/oz moh"sis, os-/, n.1. Physical Chem., Cell Biol.a. the tendency of a fluid, usually water, to pass through a semipermeable membrane into a solution where the solvent concentration is higher, thus equalizing the concentrations of materials on either side of the membrane.2. a subtle or gradual absorption or mingling: He never studies but seems to learn by osmosis.[1865-70; Latinized form of now obs. osmose osmosis, extracted from endosmose endosmosis, exosmose exosmosis < F, equiv. to end- END-, ex- EX-2 + Gk osm(ós) push, thrust + F -ose -OSIS]
* * *If a solution is separated from a pure solvent by a membrane that is permeable to the solvent but not to the solute, the solution will tend to become more dilute by absorbing solvent through the membrane. The pressure caused by the migration of solvent through the membrane is called osmotic pressure.
* * *▪ chemical processthe 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 (Graham, Thomas).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 (Hoff, Jacobus Henricus van 't) 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.
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