Quote:
Originally Posted by deltadoc  Two, if it were true that "only the water moves, not the salt", then assuming you have a closed system divided in half by the semi-permeable membrane, then eventually one side would contain "nothing" because all the water migrated to the salty side (or conversely, all the water would migrate to the water side, and eventually only dry salt molecules would be left on one side of the semi-permeable membrane). This can't possibly happen because of entropy.
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Osmotic pressure will prevent too much water from going to one side and drying the other. the combination of the differential salt concentration across the membrane and column height will hit an equilibrium in a close system (evaporation is eliminated). Entropy applies to the water movement across the membrane that is impeded on one side because of the salt.
Osmosis explains how you can purify water by pushing it trough a membrane so that salts are retain and pure water is collected on the other side (reverse osmosis). The pressure needed to push the water through the osmosis column is higher then the osmotic pressure working in reverse.
Check out:
Osmosis - Wikipedia, the free encyclopedia
under: basic explanation
Consider a permeable membrane, such as
visking tubing, with apertures small enough to allow water (solvent) molecules, but not larger solute molecules, to pass through. When this membrane is immersed in liquid it is constantly hit by molecules of the liquid, in motion due to their thermal kinetic energy. In this respect solute and solvent molecules are indistinguishable. At a molecular scale, every time a molecule hits the membrane it has a defined likelihood of passing through. Here, there is a difference: for water molecules this probability is non-zero; for solute molecules it is zero.
Suppose the membrane is in a volume of pure water. In this case, since the circumstances on both sides of the membrane are equivalent, water molecules pass in each direction at the same rate; there is no net flow of water through the membrane.
If there is a solution on one side, and pure water on the other, the membrane is still hit by molecules from both sides at the same rate. However, some of the molecules hitting the membrane from the solution side will be solute molecules, and these will not pass through the membrane. So water molecules pass through the membrane from this side at a slower rate. This will result in a net flow of water to the side with the solution. Assuming the membrane does not break, this net flow will slow and finally stop as the pressure on the solution side becomes such that the movement in each direction is equal: dynamic equilibrium. This could either be due to the water potential on both sides of the membrane being the same, or due to osmosis being inhibited by factors such as pressure potential or Osmotic pressure.
Entropy is the only <force> that explains how hot and cold water blend without agitation or other exterior force through a membrane because the water flows freely. For every molecule of water going through the membrane in one direction, hot or cold, one goes in the other direction. No column height difference, no osmotic pressure.
Deltadoc:....<These phthalates are securely entrenched in the food chain. They're everywhere, and we got lots of 'em in our bodies right now>
And also entrenched in the environment!
Luc