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From: ske@pkmab.se (Kristoffer Eriksson)
Newsgroups: sci.bio,sci.chem,sci.physics,sci.misc
Subject: Re: Osmosis - the cause at the molecular level.
Message-ID: <4396@pkmab.se>
Date: 31 Oct 90 22:14:29 GMT
References: <1990Oct28.115303.7221@newcastle.ac.uk>
Organization: Peridot Konsult i Mellansverige AB, Oerebro, Sweden
Lines: 42

In article <1990Oct28.115303.7221@newcastle.ac.uk> W.P.Coyne@newcastle.ac.uk writes:
>         |        *        |        |^^^^^^^*        |
>         |^^^^^^^^*^^^^^^^^|        |       *        | 
>         |        *        |        |       *^^^^^^^^|
>         | high   *  lo    |        |medium * medium |
>         |        *        |        |       *        |
>         ------------------          ----------------
>            BEFORE                       AFTER
>
>What mechanism is allowing the salt to cause the water to build
>up on left hand side?

The way I remember it, the key to this is that the salt ions can't pass
through the membrane, only the water can. The solutions on both sides of
the membrane tend to even out their respective concentrations, and the
only way that can be done is by raising the amount of water on the side
with the higher salt concentration, and vice versa, since the amount
of salt can't be changed.

Why does the concentrations tend to even out? There will always be a lot
of water molecules that are passing through the membrane in both directions,
since nothing is stopping them from doing so. However, on the side with
higher salt concentration, there will be fewer water molecules (and more
salt ions) adjacent to a given membrane surface area, and therefore a
slightly lower flow of water from that side to the other than in the other
direction.

>Would the container need to be very large before gravity would
>prevent the water on left hand side rising noticeably.

If the water remaind and equal levels, gravity would also act equally on
both sides, and would therefore not stop the sides from differentiating.

I suppose that if one of the sides would rise considerably above the
other side, it could be compressed enough by gravity to make the number
of water molecules to a given surface area on both sides equal slightly
before the concentrations become equal. (The water pressures would become
equal without the solution concentrations being equal.)
-- 
Kristoffer Eriksson, Peridot Konsult AB, Hagagatan 6, S-703 40 Oerebro, Sweden
Phone: +46 19-13 03 60  !  e-mail: ske@pkmab.se
Fax:   +46 19-11 51 03  !  or ...!{uunet,mcsun}!sunic.sunet.se!kullmar!pkmab!ske