Now let's put motion into the scenario and illustrate in the simulation at the top of the next page, the Brownian movement of both solvent particles and solvated solute particles. For viewing ease, the density of water molecules has been drastically reduced from the numbers shown on previous pages. At first, only distilled water is illustrated. The compartment contains 150 water molecules, which are exhibiting the same random walk behavior as shown in the Simple Diffusion model examined in the Chapter 1.
Use the choice box at the lower right of the simulation to increase the
number of solute molecules added to this compartment. Note the association
of water with solute, which leaves on average fewer free water molecules
in motion. The overall chemical activity of water decreases in direct
proportion to the amount associated with solute. In
other words, the chemical activity of water is inversely proportional to
to solute concentration! Understanding this "retarding" effect
of solute on solvent is the key to understanding osmosis.
Also notice the solute molecules, associated with water as shown, move less quickly (and over a shorter average distance) than do the free water molecules. (The effect is exaggerated here for emphasis.) The extra bulk of the solute/water complex means that it cannot move as fast or as far per unit time as the individual water molecules.