Question

how does the presence of impurities affect the boiling point and freezing point of a substance

Answer 1

if a pure solvent is heated upto its boiling point, we get a boiling point of pure solvent.
If any impurities is added to this pure solvent, it covers the upper surface of the liquid. Due to which a excess heat is required to boil the liquid.
Thus on adding any impurities in substance it increases the boiling points.

Second question :
As like previous one, adding impurities lowers the freezing point. Thus we have to supply much lower temperature to make the substance freeze.

Simple!