Diffusion, << dih FYOO zhuhn, >> in chemistry, is the mixing of the atoms or molecules of one substance with those of another. It is caused by the natural movements of atoms and molecules. It differs from the mixing caused by stirring or shaking or the blowing of wind.
Diffusion occurs readily in gases and liquids because of the constant and random motion of their atoms and molecules. The process takes place more rapidly in gases than in liquids. Molecules of gases are farther apart and collide less frequently than those of liquids–and collisions among molecules hinder diffusion. In solids, the molecules are arranged in rigid patterns and move very little. Therefore, diffusion does not occur in solids except under special conditions.
Diffusion can be demonstrated by adding ink to a glass of water. Each molecule of ink has its own constant and random motion. The motion of the ink molecules causes them to spread through the water. The water molecules also move about and become mixed with the ink molecules. After being mixed completely, the molecules of ink and water each continue to move individually. But as a result of diffusion, the color of the mixture becomes the color of the ink.
Many common occurrences involve diffusion. For example, water boiling in an uncovered pot produces steam that disappears. The disappearance results from the diffusion of steam molecules with air molecules. Odors from flowers, food, perfume, and other sources are produced by the diffusion of special gaseous odor molecules with molecules of air.
