How does reverse osmosis work to purify water?
To understand ”reverse osmosis”, it is probably best to start with normal ”osmosis” first. According to Merriam-Webster’s Collegiate Dictionary, Osmosis is the ”movement of a solvent through a semipermeable membrane (as of a living cell) into a solution of higher solute concentration that tends to equalize the concentrations of solute on the two sides of the membrane”. That’s a mouthful. To understand what it means, this picture is helpful: On the left is a beaker filled with water, and a tube has been half-submerged in the water. As you would expect, the water level in the tube is the same as the water level in the beaker. in the middle figure, the end of the tube has been sealed with a “semi-permeable membrane” and the tube has been half-filled with a salty solution and submerged. Initially the level of the salt solution and the water are equal, but over time something unexpected happens – the water in the tube actually rises. The rise is attributed to “osmotic pressure”. A sem
