When dilute solution and concentrated solution with the same volume are placed on two sides of a container respectively, and the middle is blocked by a semi-permeable membrane, the solvent in the dilute solution will naturally flow to the concentrated solution side through the semi-permeable membrane, and the liquid level on the concentrated solution side will be higher than that on the dilute solution side by a certain height, forming a pressure difference to reach the osmotic equilibrium state. This pressure difference is called osmotic pressure. If a pressure greater than osmotic pressure is applied to one side of the concentrated solution, the solvent in the concentrated solution will flow to the dilute solution, and the flow direction of this solvent is opposite to the original osmotic direction. This process is called reverse osmosis.