While melting of a substance the molecules get separated to some extent but when the same substance is vaporised the molecules get separated to a greater extent which requires more enery, i.e. more heat.

Heat of fusion (Enthalpy of fusion) :

It is the heat required to change a specific amount of substance from solid to liquid at a constant pressure and vice versa.

Heat of vaporisation (Enthalpy of vaporisation):

It is the heat required to change a specific amount of substance from liquid to gas at a constant pressure and vice versa.

A graph showing the Heat of fusion and the Heat of vaporisation of water is given below.

When a solid substance is heated its temperature rises constantly and then stops when the solid starts melting until the total amount of the solid is melted. During this phase change the substance absorbs heat energy.

The shaded portions show the heat of fusion and the heat of vaporisation.

The heat of fusion and vaporisation are usually quoted on molar basis.

The heat of fusion and vaporisation for some substances are given below:

It is obvious that the heat of vaporisation is more than that of the heat of fusion of the same substance. It is because of the following reason:

The gas molecules are more haphazard than the liquid molecules. The gas molecules are more separated from each other than those of the liquid molecules. Thus the separation energy required for a phase change from liquid to gas is always more than that for solid to liquid. Hence more energy is required in terms of heat to form a gas from liquid than from solid to liquid. Hence the Heat of Vaporisation is more than the Heat of Fusion of the same substance.

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