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.