How should water impurities affect the melting point, and boiling point of water?
Impurities in water DECREASE the melting point and should INCREASE the boiling point.
If you have ever travelled in North America during the winter months, you would have noticed trucks salting the roads during the early morning (i.e. actually spreading salt granules on the road; this is one of the reasons why you will see so many relatively new cars that are absolute rust buckets on North American roads! Salt on the car's body facilitates its corrosion).
Why should they do this? Addition of salt causes freezing point depression. Salting the roads should cause the ice to melt (if the temperature is not too far below the regular freezing point!), and the real hazard of your motor slipping on the icy road is reduced.
On the other hand, when you try to distill a water solution, the boiling point should be a bit higher than that of the pure solvent. Boiling point elevation, and freezing point depression are very commonly encountered so-called colligative properties.
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