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 earlier.
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.