Distillation is a process in which the latent heat of vaporization is initially supplied to a substance and then the latent heat of vaporization is subsequently extracted. In simple terms, it means a process in which a portion of a liquid is first evaporated and then condensed. The requirement is that the end phase must be the same as the initial phase, a liquid.
Distillation is pointless when dealing with a pure substance because the end substance is exactly the same as the beginning substance. It is used when dealing with solutions of two or more substances which may be evaporated at different pressures and temperatures. The more volatile substance will evaporate first and then can be condensed.
We are dealing with salt water where we can assume that under most ordinary conditions, the salts are completely non-volatile. Thus, only the pure water will evaporate and then condensed, resulting in pure water in liquid form. The only time that the volatility of the salts must be considered is when distillation is taking place at temperatures above 300 degrees C. For all practical purposes, we will ignore this.
Distillation is the best-developed sea-water purification method. There are several different types of distillation that have been developed, both by man and by nature.