The temperature at which a solid passes into the liquid phase is known as the melting point . In practice, it is taken as the temperature at which the solid and liquid exist in equilibrium at an external pressure of 1 atm. The heat absorbed when a gram of a solid melts or the heat liberated when freezes is known as the latent heat of fusion . The heat of fusion may be considered as the heat required to increase the interatomic or intermolecular distances in crystals, thereby allowing melting to occur. A crystal that is bound together by weak forces generally has a low heat of fusion and a low melting point, whereas one bound by strong atomic forces has a high heat of fusion and a high melting point.
Paraffins crystallize as thin leaflets composed of zig-zag chains packed in a parallel arrangement.
The melting points of normal saturated hydrocarbons increase with molecular weight, because the van der Waals forces between the molecules of the crystal become greater with an increasing number of carbon atoms. Phase Equilibria and Phase Diagrams.