Changing the concentration of solute in the right-hand
compartment of the last simulation moved the entire system away
from equilibrium. Diffusion of water then brought the system back
into equilibrium. Such differential movement of water between
compartments containing initially different solute concentrations
is called osmosis. How can we explain osmosis?
The pattern of changes you observed suggests a correlation
between the number of solute particles in each compartment and
its new volume when equilibrium is re-established. Equilibrium
occurs when both compartments contain the same concentration of
solute, and osmosis in all the simulations caused one compartment
to lose water and the other to gain it as a new equilibrium was
reached. What, in turn, causes osmosis?
In the initial simulation, when both solutions contained the same
concentration of solute, the volumes of both compartments stayed
more-or-less the same because essentially the same amount of water
diffused across the membrane in both directions. Why? Both compartments
contained approximately the same mole fractions of free and bound
solvent, and diffusion of free water was as likely to occur in
either direction. Thus, when the chemical activity of water in
both compartments was the same, no osmosis occurred.
Such was not the case when solute concentrations differed between
the compartments. In every instance where there was a lower solute
concentration, there was correspondingly a lower fraction of bound
water and a correspondingly higher fraction of freely diffusable
water. And you observed that more water always diffused out of
the compartment with higher solvent activity than into it. Recalling
that solute lowers the chemical activity of water in solution,
you can now understand how the effect of solute on solvent behavior
causes osmosis. How does this explanation of osmosis
compare with others you've learned?
How can we quantify osmosis? Go on to the next page and simulation to consider this problem.