Penetration is common in nature. For example, when a cucumber is placed in salt water, the cucumber will lose water and become smaller. The process by which water molecules in cucumber enter the brine solution is the infiltration process. If a water tank is used to permeate a pool, the water pool is divided into two parts, and pure water and brine are injected into the same height on both sides of the diaphragm.
After a while, it can be found that the pure water level is lowered, and the liquid level of the brine is raised. The phenomenon in which water molecules migrate through the membrane into the brine is called infiltration. The rise in salt water is not endless, reaching a point of equilibrium at a certain height. At this time, the pressure represented by the liquid level difference at both ends of the diaphragm is called the osmotic pressure. The magnitude of the osmotic pressure is directly related to the concentration of the brine.
After the above device reaches equilibrium, if a certain pressure is applied to the liquid surface of the brine end, the water molecules will migrate from the brine end to the pure water end. The phenomenon that the liquid molecules migrate from a dilute solution to a concentrated solution under pressure is called a reverse osmosis phenomenon.
If brine is added to one end of the above facility and a pressure above the osmotic pressure of the brine is applied at that end, we can get pure water at the other end. This is the principle of reverse osmosis water purification. There are two key points in the production of pure water by reverse osmosis facilities. One is a selective membrane, which we call semi-permeable membrane, and the other is a certain pressure.