Question

how does evaporation and condensation helps purify water?

Answer 1

Impurities in water are usually suspended molecules of another substance suspended in a liquid solution. When water evaporates, it turns to vapor. Because of the distance between molecules in vapor it can hold fewer suspended particles. So unless the substance in the water becomes vapor at the same temperature it is left behind.

To demonstrate this, leave a glass of regular tap water on a sunny windowsill. When the glass is empty from evaporation, a residue of minerals and chemicals from the water will remain in the glass.

Condensation however does not effect the purification process much at all. This process of changing from a gas to liquid actually allows the water to pick up impurities from whatever it is condensing in. (Pollution in the air in the case of rain, ground substances in the case of fog, etc.)

In theory, if you could evaporate water, and condense it in a sterile environment, the water would be purified.

Answer 2

http://education-portal.com/academy/lesson/the-water-cycle-precipitation-condensation-and-evaporation.html