Pure Substance Definition

What Is a Pure Substance?

Uncut diamonds
Diamond is an example of a pure substance. Peter/Stef Lamberti / Getty Images

You may have wondered what is meant by the term "pure substance". The term has a specific meaning in chemistry. A pure substance is a sample of matter with both definite and constant composition with distinct chemical properties. To avoid confusion, in chemistry a pure substance is often referred to as a "chemical substance".

Examples of Pure Substances

Examples of pure substances include elements and compounds. Alloys and other solutions may also be considered pure.

  • water
  • diamond
  • gold
  • table salt (sodium chloride)
  • ethanol
  • brass
  • bronze
  • saline solution

Examples of Things That Are NOT Pure

Basically, any heterogeneous mixture is not a pure substance. If you can see differences in the composition of a material, it's impure, at least as far as chemistry is concerned.

Common Definition of a Pure Substance

To a non-chemist, a pure substance is anything composed of a single type of material. In other words, it is free of contaminants. So, in addition to elements, compounds, and alloys, a pure substance might include honey, even though it consists of many types of molecules. 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.

Which Definition to Use

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.