Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

What causes osmosis?

  1. Jul 17, 2010 #1
    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 :smile:
  2. jcsd
  3. Jul 17, 2010 #2


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Most concentration gradients tend to even out when possible.* It's not necessarily a matter of free space existing or not existing. It can be shown that the Second Law requires that differences tend to smooth themselves out over time.

    *(More precisely, gradients in chemical potential always tend to even out. For salts in water, the concentration scales similarly with the chemical gradient.)
  4. Jul 29, 2010 #3
    sameeralord, it seems to me that your problem stems from two sources of misunderstanding.
    Firstly, suppose you have two liquids in contact with each other, but in such a way that there is no convection. In other words, the only way that they can mix is by their molecules (or ions, whichever particles are relevant) diffusing into each other. If you find it simpler, you might assume that their molecules have the same molecular mass, so that they both diffuse about equally fast. They might be the same chemical compound, but isotopically different for example. For a more detailed example, we might have water containing oxygen 18 and hydrogen 1, in contact with water containing oxygen 16 and deuterium.
    Now, the way you see this, it seems that you imagine that there would be no diffusion into each other's volume because there is no "free space" for the molecules to get into. That is only partly true even for solids. (It has long been known, and shown, that if a solid block of silver placed onto a block of gold, then some atoms from each block will diffuse into the other block.)
    The thing is that in each liquid the molecules are bouncing around, bumping into each other and bouncing off, but not remaining in place. There is enough room for them to pass each other part of the time and they do not hold onto each other so firmly that none can pass. For a rough comparison, suppose that you had a tray loosely filled with variously coloured, but otherwise identical marbles which you gently shook. Sooner or later you would discover that no marble was in its original position.
    That gives you some idea of what happens within a mass of liquid, but now let us consider what happens at the interface between two such masses of liquid.
    I am sure that you can see that, just as the molecules had been slipping past other molecules of the same kind, they would often slip past other molecules of the different kind. Free space does not play much of a role in this. We can assume that there will be enough free space. There will not be so much free space that the molecules will get past each other as if they were not there (there is a lot of bouncing back going on) but they will generally get past sooner or later.
    If the molecules have very much the same behaviour, then the traffic in each direction will be very much the same. But suppose that one of the liquids is a 50% mixture of molecules of types A and B, whereas the other is pure type B. Then something quietly dramatic happens. The 50% mixture of say, molecules of types A and B, only has half as many molecules of type B as the pure liquid. Therefore, purely by chance, at the start, anyway, there will be twice as many molecules of type B moving from the pure liquid to the mix, as in the opposite direction. The rest of the discrepancy is taken up by the type A molecules moving along with their type B neighbours.
    That may not yet look very important, but then ask yourself what would happen if we magically prevented the type A molecules from crossing the boundary. Suddenly that discrepancy is no longer taken up, and we have type B molecules moving twice as often in one direction as in the other.
    Get the picture?
    Stop press! Magic discovery! Someone has invented a way in which we actually can stop molecules of type A from crossing the boundary. The inventor has decided to call his discovery: a semi-permeable membrane.
    Still with me?
    Now what happens is that the molecules of type B are pressing twice as hard in one direction as the other. This is what they had been doing in the first place, but the molecules of type A had been doing the other half of the pressing.
    Surely I am talking nonsense here? We had agreed that none of the molecules was moving in any special way different from the others, yes? We even had chosen isotopes that permitted us to be sure of this. Then how can I say that the type B molecules are pressing harder one way then the other?
    Ask yourself what the pressure of a fluid amounts to. If I have two fluids of the same type and temperature, then the pressure that each exerts is the force of the molecules bouncing against the surface that takes the pressure. Double the number of molecules bouncing, and you will double the pressure. Right?
    Well then, we will have double the pressure from type B molecules on one side of that semipermeable membrane!
    Does that help?
    Call back if not.
  5. Aug 3, 2010 #4


    User Avatar
    Gold Member

    Last edited by a moderator: May 4, 2017
  6. Aug 3, 2010 #5
    Osmosis is the diffusion of water from LOW SOLUTE concentration to HIGH SOLUTE concentration.

    For example, in a solution of very salty water (80% water and 20% salt) and pure H2O (100% water) separated by a semi-permeable membrane, water will diffuse to the salty side.
    The pure water still exists a 100% water but has decreased volume, and the salt solution absorbed water and now has a relatively lower salt concentration.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook