At the melting point enough heat is available to begin to overcome the intermolecular forces which hold the particles in the lattice arrangement. As the particles gain potential energy they generally move apart some distance and this also allows more freedom of motions beyond the simple "whole particle" vibrations possible in the solid state. In molecules vibrations become possible in which one part of the molecule moves relative to another. Limited rotations of the particles become possible as well and some limited "whole body motion" or translation also begins to occur. All of these added modes of motion are ways of dispersing energy and so the entropy rises significantly during melting.

Vaporization is an analogous process. Although it might seem like the largest change in energy and entropy would occur between solid and liquid, the volumes of gases are so large compared to corresponding samples of liquid that tremendous potential energy increases are the norm during vaporization. The heat of vaporization is therefore always considerably larger than the heat of fusion.