Dehydration is the removal of water from a compound, set of compounds, or other material. Dehydration refers not only to the removal of pre-existing water molecules from a solution, but also to the chemical formation of water by its removal from a compound to form a new compound.

The simplest form of dehydration is evaporation, in which water spontaneously leaves the liquid phase. The evaporation rate can be increased with heat. As water leaves a solution, the remaining solution becomes increasingly concentrated. At sufficiently high concentrations, solute molecules or ions may exceed their solubility, and begin to crystallize. This process is one of the most common means of obtaining crystals, both in the laboratory and in nature.

Dehydration for the purpose of crystallization may be aided by vacuum, which lowers the vapor pressure above the solution to increase the rate of evaporation. Freeze-drying uses evacuation and cold to...