The melting point is an important physical property of a compound. The melting point can be used to identify a substance and as an indication of its purity. A pure crystalline compound usually possesses a sharp melting point and it melts completely over a narrow range of temperature of not more that 0.5 -1 °C. The presence of even small amount of impurities usually produces a depression in the freezing points and shows a marked increase in the width of the melting point range. The melting point range of greater than 5 °C indicates that the substance is impure. For a material whose identity is known, an estimate of degree of purity can be made by comparing its melting point with that of a pure sample.
This video explains how to determine the melting point of an organic compound.