Impurities in the solution increase the boiling point. This is because impurities decrease the water molecules available for vaporisation during boiling. A greater amount of heat is needed to make the same amount of impure solution to vapourize than the heat that is required to make a pure solution vaporize.

Why do impurities increase boiling point and decrease melting 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).

Does impurity 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.