How should water impurities affect the melting point, and boiling point of water?

If you have ever traveled through North America in the winter, you have probably seen trucks salting the roads in the early hours of the day. This is the process of actually spreading salt granules on the road, which is one of the reasons why you see so many relatively new cars on North American roads that are absolute rust buckets! The salt on the car's body facilitates corrosion.

Why should they do this? Because salt depresses the freezing point, salting the roads should melt the ice (assuming the temperature is not too low below the average freezing point) and lessen the actual risk of your car sliding on the icy surface.

Conversely, when attempting to distill a water solution, the boiling point needs to be somewhat greater than that of the pure solvent. The terms "colligative properties" are frequently encountered and include freezing point depression and boiling point elevation.

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Impurities lower the freezing point and elevate the boiling point of water.