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Assume that we have a semi-permeable membrane with water on both sides. First, straightforward, case: On the right side, we have a concentration of the substance A, and on the left side, we have pure water. Obvious effect: water flows from left to right.

Now, imagine we have a second case. We have the same membrane, and a water solution of A to the right. To the left, we have a water solution of a different substance, let's call it B. Now, what happens? I can imagine a few possibilities:

  • the system behaves exactly as in the first case, because the substances have no influence on each other
  • the system behaves as if it was a solution of A on both sides, with water flowing in the direction of higher concentration (because it only matters if there is something dispersed between the water molecules, but it does not matter what)
  • some interesting cross-influence effect happens
  • either one of the three things above can happen, depending on the particular combination of substances A and B

Which one is the true effect? What happens, and why?

If you need an example of the actual substances, let B be thiamine and A be plain sodium chloride, but I am interested in the general case.

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up vote 3 down vote accepted

If A and B are at the same concentration there will be no net movement of water.

Everything is down to diffusion. In your example of solution A/pure water, water molecules on both sides of the membrane are diffusing through the membrane to the other compartment. However the "concentration" of water is less in solution A, so the rate of movement is less from solution A to water than it is from water to solution A. Water will continue to move into the solution until another force, such as hydrostatic pressure, counteracts the "osmotic pressure".

If solution A and solution B have solute at the same concentration then the rate of movement of water will be the same in both directions, so no net movement will occur.

(In a typical experimental set up using dialysis membrane none of this would apply with the solutes that you mention since they would also be able to move through the pores and you would end up with perfect mixing. You would have to use macromolecular solutes.)

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To make sure I understand your answer: you are saying that if we have a high concentration of B on the right, and low concentration of A on the left, water will flow from left to right? – rumtscho Jun 30 '13 at 12:01
Yes, that's correct. – Alan Boyd Jun 30 '13 at 12:13
If one solution can permeate and the other can't, would there be any net movement of particles? – Cayetano Gonçalves Mar 2 at 1:40

Actually, the so-called osmolarity of a solution is only determined by the concentration and number of dissociating species in the solute, and is irrespective of the chemical composition of the solute. This is an example of what is known as a colligative property in chemistry.

The van't Hoff equation gives the osmotic pressure exhibited by a solution:

P = icRT

where P is the osmotic pressure, i is the coefficient associated with the number of dissociating units in the solute, c is the molar concentration of the solute, R is the universal gas constant, and T is the absolute temperature. A nonionic species, such as glucose, would have a value of 'i' close to one as the molecule does not dissociate in solution, while NaCl in relatively dilute concentrations should have an 'i' value close to 2, as it dissociates into sodium and chloride ions in solution.

Thus in the case of the two solutions on opposite sides of the semipermeable membrane as you described, water would flow from the region of lower osmolarity to the region of higher osmolarity, until the osmotic pressure on both sides of the membrane come to equilibrium (due to the changes in concentration of the solutes by the net water movement).

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