What Is a Pure Substance?

Scientific Definition of Purity

Honeycomb with honey dripping from it
Honey is an example of a pure substance. Tony Hutchings / Getty Images
by Anne Marie Helmenstine, Ph.D.
Dr. Helmenstine holds a Ph.D. in biomedical sciences and is a science writer, educator, and consultant. She has taught science courses at the high school, college, and graduate levels.
Updated January 17, 2019

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.

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 an alcohol), you no longer have a pure substance. Got it?

Now, it's worth keeping in mind, some people define a pure substance to be a material that consists of one type of "building block" of matter. If this definition is used, only elements and compounds are pure substances, while homogenous mixtures are not considered to be pure substances. For the most part, it does not matter which definition you use, but if you are asked to give examples of pure substances as a homework assignment, go with examples that meet the narrow definition: gold, silver, water, salt, etc.