relatively lower when compared to solids.

An example of the latent heat of solidification can be observed in the cooling of water. When water, initially kept in the liquid state, is cooled to temperatures below zero degrees celsius, the temperature of the liquid water steadily drops until it approaches 0oC. At this temperature, the water undergoes crystallization and becomes a solid. Once the water is completely frozen, the temperature of the solid continues to drop below zero degrees celsius as it is cooled further.

The latent heat of fusion of a solid is almost always positive in value. One of the few known exceptions to this is the element helium. Below temperatures of 0.3K (at temperatures approaching absolute zero), the helium-3 isotope has a negative value for the latent heat of fusion.

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