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Q & A: osmotic pressure and flow

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Most recent answer: 12/25/2015
Q:
What is the physics behind osmotic pressure ie the pressure applied on the surface of a solution (separated from a pure solvent by semipermeable membrane) to stop osmosis?
- Sanjukta (age 20)
Asansol,West Bengal,India
A:

There are two causes of the flow through the membrane:

1) the osmotic pressure difference caused by different solute concentrations
2) the actual pressure difference.

Flow stops when these two effects cancel. 

I guess you don't need an explanation for (2), but do want one for (1). Why does the solvent flow from the side with low solute concentration to the side with high solute concentration? Nature always flows toward conditions of high total entropy. (That universal rule is called the Second Law of Thermodynamics.) Higher entropy conditions mean conditions which describe larger numbers of quantum microstates. 

So the question is: why would a solvent molecule make more microstates by flowing to the high-solute-concentration side? the essential point is that the more solvent molecules, the more room is available for the solute particles to roam around. On the high-solute-concentration side, the solvent particle contribute more to the roaming-around entropy of the solute. So the solvent flows to that side.

The fundamental physics is very closely related to the reason why solute lowers the freezing point and raises the melting point. See espececially Follow-Up #5 here: https://van.physics.illinois.edu/qa/listing.php?id=1608.

Mike W.

 


(published on 12/25/2015)

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