---Melting point of a crystalline organic compound is defined as a definite and sharp temperature at which a solid starts melting .Actually in practice, we can only observe the melting range i.e. the difference between the temperature at which the collapse of the crystals is observed and the temperature at which the sample becomes completely liquid. In pure crystalline organic substances the range is nearly 0.5 deg C.
Hence actually speaking the melting point is a range over which the melting process is taking place and is not a fixed temperature .As the amount of impurities in the substance  increases ,it also increases the range of melting point.