When a solution of higher concentration and a solution of lower concentration are separated by a semipermeable membrane, one through which solvent molecules can pass but through which solute particles cannot, solvent molecules will pass through the membrane from the side containing the more dilute solution to the side containing the more concentrated solution. This process is called osmosis. Solvent molecules actually move through the membrane in both directions but because the concentration of the solvent is higher on the right, solvent molecules on the right collide with the membrane more frequently than on the left resulting in a net flow of solvent from the right to left. If we apply pressure to the side containing the more concentrated solution, we can slow or stop the flow of solvent. This is due to the fact that more solvent molecules in the concentrated solution are colliding with the membrane since the pressure or force per unit area is increasing. The precise amount of pressure necessary to stop the flow of solvent is called the osmotic pressure. A pressure greater than the osmotic pressure will cause solvent molecules to flow from the more concentrated solution to the more dilute solution. This process is called reverse osmosis.