Explain why the latent heat of vaporization of a substance might be expected to be considerably greater than its latent heat of fusion.

Specific latent heat of fusion of a substance is defined as the amount of heat required to convert 1 gram of substance from solid to liquid state, at the melting point, without any change of temperature. After whole of the solid has been converted into liquid, any further supply of heat results in rise of temperature of the liquid.
Specific heat of vaporization of a substance is defined as the amount of heat required to convert 1 gram of liquid into its vapors at its boiling point without any rise of temperature.
In solids the intermolecular force of attraction between the molecules are very strong than liquid and gas. The molecules are very closely placed in solids whereas in liquids are little closely spaced. But in the case of gases the molecules are randomly moved. So the energy required to separate completely the molecules from liquid to gas is greater than from solid to liquid. That is why the latent heat of vaporization of a substance might be expected to be considerably greater than its latent heat of fusion.