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From: william@lorien.newcastle.ac.uk (William Coyne)
Newsgroups: sci.bio,sci.chem,sci.physics,sci.misc
Subject: Osmosis - the cause at the molecular level.
Summary: osmosis - the cause at the molecular level
Message-ID: <1990Oct28.115303.7221@newcastle.ac.uk>
Date: 28 Oct 90 11:53:03 GMT
Sender: news@newcastle.ac.uk
Reply-To: W.P.Coyne@newcastle.ac.uk
Organization: Chemical & Process Engineering Dept, University of Newcastle, UK.
Lines: 32

Thanks to those who replied to my earlier question on osmosis, 
but none explained what was going on at the atomic or molecular
level.  Perhaps the following will make my question clearer.

Imagine a container with a semi-permiable membrane down the middle
with a high salt concentration solution on one side and a low
conc solution on the other.  Initially both solutions are the
same depth, then after the membrane is uncovered osmosis begins.
Eventually the depths of the water on either side will stop 
changing and one side will be deeper than the other(see below)

         |        *        |        |^^^^^^^*        |
         |^^^^^^^^*^^^^^^^^|        |       *        | 
         |        *        |        |       *^^^^^^^^|
         | high   *  lo    |        |medium * medium |
         |        *        |        |       *        |
         ------------------          ----------------
            BEFORE                       AFTER

What mechanism is allowing the salt to cause the water to build
up on left hand side?  Are the salt molecules forming weak bonds
with the water molecules so reducing the number of molecules
passing from the high to low concentration?
Would the container need to be very large before gravity would
prevent the water on left hand side rising noticeably.
 
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