eli5: if particles want to move from high concentrations to low, how does reverse osmosis happen?


eli5: if particles want to move from high concentrations to low, how does reverse osmosis happen?

In: 2

because of outside forces. if you’re refering to the water purification method, we add a pressure to one side of the membrane so that particles move and we end up with the good water on one side and bad water on the other.

You have to physically force it, using an input of energy. Reverse osmosis is never spontaneous.

Example: Water molecules are very small, even compared to other molecules. When they do “reverse osmosis” to water to purify it – what’s actually happening is they use super high pressure pumps to force the water-with-dissolved-stuff through a sieve or filter with holes *so* small that the water molecules can pass through and the dissolved stuff can’t. As you can imagine, forcing water through a mesh with holes barely bigger than water molecules themselves takes a ton of force and energy.

The result of this process is the water going through the “mesh” comes out with a much lower concentration of dissolved stuff. Since the concentration of stuff is going from high to low, that’s like the “reverse” of osmosis, where water *naturally on its own* moves to reduce high concentrations of dissolved stuff. So that’s why we call the process “reverse osmosis”. Because *the net result* is the opposite of what osmosis does. But the process is **not** just “the same thing happening in reverse”. It’s an entirely human made mechanical and physical separation process requiring an energy input. Vs osmosis, which is something that naturally happens on its own.

Most of the time when you hear the phrase “reverse osmosis” it is just a slightly fancy way of saying “filter.”

You apply pressure to one side of a porous screen where the holes are big enough to allow water to seep through but not the particulates and the particles are left on one side as the water is pushed through to the other.

It’s “reverse” osmosis because if the pressure was not there then water from the clean side would tend to flow across the barrier to dilute the impure side in order to try to achieve equilibrium. That’s regular osmosis. So if you put pressure in to force the water back through the barrier it’s reversing the process.

Which sounds a lot more scientific than saying “we poured it through a cheap filter.”