The melting point of a solid is the temperature at which it changes from solid state to liquid state. At the melting point the solid and liquid phase exists in equilibrium (balance). The melting point of a substance depends (usually slightly) on pressure and is usually specified at standard atmospheric pressure. When considered as the temperature of the reverse change from liquid to solid, it is refereed to as the “Freezing point or crystallization point”. Due to the ability of some of the substances to super cool, the freezing point is not considered as the characteristic property of a substance. Super cooling a liquid is the process of cooling a liquid past its freezing point without it turning into a solid. When the “characteristic freezing point” of a substance is determined, the actual methodology is almost always “the principle of observing the disappearance rather than the formation of ice&rdquo;, that is the “Melting Point”.
Many laboratory methods exist for the determination of melting points.
From thermodynamics point of view, at the melting point the change in Gibbs free energy (?G) of the material is zero, but the enthalpy (H) and the entropy (S) of the material are increasing. Gibbs free energy also known as free enthalpy is a thermodynamic potential that measures the useful work obtainable from a thermodynamic system. Enthalpy is a measure of total energy of a thermodynamic system. Entropy is the measure of disorder of a thermodynamic system. Melting phenomenon happens when the Gibb’s free energy of the liquid becomes lower than the solid for that material. At various pressures, this happens at a specific temperature. It can be shown that
?S = ?H/T, where T, S and ?H are respectively temperature at the melting point, change in entropy of melting and the change in enthalpy of melting. Unlike boiling point, the melting point is relatively insensitive to moderate changes in pressure because the solid/liquid transition represents only a small change in volume.
In organic chemistry, Carnelley’s rule established by Thomas Carnelley, stated that high molecular symmetry is associated with high melting point. Molecular symmetry is the symmetry (evenness) present in molecules and the classification of molecules according to their symmetry. A high melting point results from a high enthalpy of fusion, low entropy of fusion or both. In highly symmetrical molecules the crystal phase is densely packed with many efficient intermolecular interactions in a higher enthalpy change on melting.
An attempt to find out the bulk melting point of crystalline materials was first made by Fredrick Lindemann, an English physicist in 1910. The idea behind the theory was the observation that the average amplitude of thermal vibration increases with increasing temperature. Melting initiates when the amplitude of vibration becomes large enough for adjacent atoms to partly occupy the same space. The Lindemann criterion states that melting is expected when the root mean square vibration amplitude exceeds a threshold value. Assuming that all atoms in a crystal vibrate with the same frequency ‘v’, the melting point is estimated using the expression for the Lindemann criterion as given below:
Tm = 4 ?2 m v2 c2 a2/kB, where