When a solution (say of sugar) is separated from the pure solvent (water in present case) by means of a semipermeable membrane (a membrane which allows only the solvent molecules but not the solute molecules to pass through it), the pure solvent passes through the membrane and goes to solution (osmosis)1 till the hydrostatic pressure of the liquid column exactly balances the tendency of water to pass inward through the semipermeable membrane2. The hydrostatic pressure set up as a result of osmosis is a measure of the osmotic pressure of the solution. For instance, if the solution of density d rises to height h, then osmotic pressure
\(\pi = h \times d \times g\), where g is the acceleration due to gravity.
Thus osmotic pressure may be defined as the excess pressure which must be applied to a solution in order to prevent flow of solvent into the solution through the semipermeable membrane.
Osmotic pressure may further be defined in several other ways.
(i) Osmotic pressure is the excess pressure which must be applied to a given solution in order to incresase its vapour pressure until it becomes equal to that of the solution.
(ii) Osmotic pressure is the negative pressure which must be applied to (i.e. the pressure which must be withdrawn from) the pure solvent in order to decrease its vapour pressure until it becomes equal to that of the solution.
(iii) Osmotic pressure is the hydrostatic pressure produced when a solution is separated from the solvent by a semipermeable membrane.
