Assume that we have a semi-permeable membrane with water on both sides. First, s
ID: 32379 • Letter: A
Question
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:
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.
Explanation / Answer
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.)