When substance contain an impurity,it melting point...........and its boiling point ..............

When a substance contains an impurity, its melting point tends to decrease and its boiling point tends to increase. This is because the impurity disrupts the crystal lattice structure of the substance, making it easier to overcome the forces holding the molecules together in a solid state (lowering the melting point) and harder to overcome the intermolecular forces holding the molecules together in a liquid state (raising the boiling point).