When a solution and pure solvent or two solutions of different concentrations are separated by a semi- permeable membrane, the solvent molecules pass through the membrane.
What is the direction of flow of solvent molecules? It is important to understand that the passage of solvent molecules through the semipermeable membrane takes place in both directions, since solvent is on both sides of the membrane. However, the rate of passage of solvent molecules into the solution or from more dilute solution to more concentrated solution is found to be greater than the rate in the reverse direction. This is favourable sincethe vapour pressure of solvent is greater than that of solution. The net spontaneous flow of solvent molecules into the solution or from more dilute solution to more concentrated solution through a semipermeable membrane is called osmosis.
As a result of osmosis, the amount of liquid on the pure solvent side or more dilute solution side decreases. Consequently, the amount of liquid on the other side increases. This results in decrease of the concentration of solution.
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