Solutions and Colloids


Colloids are heterogeneous (unevenly distributed) mixtures in which suspended particles do not settle.
A colloid is a mixture containing particles that do not settle. The simplest colloids are mixtures where one substance, the dispersed phase, is suspended in another substance, the dispersion medium. A colloid cannot be separated by just letting it stand undisturbed until the particles settle out. The dispersion medium is the substance in which the particles are suspended in a colloid, analogous to the solvent. Particles in a colloid are spread out evenly in a dispersion medium and must be in the size range of 1 to 1,000 nm. The dispersion medium can be solid, liquid, or gas. The dispersed phase is the substance with particles that are spread out and suspended in a colloid, analogous to the solute.

Types of Colloids

Dispersed Phase Dispersion Medium Example
Liquid Gas Fog
Solid Gas Dust
Gas Liquid Shaving cream
Liquid Liquid Milk
Solid Liquid Blood
Gas Solid Pumice (volcanic rock)
Liquid Solid Gelatin
Solid Solid Pigmented glass

Colloidal particles are able to remain suspended in the dispersion medium despite the force of gravity because the particles are surrounded by oppositely charged ions in the dispersion medium. The particles therefore acquire a net charge and repel the other particles because they all have the same net charge. These repulsive forces repel gravity and keep the particles from settling.

A colloid appears macroscopically to be homogeneous, but it is not. A simple test to determine whether a mixture is colloidal or a solution is to shine a light through it. In a colloidal dispersion, light is scattered by the particles, and the beam of light can be seen passing through the medium. The phenomenon of light being scattered by a colloid is known as the Tyndall effect. This effect is evidence that a colloid is not homogeneous. A solution that is homogeneous does not scatter the light.

The Tyndall Effect

When a beam of light shines through a colloid, the particles scatter the light so that the beam is visible. The particles in a solution are too small to scatter the light, so the beam is not visible. Sunbeams shining through trees on a sunny morning are sometimes visible because air is a colloid containing many particulates such as dust.