How does reverse osmosis work? – MC
Normal osmosis in water is a process in which pure water flows through a semi-permeable membrane to dilute a concentrated solution on the other side. It is driven by statistics—it’s much more likely for a water molecule on the fresh water side to pass through the membrane than it is for a water molecule on the concentrated solution side to pass through the membrane. There are simply more water molecules trying to cross the membrane from the fresh water side! In fact, water molecules will continue to flow from the fresh water side to the concentrated solution side until the solution has been highly diluted or an accumulation of pressure on the solution side slows the passage of water and brings it to a halt.
Reverse osmosis occurs when the pressure on the solution side is raised so high that the movement of water reverses directions. If you squeeze the concentrated solution hard enough, you can drive additional water molecules from that solution through the semi-permeable membrane and into the fresh water on the other side. The raised pressure on the solution changes the statistics, making it more likely for water molecules to go from the solution side to the fresh water side. This technique is used to purify water in homes and to desalinate water in desert countries.