1
$\begingroup$

How do the molecules of one type 'know' there are less number of the same type on the other side of a semipermeable membrane? Does it have to do with the overall energy state of the system?

$\endgroup$
  • $\begingroup$ 1. What about e.g. the mechanism section of the Wikipedia article is unclear to you? 2. What is this supposed to have to do with quantum mechanics? $\endgroup$ – ACuriousMind May 22 '16 at 17:35
  • $\begingroup$ Related: Is osmosis a kind of thermal machine? $\endgroup$ – John Rennie May 22 '16 at 19:07
  • $\begingroup$ @ACuriousMind sorry for the late reply. Here's what I understood of the mechanism from the wiki: $\endgroup$ – Rorschach 0007 May 23 '16 at 5:22
  • $\begingroup$ The problem I had is why would the membrane repel the solute molecule? Also isn't it just as likely or more likely for water molecules from the same side to move into neutralize the vacuum created, rather than water molecules being sucked in from the other side? Also if we're thinking of molecules as little balls why wouldn't the repelled solute molecule form a snug fit in the hole of the semipermeable membrane? $\endgroup$ – Rorschach 0007 May 23 '16 at 5:27
1
$\begingroup$

Solvent molecules are in higher concentration on one side of the barrier, so they are more likely to hit and pass through the barrier, until the concentrations equalise.

$\endgroup$
1
$\begingroup$

It's ultra simple: for molecules small enough to pass through, a given proportion of them on one side will pass through. Same "law" on each side. So, naturally, the most populated side (i.e., largest number of the given kind of molecule per unit volume) will invade more and thus win the balance of transfer.

Now, the density of a kind of molecule on one side also depends of how many competitors fight for the same space there. And if on one side lays molecules too large to pass through, there will be less room here for the small ones, and thus a smaller density of them. Which will mechanically settle an unbalance resulting as a (virtual) sucking of small molecules.

That why pure water injected in biologic tissues tends to explode the cells, while very salty water tend to dry them.

$\endgroup$
-3
$\begingroup$

Osmosis is not water moving down a concentration gradient. Consider dextrose 5.4% Vs saline 0.9%. there is equilibrium but the water concentrations are quite different. Osmosis is an empirical law related to the gas laws. Vant Hoff did not understand it and he got the first novel chemistry prize for describing it.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.