Osmosis: How to Go Against It
By William Chen '25
If you’ve learned about biology, chemistry, or whatever science course, chances are you know what osmosis is.
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Basically, osmosis is the movement of solvent molecules (which, in most cases, is water), through a semipermeable membrane (a membrane that only allows the passing of water but not the solute), from the region with lower solute concentration to the region with higher solute concentration.
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The pressure required to prevent the effects of osmosis is called the osmotic pressure, also represented by the Greek letter Π (yay our theme!). That is, if we don’t want the water to flow towards the higher concentration side, we would have to at least exert that amount of pressure.
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So how can we use this to do stuff?
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One of the most common applications of osmotic pressure is in water purification, a process called reverse osmosis. Unpurified water has a lot of things in it, such as dissolved ionic substances (such as chlorine and other substances used for disinfection), some solid impurities, microorganisms (and their corpses), and also other molecules.
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So basically what we have is water and a semipermeable membrane or membranes. For the most part, if we just leave it be, then some water’s going to go through the membrane, and eventually, they form an equilibrium. That way, most of the water will be wasted. So how do we get over the osmosis problem? Here comes the pressure.
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Remember the osmotic pressure? Any pressure greater than that is going to let the water go the other way, pushing the water away from the source of pressure. That is exactly what happens in reverse osmosis purification systems. Unpurified water is given a higher pressure (or hydraulic pressure), and then that pressure drives water to go against its osmotic gradient, as other substances are left behind.
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Despite being able to filter water efficiently, it requires a lot of energy to generate the water pressure, making it not really sustainable to use. That’s why there are a lot of other developing water filtering technologies being developed.
So, how do you go against osmosis? Simple pressure will do.