Why is osmosis is a special kind of diffusion?
Semipermeable membrane is the membrane that allows only water through pass through and not the solute.
When salt solution is separated from pure water by semipermeable membrane, water molecules from pure water move through it into salt solution. The salt molecules will not be able to move through semipermeable membrane.
However, when water is separated from salt solution by permeable membrane, both water and salt molecules will diffuse through it.
Thus diffusion through semipermeable membrane is a special type of diffusion and it is termed osmosis . It is different from diffusion of water through permeable membrane.