Physical Properties of Liquids

.
G. Osmosis

When a substance dissolves in a liquid, the resulting mixture is called a solution. Osmosis occurs when molecules of the initial liquid pass through a membrane, but molecules of the dissolved substance do not. The molecules of the initial liquid can pass through the membrane because they are relatively small. Osmosis tends to equalize the concentration of the solutions on both sides of a membrane. The membrane in this case is called semipermeable, because it allows one part of the mixture to pass through but not another. Cells in living organisms consist mostly of water, and they are surrounded by a watery environment. If the concentration of a dissolved substance, such as sugar or salt, differs inside and outside a cell, osmosis causes water to pass through the cell’s membrane from the area of lower concentration to the area of higher concentration, until the concentration on each side of the membrane is equal. Osmosis makes sugar and salt good food preservatives. When harmful bacteria encounter sugary or salty foods, water flows from the area of lower concentration—the cells of the bacteria—to the area of higher concentration—the food. The flow of water out from the bacteria’s cells dehydrates the bacteria, which kills it.

Osmosis

Osmosis, in botany and chemistry, the flow of one constituent of a solution through a membrane while the other constituents are blocked and unable to pass through the membrane. Experimentation is necessary to determine which membranes permit selective flow, or osmosis, because not all membranes act in this way. Many membranes allow all or none of the constituents of a solution to pass through; only a few allow a selective flow. In classic demonstration of osmosis, a vertical tube containing a solution of sugar, with its lower end closed off by a semipermeable membrane, is placed in a container of water. As the water passes through the membrane into the tube, the level of sugar solution in the tube rises visibly. A semipermeable membrane that may be used for such a demonstration is the membrane found just inside the shell of an egg, that is, the film that keeps the white of the egg from direct contact with the shell. In this demonstration, the water moves in both directions through the membrane; the flow is greater from the vessel of pure water, however, because the concentration of water is greater there, that is, fewer dissolved substances exist in this solution than in the sugar solution. The level of liquid in the tube of sugar solution will eventually rise until the flow of water from the tube of sugar solution, under the influence of hydrostatic pressure, just equals the flow of water into the tube. Thereafter, no further rise in level will occur. The hydrostatic pressure establishing this equality of flow is called osmotic pressure. A variety of physical and chemical principles is involved in the phenomenon of osmosis in animals and plants.