The fourth colligative property is osmotic pressure, and it is useful in molecular-weight determinations when freezing point depressions are not.

Many membranes have pores large enough to let some molecules pass through, but too small to pass others. These are known as semipermeable membranes. Some will permit water to pass, but not ions or salts. Others, with larger pores, will be permeable to water, salts, and small molecules, but not to protein molecules with molecular weights in the thousands.

This selective passage of ions and small molecules but not proteins is called dialysis, and is a common biochemical method of separation and purification. Our kidneys essentially are fine networks of dialysis tubing, excreting liquids, salts, and small waste molecules, but at the same time preventing the loss of proteins from body fluids.

Artificial kidney machines simulate this blood-purification process with man-made dialysis tubing, in which the blood from the patient flows across one side of semi-permeable membranes, and wash fluid flows across the other.

Picture of a dialysis machine