On adding an impurity, the vapor pressure of solution decreases. With an increase in concentration of solute, vapour pressure decreases, hence boiling point increases. This phenomenon is known as 'elevation of boiling point'. For example – adding salt to water will lead to increase in its boiling point.

Do impurities increase melting point?

The presence of even a small amount of impurity will lower a compound's melting point by a few degrees and broaden the melting point temperature range. Because the impurity causes defects in the crystalline lattice, it is easier to overcome the intermolecular interactions between the molecules.

How does impurity affect melting and boiling point?

The reason for impurities lowering the melting point yet increasing the boiling point is because the impurities stabilise the liquid phase, making it more energetically favourable. This extends the liquid range to lower temperatures (lowering the melting point) and to higher temperatures (raising the boiling point).