The presence of impurities in a substance results in a lower melting point due to a process called melting point depression. Melting point depression is the reason why adding salt to frozen streets helps to melt the ice.
Melting point depression occurs due to the nature of a material's solid state. Most solids, such as ice, form as crystalline lattices of repeating ions or molecules. This lattice is held together by intermolecular forces that create a strong, stable structure. The solid must be heated to a certain temperature in order for this structure to be disrupted and for the solid to melt. However, the presence of impurities weakens the lattice, making it less stable. As a result, the compound melts at a lower temperature.
When working with organic compounds in a lab, the purity of the compound can be partially determined through the use of a precise measurement of the melting point. If the melting point is within the scientifically accepted range of the material's melting temperature, then the material is presumed to be pure. However, if the observed melting temperatures are outside of this range, then the compound is not pure.
One interesting effect of this process is that sea water freezes at a lower temperature than pure water. Pure water freezes at 32 degrees Fahrenheit. Sea water freezes at 28.4 degrees Fahrenheit.