Osmosis

Osmosis is the movement of solvent, such as water, through a barrier from a less concentrated solution into a more concentrated solution. It occurs when two solutions are separated by a semipermeable membrane which allows only the solvent to pass through. Osmosis plays a major role in the chemistry of living things and also has applications in medicine and technology.

Osmosis was first described in 1748 by J. A. Nollet. He placed a solution of "spirits of wine" (ethyl alcohol and water) in a bottle and sealed it with a piece of pig's Figure 1. Random collisions with semipermeable membrane. Illustration by Hans & Cassidy. Courtesy of Gale Group.
bladder. The bladder, like most biological membranes, was semipermeable. Upon immersion of the bottle in a larger container of pure water, the bladder swelled, and eventually burst, as water moved into it from outside. Nollet concluded that the solution exerted a kind of pressure, now called osmotic pressure. Later work by H. Dutrochet (1826) and J. H. van't Hoff (1885) led to mathematical formulas describing osmosis as a physical property of solutions.

Although osmosis moves solvent in one direction, its cause is the random motions of molecules in all directions. The driving force is the difference in concentration of solute on either side of the membrane. Suppose that two solutions, one more concentrated and the other more dilute, are separated by a semipermeable membrane.

On the dilute side, almost all of the molecules hitting the membrane at any moment are solvent, and can pass through. But on the concentrated side, more of the "hits" are from solute particles, which cannot pass through. Therefore, at any moment, more molecules enter the concentrated side than leave it. As a result, the volume of the concentrated side grows. But change in volume also changes the concentration. As the trapped solute particles spread over a larger volume, they become more dilute. The other side, which shrinks, becomes more concentrated. The process continues until both sides reach equal concentration.

Preserving food

For thousands of years, perishable foods such as fish, olives, and vegetables have been preserved in salt or brine. The high salt concentration is hypertonic to bacteria cells, and kills them by dehydration before they can cause the food to spoil. Preserving fruit in sugar (jams, jellies) works on the same principle.

Oceans hold about 97% of the earth's water supply, but their high salt content makes them unsafe for drinking or agriculture. Salt can be removed by placing the seawater in contact with a semipermeable membrane, then subjecting it to pressures greater than 60 atmospheres. Under these conditions, reverse osmosis occurs, pushing the water molecules out of the seawater into a reservoir of pure water.