A semipermeable membrane permits solvent and some but not all solutes to pass through its pores. Solvent will spontaneously pass through a semipermeable membrane in an attempt to make impermeable solutes infinitely dilute. This phenomenon, called osmosis [local], can be used to create a pressure which, when applied, holds the diluting process in check.

Osmosis is important in biological cells. For example, when placed in pure water, red blood cells rupture due to passage of the water into the cells. When placed in concentrated salt solutions, they shrivel due to net passage of solvent out of the cell.

By placing contaminated water in contact with properly designed semipermeable membranes, and applying pressure in excess of the osmotic pressure, it is possible to force 'pure' solvent through the membrane. This process, called reverse osmosis, is used to purify water [local].