Chapter 18 Laboratory: Colloids and Suspensions 317

Laboratory:

Colloids and Suspensions

A colloid, also called a colloidal dispersion, is a two-phase heterogeneous mixture that

is made up of a dispersed phase of tiny particles that are distributed evenly within a

continuous phase. For example, homogenized milk is a colloid made up of tiny particles

of liquid butterfat (the dispersed phase) suspended in water (the continuous phase). In

comparison to true solutions, the continuous phase can be thought of as the solvent-like

substance and the dispersed phase as the solute-like substance.

Each type of colloid has a name. A solid sol is one solid dispersed in another solid, such

as colloidal gold particles dispersed in glass to form ruby glass. A solid emulsion is a liquid

dispersed in a solid, such as butter. A solid foam is a gas dispersed in a solid, such as

Styrofoam or pumice. A sol is a solid dispersed in a liquid, such as asphalt, blood, pigmented

inks, and some paints and glues. An emulsion, sometimes called a liquid emulsion, is a liquid

dispersed in another liquid, such as mayonnaise or cold cream. A foam is a gas dispersed

in a liquid, such as whipped cream or sea foam. A solid aerosol is a solid dispersed in a gas,

such as smoke and airborne particulates. An aerosol, sometimes called a liquid aerosol,

is a liquid dispersed in a gas, such as fog, which is tiny water droplets suspended in air. All

gases are inherently miscible (completely soluble in each other), so by definition there is no

such thing as a gas-gas colloid. Some colloidal substances are a mixture of colloid types.

For example, smog is a combination of liquid and solid particles dispersed in a gas (air), and

latex paint is a combination of liquid latex particles and solid pigment particles dispersed in

another liquid. Table 18-1 summarizes the types of colloids and their names.

18

wHATA BoUT GELS?

Many reference sources incorrectly list gel as a type of colloid, describing a gel as a liquid

dispersed phase in a solid continuous phase, which is properly called a solid emulsion. In

fact, a gel is a type of sol in an intermediate physical phase. The density of a gel is similar

to the density of the dispersing liquid phase, but a gel is physically closer to solid form

than liquid form. Prepared gelatin is a good example of a typical gel. Dr. Mary Chervenak

adds, “I think toothpastes are defined as colloidal gels with viscoelastic properties.”