Semi-permeable Membrane: Membrane bombardment theory
A membrane that allows only the solvent molecules to pass through it, but not the solute molecules is known as a semi-permeable- membrane. Natural membranes are mostly semi-permeable. For examples, skin inside the egg shell, membranes around the red blood corpuscle, animal bladder, and vegetable tissue are all semi-permeable.
Membrane bombardment theory: The fundamental idea behind this theory is the unequal bombardment of the solvent molecules on the two sides of the semi-permeable membrane. Since there are fewer solvent molecules on the solution side of the membrane compared to the solvent side, there are fewer bombardments per unit area of the surface on the solution side than on the solvent side. Thus, solvent molecules will diffuse slowly through the membrane from the solution side than on the solvent side. The net result is the flow of solvent from the pure solvent to the solution across the membrane.