What causes osmosis?

[scilearner]

Hello everyone,

 

I'm getting the feeling I have not understood osmosis all this time even though I thought I did. So if 2 solutions are separated by a water permeable membrane, and one solution has less solutes than the other, water would move to less concentrated area right. Now why does this occur. In the region where there are more solutes are their polar bonds between water and solutes restricting the movement of water molecules in that region, does this create free space for water from high concentration area to move in? If there is more water in one area I can understand how it would diffuse to the less water area but if I consider the whole thing as particles, if both sides have same amount of particles why would anything diffuse to the other side. I mean first side has more water+less solutes and other side less water+more solutes, so particles in both sides are similar so where is the free space for water to diffuse.Sorry if I made a mess of this. Thanks

Edited July 17, 2010 by scilearner

[Physicsfan]

Diffusion of water molecules through a semipermeable membrane does not mean a one way flow of water molecules from the region of higher concentration of water molecules(more dilute solution) to a region of lower concentration(less dilute solution). When two solutions of different solute concentrations are separated by a semipermeable membrane,the number water molecules flowing through semipermeable membrane from more dilute solution is greater than the number of water molecules flowing from less dilute region. The reason for flow on both sides is because the water molecules as well as solute molecules hit against the membrane uniformly, but on the side of the less dilute solution, more solute molecules hit the membrane per unit area.On the side of more dilute

solution its the other way round.

[scilearner]

Thanks Physics fan, that was quite a good answer.

[cypress]

In addition to the water concentration or density differential there is also typically a pressure differential providing the driver for preferential flow of water out of the concentrated side of the membrane and into the dilluted side.