After purification, the test of purity of an organic compound is the nest important step. A pure organic compound possesses characteristic physical properties such as refractive index, specific gravity, boiling point, crystalline structure and melting point. If a purified sample shows the same properties that the pure substance is known to possess, it may be considered as pure and no further purification is required. However for practical purpose in laboratory only melting point and boiling point are used as criteria for purity.
A pure organic solid has a definite and sharp (sudden, rapid and complete) melting point, while an impure substance has a lower and indefinite melting point.
The melting point of a solid may be defined as the temperature at which the solid and liquid states of the compound are in equilibrium with each other at an external pressure of l atmosphere.