Colloid
Colloid, a mixture consisting of tiny particles dispersed in a solid, liquid, or gas. Colloids make up one of the three major groups of mixtures, the others being solutions and suspensions. The three groups differ primarily in the sizes of the dispersed particles contained in each. Colloidal particles are intermediate in size between the atom- or molecule-size particles of solutions and the much larger particles of suspensions.
When a beam of light is passed through a liquid or gaseous colloid, some of the light is scattered by the dispersed particles, making the beam visible from the side. This phenomenom is called the Tyndall effect, after its discoverer, the British physicist John Tyndall. The particles dispersed in a colloid are too small to be seen directly with an optical microscope. However, they can be seen as points of light when the light scattered due to the Tyndall effect is viewed through a microscope. Observed in this manner, the particles appear to move about erratically. This motion is a result of the random bombardment by molecules in the surrounding fluid and is called Brownian movement, after the British botanist Robert Brown, who first observed it.
An understanding of colloids is important to chemists and other scientists. Most bodily fluids and tissues are colloidal, and the digestion of food partially involves colloidal chemistry. Colloids that consist of solid or liquid particles dispersed in a gas are called aerosols. Common examples of aerosols are fog and clouds, which consist of water droplets dispersed in air, and smoke, which consists of ash dispersed in air. Emulsions are colloids formed by the dispersion of one liquid in another. A common example is mayonnaise. Other colloids include glues, jelly, gelatin, cheese, and colored glass.