Beer glossary

Osmosis

Osmosis is a very important process in nature. It is closely associated with the exchange of water and minerals through cell walls. Osmotic forces are of great importance for the exchange of water between the inside of a cell and the extracellular environment.

While the cell walls allow water to pass through unhindered, they are quasi only semi-pervious to minerals. Therefore, such membranes are also called “semi-permeable” in living systems.

When two chambers A and B with the same volume (100 ml) with a different concentration of particles (20 particles in chamber A and 10 particles in chamber B) are brought into contact with each other through a semi-permeable membrane, this membrane acts selectively. It allows the solvent to pass, but none or only a few of the dissolved particles. Since the different solutions strive to equalise their concentrations, solvent diffuses from chamber B to chamber A until the particle concentration is the same in both.

This process of a membrane’s permeation by the solvent is called osmosis. The osmosis process ends when the particle concentration is the same in both chambers. When 33.3 ml of liquid has passed from chamber B to chamber A, the concentration has equalised. The water column in chamber A exerts hydrostatic pressure on the membrane, referred to as osmotic pressure.

The reverse osmosis principle
The osmotic process can also take place in reverse. When pressure is built up on the side with the concentrated solution (e.g. tap water) that is able to overcome the osmotic pressure, the solution flows through the membrane in the opposite direction. Only the solvent (e.g. water) can pass through, but not the dissolved substances.

(Source: Gesellschaft für Wasser und Gesundheit mbH)