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 on October 04, 2021In chemistry, a pure substance is a sample of matter with both definite and constant composition and distinct chemical properties. To avoid confusion, a pure substance is often referred to as a "chemical substance."
Examples of pure substances include chemical elements and compounds. Alloys and other solutions may also be considered pure if they have a constant composition.
How do you test whether or not a substance is pure?
So, elements are easy examples of pure substances. It does not matter whether they consist of individual atoms, ions, or molecules.
Compounds are pure substances, too. Sodium chloride (NaCl), water (H2O), methane (CH4), and ethanol (C2H5OH) have chemical formulas that specify a relationship between the number and type of atoms they contain.
Be careful, though, because elements and compounds containing impurities might not be considered pure substances. Is tap water a pure substance? Probably not. Distilled, deionized water is a pure substance.
Alloys and solutions may or may not be pure, depending on who you ask. On the one hand, alloys like steel, brass, and bronze have a set composition. On the other hand, if you examine these metals closely, they have different phases and structures within them.
Usually, a solution of salt or sugar in water is a pure substance. The concentration of the solution is the same no matter where you take a sample. In other words, the number and type of atoms remains constant. The phase of matter also remains the same throughout the composition.
In general, 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.
Some substances don't have a chemical formula, but may have a more or less consistent composition. Whether or not you consider them to be pure substances is a matter of interpretation.
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 different 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.
For the most part, it does not matter which definition you use, but if you are asked to give examples of pure substances as part of a homework assignment, go with examples that meet the narrow chemical definition: gold, silver, water, salt, etc.
Helmenstine, Anne Marie, Ph.D. "Pure Substance Definition in Chemistry." ThoughtCo, Oct. 4, 2021, thoughtco.com/definition-of-pure-substance-605566. Helmenstine, Anne Marie, Ph.D. (2021, October 4). Pure Substance Definition in Chemistry. Retrieved from https://www.thoughtco.com/definition-of-pure-substance-605566 Helmenstine, Anne Marie, Ph.D. "Pure Substance Definition in Chemistry." ThoughtCo. https://www.thoughtco.com/definition-of-pure-substance-605566 (accessed September 8, 2024).
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