According to the definition, a semi-permeable membrane is a membrane that allows only solvent molecules to pass through it; solute molecules are blocked. Solvent molecules flow from the membrane side that has low solute concentration (i.e. high solvent concentration) to high solute concentration (i.e. low solvent concentration).
Lets take a case:
- I take oil as a solvent in one side and water as a solvent in other side. I take the same solute in both sides.
- Initially, the water side is filled to height $h$ and the oil side to height $2h$. The amount of solute is same in both the sides. So the solute concentration is more in water than in the oil. Accordingly, oil will flow from the oil side to water side. Now, I add more water on the water side to reduce the solute concentration on the oil side. Accordingly, water should now flow towards the oil side.
So what will happen? Will this happen? What makes the semi-permeable membrane so smart or intelligent?