Can you clearly explain osmotic pressure for me?

[Misr]

Hello,world..missed you really

I've searched the internet for a good explanation for osmotic pressure
see this page for exapmle
http://chemed.chem.wisc.edu/chempaths/GenChem-Textbook/Osmotic-Pressure-854.html
the idea I don't understand here Is why applying a pressure to the solution of higher concentration prevents water flow from the solution of lower concentration? that's my problem with osmotic pressure which is purely a problem with physics
thanks in advance

 

[CWatters]

I think you first need to understand Osmosis..

http://en.wikipedia.org/wiki/Osmosis

When the membrane has a volume of pure water on both sides, water molecules pass in and out in each direction at exactly the same rate; there is no net flow of water through the membrane.

Osmosis can be explained using the concept of thermodynamic free energy: the less concentrated solution contains more free energy, so its solvent molecules will tend to diffuse to a place of lower free energy in order to equalize free energy. Since the semipermeable membrane only allows solvent molecules to pass through it, the result is a net flow of water to the side with the more concentrated solution.

If you understand that then it's not hard to see how increasing the pressure on the more concentrated side pushes more molecules back the other way slowing or stopping the process.

 

[0xDEADBEEF]

If you have multiple substances in a solution, and you look at what happens at a semi-permeable membrane, you can treat the system as if the pressure is split equally among the substances (it's called partial pressure). So when you have water with say salt on one side and water without salt on the other side both at the same pressure, water will flow towards the salt water, because the pressures in the water system are different: The partial pressure of the water in the salt water is smaller because some part is going to the salt. Maybe you already see the answer to your question now. If you increase the pressure of the salt water the partial pressure of the water increases, it may still only be say 99% of the total pressure but these 99% are as high as the pressure on the water side, so no water flows. If you increase the pressure even more, you can do reverse osmosis and produce fresh water.

 

[Misr]

If you understand that then it's not hard to see how increasing the pressure on the more concentrated side pushes more molecules back the other way slowing or stopping the process.

I still can't understand osmosis in this thermodynamical point of view,I've never studied thermodynamics before
but can you tell me what makes he less concentrated solution contains more free energy?I've read this somewhere on the internet n copuldn't understand why

The partial pressure of the water in the salt water is smaller because some part is going to the salt

both of you are introducing all brand new conceptions for me I don't understand this at all but I would be very happy If you explain this to me in a bit simpler way..
my textbook is not dealing with thermodynamical views or partial pressure which makes me unable to understand you
I don't undestand this partial pressure>>
thanks

 

[Studiot]

Have you covered the kinetic theory of fluid pressure?

That is the theory that fluid pressure is the average effect of all the fluid particles bumping into the container walls?