A pure substance will have a melting point that is generally within a 1 °C range of the literature melting point of that substance. An impure substance will typically have a broad melting point range and it will also be at a lower temperature than the literature melting point. We call this a melting-point depression.
A pure crystalline substance will have the particles packed in repeating patterns and these forces hold the crystals together. In an impure substance, the impurities disrupt these forces holding the solid together, therefore, requiring less energy to melt the portion of the solid containing the impurities. Less energy required results in a lower melting point.
