What Is a Pure Substance?

You may have wondered what is meant by the term "pure substance." Here's a look at what a pure substance is and how you can tell if a substance is pure or not.

Any Material Free of Contamination

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Pure Substance Definition in Chemistry

By Anne Marie Helmenstine, Ph.D.

In a nutshell, a pure substance is any single type of material.

A substance can be anything. It doesn't have to consist of a single element or type of molecule. Pure hydrogen is a pure substance. So is pure honey, even though it consists of many different types of molecules. What makes both of these materials pure substances is that they are free from contamination.

If you add some oxygen to the hydrogen, the resulting gas is neither pure hydrogen nor pure oxygen. If you add corn syrup to the honey, you no longer have pure honey. Pure alcohol could be ethanol, methanol, or a mixture of different alcohols, but as soon as you add water (which is not alcohol), you no longer have a pure substance.

"Building Block" of Matter

Some people define a pure substance to be a material that consists of one type of "building block" of matter. By this definition, only elements and compounds are pure substances, while homogenous mixtures are not. Generally, it does not matter which definition you use, but if you are asked to give examples of pure substances as a homework assignment, use those that meet the narrow definition, such as gold, silver, water, and salt.