Diffusion is the movement of molecules from a region of higher concentration to a region of lower concentration.
Osmosis
is a special case of diffusion in which the molecules are water and the
concentration gradient occurs across a semipermeable membrane. The
semipermeable membrane allows the passage of water, but not ions (e.g.,
Na+, Ca2+, Cl-) or larger molecules (e.g., glucose, urea, bacteria).
Diffusion and osmosis are thermodynamically favorable 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 the water
is moved across the membrane against the concentration gradient, from
lower concentration to higher concentration.
To illustrate, imagine a
semipermeable 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 fresh water 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).
