Question

The purity of a substance will often affect the melting and boiling points. this is used by chemists to assess the purity of a product. how does the purity of the substance affect the melting and boiling points?

Answer 1

Answer is: impurities lower melting point (melting point depression) and increase boiling point (boiling point elevation).
For example pure water has lower boiling point than solution of water and some salt, because partial pressure is increased. Impurities in solids lower the melting point because they disrupt crystal structure of substance.