Osmosis was studied by Thomas Graham, who coined the term (1858); in 1886 Van't Hoff showed that, for dilute solutions (obeying Henry's law), the osmotic pressure varies with temperature and concentration as if the solute were a gas occupying the volume of the solution. This enables molecular weights to be calculated from osmotic pressure measurements, and degrees of ionic dissociation to be estimated. Osmosis is important in dialysis and in water transport in living tissues.
Osmosis of a sugar solutionSemipermeable membranes have small pores in them. While the pores allow small molecules through (1, in the second diagram from the top), they also block larger molecules (2).
If a semipermeable membrane is used to separate a sugar solution from a container of water (3), initially the concentration of water molecules in the solution will be lower than that in the pure water. Over time, water molecules will pass through the membrane, diffusing into the container, but as the pores are too small to allow the sugar solution to flow the other way, the level of the sugar solution rises.
Osmosis in plant cellsOsmosis can be studied quite easily in plant cells. The cells are surrounded by a stiff outer wall which is lined with cytoplasm. Inside the cytoplasm is a space (vacuole) full of liquid (sap). The pressure of the sap presses the cytoplasm against the cell wall just like air in a leather football presses the bladder against the case. The cytoplasm is selectively semipermeable and lets water and some dissolved salts in. If the cell is put into a strong solution, water will pass out of the cell which will then lose its turgor and the cytoplasm will shrink away from the cell wall. If the cell is then put into pure water, water will pass into the sap and push the cytoplasm against the wall again. If the cell sap is very concentrated it may absorb enough water to burst the cell wall. Normally the cell wall is strong enough to overcome the osmotic force.
All water-living animals, unless they have impermeable skins, must have some method of getting rid of the excess water that they absorb. The process is called osmoregulation and is found in many types of animal. If they did not get rid of the water they would burst because, unlike the case in plants, the cells have no cellulose walls.
Reverse osmosisReverse osmosis is the movement of a liquid through a semipermeable membrane from a concentrated solution to a less concentrated solution. This movement is in the opposite direction to that in normal osmosis and can be achieved only by applying external pressure to the concentrated solution.
It is used in desalination plants to produce drinking water from seawater. The process is generally employed only on a small scale because the pressures involved are so high (up to 25 atmospheres).
Related categories PHYSICAL CHEMISTRY
PROPERTIES OF MATTER
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