Chemistry: Colloids: Somewhere in Between

Colloids: Somewhere in Between

Earlier, I said that something was probably a homogeneous mixture if it had more than one component and appeared uniform to the naked eye. However, let's imagine that you make a watercolor paint by combining paint powder with water. When the two have mixed, the resulting paint consists of fine pigmented particles floating around in water. Because these paint particles are so small, they never settle at the bottom of the container.

You've Got Problems

Colloids are materials in which one type of particle is suspended in another without actually having been dissolved.

So what's the deal with this stuff? It's hard to say that it's a solution because it has both solid and liquid components that can be separated from each other by filtering. On the other hand, if you were to look at paint under a microscope, it would still look completely uniform. As a result, this material is somewhere on the borderline between a solution and a heterogeneous mixture. Such materials are referred to as colloids.

There are many types of colloids, categorized by the phases of matter they contain. The main types are listed below:


One easy way to distinguish colloids from solutions is to shine a light through them. Because the molecules in a solution are very small (usually single molecules), they don't reflect light. Colloids, on the other hand, have particles large enough to reflect light. As a result, light beams are visible within colloids but not within solutions. This effect is referred to as the Tyndall effect. In some cases, this effect is so pronounced that the milkiness of the liquid makes it impossible to see through it at all.

  • Aerosols consist of liquid or solid particles suspended in a gas. Examples of aerosols include smoke and fog.
  • Foams are formed when a gas is suspended in a liquid or a solid. Common examples are shaving cream and marshmallows.
  • Emulsions are formed when particles of a liquid are suspended in another liquid or a solid. Mayonnaise and butter are common examples of emulsions.
  • Sols are made when solid particles are suspended in a liquid. Examples include paints, gelatin, and blood.
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Excerpted from The Complete Idiot's Guide to Chemistry © 2003 by Ian Guch. All rights reserved including the right of reproduction in whole or in part in any form. Used by arrangement with Alpha Books, a member of Penguin Group (USA) Inc.

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