Osmotic Pressure

Osmotic pressure is a colligative property, which means its magnitude depends on how many solute particles are present rather than on what they are. For dilute solutions the van 't Hoff relation gives it as the product of molar concentration, the gas constant and absolute temperature, with a factor accounting for any dissociation of the solute into ions. The pressures involved are larger than intuition suggests: a solution of ordinary physiological concentration develops several atmospheres against pure water. Biology exploits the effect continuously, since cell membranes behave as semipermeable barriers and cell volume is set by the balance of internal and external solute concentrations. Reversing the flow by applying pressure above the osmotic value is the operating principle of reverse-osmosis desalination.