Why does osmosis require a semipermeable membrane?

Answer 1

Semi permeable membrane means a very thin layer of material which allows some molecule to pass through them and prevent some molecule to pass through them.

Osmosis is the passage of water from a dilute solution via a membrane which is semi permeable to a more concentrate solution.

So the membrane should be semi-permeable so as to only allow the movement of H2O molecules and not the other molecules of the mixture.

Answer 2

Osmosis involves the movement of solvent molecules from an area of higher concentration to an area of lower concentration across a semipermeable membrane. The membrane allows the passage of solvent molecules but restricts the passage of solute molecules, which drives the movement of solvent to balance the concentration gradient.