Pure Substance Definition in Chemistry

Definition of a pure substance

ThoughtCo / Hilary Allison

In 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."

Key Takeaways: Pure Substance Definition in Chemistry

  • In everyday usage, a pure substance is a material that is free of impurities or contaminants. However, the definition of a pure substance is more narrow in chemistry.
  • In chemistry, a pure substance has a constant chemical composition. No matter where you sample a substance, it is the same.
  • For chemistry homework, the safest examples of pure substances are elements and compounds. So, examples include gold, silver, helium, sodium chloride, and pure water.

Examples of Pure Substances

Examples of pure substances include chemical elements and compounds. Alloys and other solutions may also be considered pure if they have a constant composition.

Why Are These Substances Pure?

How do you test whether or not a substance is pure?

  • Does it consist of only one kind of atom?
  • If not, does it have a chemical formula?
  • If you take a sample from one part of the substance, is it identical in composition to a sample taken from another location?

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.

Examples of Substances That Are Not Pure

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.

  • Rocks
  • Oranges
  • Wheat
  • Light bulbs
  • Shoes
  • Sandwiches
  • A cat
  • A computer
  • A house
  • Sand

The Gray Area

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.

  • Air
  • Milk
  • Honey
  • Soft drink
  • Coffee
  • Tea

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 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.

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 part of a homework assignment, go with examples that meet the narrow chemical definition: gold, silver, water, salt, etc.

Sources

  • Hale, Bob (2013). Necessary Beings: An Essay on Ontology, Modality, and the Relations Between Them. OUP Oxford. ISBN 9780191648342. 
  • Hill, J. W.; Petrucci, R. H.; McCreary, T. W.; Perry, S. S. (2005). General Chemistry (4th ed.). Upper Saddle River, New Jersey: Pearson Prentice Hall.
  • Hunter, Lawrence E. (2012). The Processes of Life: An Introduction to Molecular Biology. MIT Press. ISBN 9780262299947.
  • IUPAC (1997). "Chemical Substance." Compendium of Chemical Terminology (the "Gold Book") (2nd ed.). Blackwell Scientific Publications. doi:10.1351/goldbook.C01039
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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 June 26, 2022).