Osmosis

A selectively (differentially) permeable membrane will only allow certain molecules to pass through it. Let us see when equal volumes of a weak and a strong sugar Solution are separated by a selectively permeable membrane.

    Water molecules can pass through a selectively permeable membrane easily but sugar molecules cannot. The weak sugar solution contains a higher concentration of water molecules than the strong one. A diffusion gradient exists between the water molecules in two solutions. As a result, water moves from one weak sugar solution tot he strong one through the selectively permeable membrane. This movement of water molecules continues until the concentration of the sugar solution is the same on both sides of the membrane. In this case, equilibrium had been achieved by the movement of water molecules only. Thermal movement of water molecules through a selectively permeable or a semi permeable membrane is known ad osmosis. It is a special case of diffusion in which only water molecules move from a region where they are in high concentration to one where they are in low concentration.