Let's now consider osmosis in a more complex system. Two compartments containing identical solutions are separated by a membrane permeable only to water. Such a system is simulated at the top of the next page. Note both water molecules and solvated solute particles exhibit Brownian motion, but the larger, solvated solute particles are moving less rapidly and traversing a shorter path than the smaller water molecules.
Occasionally, water diffuses across the membrane, but solute diffusion
is restricted by the membrane to one compartment or the other.
A membrane that restricts the flow of one substance - in this
case, solute - relative to that of another such as solvent is
said to be selectively permeable.
The selectively permeable membrane in the simulation on the next
page exhibits one more interesting feature: it moves! And its
relative location reflects the volume of each compartment, that
is, the total number of particles in each. Initially, both compartments
contain the same number of particles and are therefore at equilibrium
with respect to each other. Ideally no net movement of solvent
occurs in either direction across the membrane: on balance, an
equal amount of water moves out of a compartment as moves into
it.