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View submission: Ask Anything Wednesday - Physics, Astronomy, Earth and Planetary Science
How does diffusion lead to osmotic pressure? We’re told that the random “Brownian” movements of particles (molecules, ions etc) will lead to a homogeneous concentration over time. Fine. But with the introduction of a semipermeable membrane, this tendency will lead to osmosis, and also osmotic pressure. How can random movements of particles counteract e.g. gravity? Where does the actual force come from here?
Comment by Flannagill at 06/12/2023 at 23:15 UTC
2 upvotes, 0 direct replies
This question is quite complicated to answer in one go, but I will try my best.
First things first, a system will always try to attain the lowest energy state possible.
If we are talking about osmotic pressure, we often have an ion, a charged particle, that wants to be surrounded by as many water molecules as possible. Water helps stabilize the charge in the particle which lowers the energy of the system. If we have a lot of ions in a semi permeable membrane with not that much water, the ions want to have more water around them. I hope it is now clear that having more water around the ions would lead to lower energy.
But how does the water move? Where does the energy come from? Every particle that is not at absolute 0 temperature has some thermal energy. This thermal energy shakes around the particle. This random shaking will sometimes allow the particle to move a small distance, although it will often just move back as well. In the case of osmotic pressure this random hopping particle has a change to find a place where it is more happy (lower energy) than before, which make it more likely to stay there. If many particle do this at the same time we observe this as osmotic pressure.