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Introduction to Chemical Mixtures

Author(s): David R. Caprette, PhD

Microscopic Characteristics of Mixtures

Wilhelm Ostwald, Nobel Laureate in Chemistry in 1909, was one of the founders of modern physical chemistry. He is reported to have said, "There are no sharp differences between mechanical suspensions, colloidal solutions, and molecular [true] solutions. There is a gradual and continuous transition from the first through the second to the third."

A mixture, regardless of type, is described as "uniformly dispersed." This means that one or more minor components are evenly distributed throughout a major component.  The major component is the substance that is present in the greatest proportion. In the biology laboratory, the major component often is a liquid, and minor components can be solids, other liquids, or even gases.

The "mechanical suspension" to which Ostwald referred is the easiest to describe. The minor component in a suspension is typically visible in an optical microscope and often is visible to the naked eye.

A colloidal mixture is sometimes called a colloidal system, a colloidal suspension, or simply a "colloid." The smallest dimension of the minor component of a colloidal mixture can range from approximately one nanometer (1 billionth of a meter) to one micrometer (1 millionth of a meter). Examples of liquid colloidal mixtures are milk, paints, and muddy water. The medium can be a gas, in the cases of smog, smoke, or aerosol sprays.  Some solids are considered to be colloidal mixtures, as in steel or foam rubber.

In a true solution, one or minor components interact at the molecular level or ionic level with the major component. The minor components are atoms or molecules, and are not distinguishable in any optical microscope.