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