Colloids

Milk is a mixture of substances including water, sugar, proteins, and fats. When fresh cow's milk is allowed to stand, a layer of cream rises to the top. This layer contains much of the fat in the milk. In the milk you buy at the store, the cream does not form a separate layer. The milk has been processed so that the fat remains dispersed throughout the milk. The result is homogenized milk, which is a colloid.

A colloid contains some particles that are intermediate in size between the small particles in a solution and the larger particles in a suspension. Like solutions, colloids do not separate into layers. You cannot use a filter to separate the parts of a colloid.

Fog is a colloid of water droplets in air. Figure 8 shows how fog affects which headlights a driver uses. Automobiles have headlights with low beams for normal driving conditions and high beams for roads that are poorly lit. With the high beams, a driver can see a bend in the road or an obstacle sooner. But the high beams are not useful on a foggy night because the water droplets scatter light back toward the driver and reduce visibility. With the low beams, much less light is scattered. The scattering of light is a property that can be used to distinguish colloids and suspensions from solutions.