Diffusion is the movement of molecules from a region of higher concentration to a region of lower concentration. Osmosis, however, is a special case of diffusion in which the molecules are of water and the concentration gradient occurs across a semi- permeable membrane. The semi-permeable membrane allows the passage of water, but not of ions (e.g. Na +, Ca 2+, Cl -) or larger molecules (e.g. glucose, urea, bacteria). Diffusion and osmosis are thermodynamically favourable and will continue until equilibrium is reached. Osmosis can be slowed, stopped, or even reversed if sufficient pressure is applied to the membrane from the ‘concentrated’ side of the membrane.
Reverse osmosis occurs when water moves across the membrane against the concentration gradient –– from a region of lower concentration to one of higher concentration. To illustrate, imagine a semi-permeable membrane with fresh water on one side and a concentrated aqueous solution on the other side. If normal osmosis takes place, the fresh water will cross the membrane to dilute the concentrated solution. In reverse osmosis, pressure is exerted on the side with the concentrated solution to force the water molecules across the membrane to the freshwater-containing side. Reverse osmosis is often used in commercial and residential water filtration. It is also one of the methods used to desalinate seawater. Sometimes reverse osmosis is used to purify liquids in which water is an undesirable impurity (e.g. ethanol).
0 Comments