Question

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

Answer 1

Presence of impurities make the boiling or freezing point as high or low. Only pure substances have proper boiling and freezing point. From the value we can say whether impurities are present or not can be decided.
For ex.,
Water has boiling point of 100 ⁰C. If the boiling point is higher or lower it may be > or < 100 ⁰C