Colloid

Colloids are mixtures whose particles are larger than the size of a molecule but smaller than particles that can be seen with the naked eye. Colloids are one of three major types of mixtures, the other two being solutions and suspensions. The three kinds of mixtures are distinguished by the size of the particles that make them up. The particles in a solution are about the size of molecules, approximately 1 nanometer (1 billionth of a meter) in diameter. Those that make up suspensions are larger than 1,000 nanometers. Finally, colloidal particles range in size between 1 and 1,000 nanometers. Colloids are also called colloidal dispersions because the particles of which they are made are dispersed, or spread out, through the mixture.

Types of colloids

Colloids are common in everyday life. Some examples include whipped cream, mayonnaise, milk, butter, gelatin, jelly, muddy water, plaster, colored glass, and paper.

Every colloid consists of two parts: colloidal particles and the dispersing medium. The dispersing medium is the substance in which the colloidal particles are distributed. In muddy water, for example, the colloidal particles are tiny grains of sand, silt, and clay. The dispersing medium is the water in which these particles are suspended.

Colloids can be made from almost any combination of gas, liquid, and solid. The particles of which the colloid is made are called the dispersed material. Any colloid consisting of a solid dispersed in a gas is called a smoke. A liquid dispersed in a gas is referred to as a fog.

Types of Colloids

Dispersed Material	Dispersed in Gas	Dispersed in Liquid	Dispersed in Solid
Gas (bubbles)	Not possible	Foams: soda pop; whipped cream; beaten egg whites	Solid foams: plaster; pumice
Liquid (droplets)	Fogs: mist; clouds; hair sprays	Emulsions: milk; blood; mayonnaise	butter; cheese
Solid (grains)	Smokes: dust; industrial smoke	Sols and gels: gelatin; muddy water; starch solution	Solid sol: pearl; colored glass; porcelain; paper

Properties of colloids

Each type of mixture has special properties by which it can be identified. For example, a suspension always settles out after a certain period of time. That is, the particles that make up the suspension separate from the medium in which they are suspended and fall to the bottom of a container. In contrast, colloidal particles typically do not settle out. Like the particles in a solution, they remain in suspension within the medium that contains them.

Colloids also exhibit Brownian movement. Brownian movement is the random zigzag motion of particles that can be seen under a microscope. The motion is caused by the collision of molecules with colloid particles in the dispersing medium. In addition, colloids display the Tyndall effect. When a strong light is shone through a colloidal dispersion, the light beam becomes visible, like a column of light. A common example of this effect can be seen when a spotlight is turned on during a foggy night. You can see the spotlight beam because of the fuzzy trace it makes in the fog (a colloid).

Light shining through a solution of sodium hydroxide (left) and a colloidal mixture. The size of colloidal particles makes the mixture, which is neither a solution nor a suspension, appear cloudy. (Reproduced by permission of

Photo Researchers, Inc.

)