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Osmosis creates pressure on the side of the membrane with higher concentration. But where does the energy for this come from?

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All things naturally tend towards equilibrium, moving from high energy to low energy. Osmotic pressure is the force that helps achieve osmotic equilibrium, so it is really just a manifestation of that natural tendency. You don't really need energy to create osmotic pressure, the osmotic pressure will be present until equilibrium is reached. It comes from the higher concentration itself, not some other force. Osmosis is a sort of 'gradient-driven' process, which comes about through entropy.

https://courses.lumenlearning.com/introchem/chapter/osmotic-pressure/ https://simple.wikipedia.org/wiki/Osmosis

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  • $\begingroup$ I think this answer skirts around and shows an unawareness of chemical potential energy, might be exactly what he is looking for (or at the very least, related and worth mentioning). $\endgroup$ Commented Oct 22, 2020 at 19:33
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Consider a U shaped tube with a membrane in the middle, permeable to water but not to salt. It is partially filled with water. The height of the water surface on both sides of the tube will be the same, as they both feel the same atmospheric pressure.

Now we add salt to the left side of the tube, which fully dissolves (and it can't go through the membrane so it all stays there). We observe that the water level on the left rises and on the right side it falls. So there was a net movement of water molecules from right to left.

This movement went against gravity, as it lifted some of the solution, which is now weighting down on the rest of the liquid, on top of the atmospheric pressure already there. Yet the solution stays risen.

Such net movement, implies an attraction between water and salt. Salt dissolves mostly because of entropic effects (rather than, say, forming more stable bonds). Once dissolved, the water molecules are attracted to the charges of the ions, which are stronger than the mere dipole moment other water molecules have.

It is this attractive force that pushes water to the left, and causes the solution to rise. In other words, this attraction supplies the necessary energy to make the solution rise.

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  • $\begingroup$ No. If we use sugar to simplify (no dissociation) the water will not move from right to left as the pressure due to the water remains the same on both sides. The side with the sugar will have higher pressure because that side of the membrane is being hit by the sugar molecules as well as by the water molecules. In equilibrium that extra pressure is equal to the weight of the added sugar. With dissociation there are extra factors, but they are complications which obscure simplicity of the basic osmosic mechanism. $\endgroup$
    – mike stone
    Commented Oct 27, 2020 at 15:46

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