Osmosis | Definition, Examples, & Facts (2024)

osmosis, the spontaneous passage or diffusion of water or other solvents through a semipermeable membrane (one that blocks the passage of dissolved substances—i.e., solutes). The process, important in biology, was first thoroughly studied in 1877 by a German plant physiologist, Wilhelm Pfeffer. Earlier workers had made less accurate studies of leaky membranes (e.g., animal bladders) and the passage through them in opposite directions of water and escaping substances. The general term osmose (now osmosis) was introduced in 1854 by a British chemist, Thomas Graham.

If a solution is separated from the pure solvent by a membrane that is permeable to the solvent but not the solute, the solution will tend to become more dilute by absorbing solvent through the membrane. This process can be stopped by increasing the pressure on the solution by a specific amount, called the osmotic pressure. The Dutch-born chemist Jacobus Henricus van ’t Hoff showed in 1886 that if the solute is so dilute that its partial vapour pressure above the solution obeys Henry’s law (i.e., is proportional to its concentration in the solution), then osmotic pressure varies with concentration and temperature approximately as it would if the solute were a gas occupying the same volume. This relation led to equations for determining molecular weights of solutes in dilute solutions through effects on the freezing point, boiling point, or vapour pressure of the solvent.

While osmosis naturally moves solvents across a membrane from the side of higher concentration to the side where the concentration is lower, the application of pressure in reverse osmosis forces the solvent to flow in the opposite direction—against the concentration gradient—thus resulting in the filtration of the solute from the solvent, leaving behind the solutes. The membrane allows passage of small solution components, such as fresh water, while preventing passage of larger molecules, like salts and other impurities. Reverse osmosis is an effective means of concentrating impurities, recovering contaminated solvents, cleaning up polluted streams, and desalinizing seawater and is often used as an alternative to distillation for water purification.