Osmosis Definition in Chemistry

What Is Osmosis?

In osmosis, water moves from a lower concentration region to higher concentration across a semipermeable membrane.

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Two important mass transport processes in chemistry and biology are diffusion and osmosis.

Osmosis Definition

Osmosis is the process where solvent molecules move through a semipermeable membrane from a dilute solution into a more concentrated solution (which becomes more dilute). In most cases, the solvent is water. However, the solvent may be another liquid or even a gas. Osmosis can be made to do work.


The phenomenon of osmosis was first documents in 1748 by Jean-Antoine Nollet. The term "osmosis" was coined by French physician René Joachim Henri Dutrochet, who derived it from the terms "endosmose" and "exosmose."

How Osmosis Works

Osmosis acts to equalize concentration on both sides of a membrane. Since the solute particles are incapable of crossing the membrane, its the water (or other solvent) that needs to move. The closer the system gets to equilibrium, the more stable it becomes, so osmosis is thermodynamically favorable.

Example of Osmosis

A good example of osmosis is seen when red blood cells are placed into fresh water. The cell membrane of the red blood cells is a semipermeable membrane. The concentration of ions and other solute molecules is higher inside the cell than outside it, so water moves into the cell via osmosis. This causes the cells to swell. Since the concentration cannot reach equilibrium, the amount of water that can move into the cell is moderated by the pressure of the cell membrane acting on the contents of the cell. Often, the cell takes in more water than the membrane can sustain, causing the cell to burst.

A related term is osmotic pressure. Osmotic pressure is the external pressure that would need to be applied such that there would be no net movement of solvent across a membrane.

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Helmenstine, Anne Marie, Ph.D. "Osmosis Definition in Chemistry." ThoughtCo, Aug. 27, 2020, thoughtco.com/definition-of-osmosis-605890. Helmenstine, Anne Marie, Ph.D. (2020, August 27). Osmosis Definition in Chemistry. Retrieved from https://www.thoughtco.com/definition-of-osmosis-605890 Helmenstine, Anne Marie, Ph.D. "Osmosis Definition in Chemistry." ThoughtCo. https://www.thoughtco.com/definition-of-osmosis-605890 (accessed April 22, 2021).