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Osmosis seems very counter-intuitive to me. I will try to explain my doubt with the following thought experiment:

Consider a huge container divided into two by a semi-permeable membrane. We add equal levels of water on both sides. On one side, dissolve some sugar. After some time, there will be a net movement of water to the sugar containing side of the container. This will increase the level of water on one side and decrease it on the other. If we create a huge hole in the membrane (large enough for both water and sugar molecules), then water will rush from one side to the other, equalizing the levels. If we were to somehow place a turbine at this hole, we could generate quite a bit of energy. Where did this energy come from?

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Liked the way you presented it –  WYSIWYG Jul 15 '13 at 19:32
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up vote 7 down vote accepted

I'm no thermodynamics expert, but Ill have a go at this.

The energy comes from the original set up, in which you have created a low entropy state. As the diffusion of water molecules equalises their concentration across the membrane so the entropy of the system will increase. This translates to a negative free energy change. That manifests as potential energy stored in the hydrostatic pressure resulting from the changes in volume, which in turn is available to turn the turbine.

So in the case of an experiment of the type described, the energy is derived from the experimenter who sets up the experimental situation in the first place.

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Small Addition: free energy changes and the reaction spontaneity is governed by ambient conditions. The semi permeable membrane poses an energy barrier. Given this barrier, osmotic motion, brings down the free energy by entropy maximization (as Alan Boyd said). –  WYSIWYG Jul 15 '13 at 19:41
    
I understand now, thanks a ton! –  Gerard Jul 16 '13 at 15:35
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