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Pure Substance in Chemistry: Definition, Properties & Examples

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  • 0:04 Pure Substances:…
  • 2:20 Concept vs. Reality
  • 3:10 Lesson Summary
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Lesson Transcript
Instructor: Nathan Crawford

Nathan, a PhD chemist, has taught chemistry and physical science courses.

This lesson introduces the concept of the pure substance. The impact of purity on the physical and chemical properties are discussed and several examples are explored along the way.

Pure Substances: Concept and Properties

Imagine a cube of sugar that you might add to a cup of hot coffee or tea. If you look at the box of sugar cubes, you might see on the label the words 'pure cane sugar.' Does the word 'pure' mean the same in all cases, and what does this word mean within the context of chemistry?

The pure substance within chemistry is a very simple concept to grasp. Pure substances are defined as substances that are made of only one type of atom or only one type of molecule (a group of atoms bonded together). The measure of whether a substance is pure is known as purity. For example, pure iron would only contain iron atoms, and, as in the sugar cube mentioned above, pure sugar would only contain molecules of the substance called sucrose.

Pure substances exhibit very well-defined physical properties, or properties that are not connected with the substance's ability to combine with different substances. The temperatures where pure solids melt, known as melting points, are particularly sharp, meaning the melting occurs at a single temperature. Likewise, the temperatures where pure liquids begin to boil, or boiling points, occur at single temperatures when other factors, like air pressure, are controlled. The ability of substances to conduct electricity, or conductivity, is profoundly affected by purity. For example, copper that is used in electrical wiring must have very high purity in order to provide the most efficient conductivity. On the other hand, extremely pure liquid water is a very poor conductor of electricity because there are no dissolved electrical conductors, called electrolytes, to allow conduction to occur.

Purity can also have an important impact on the chemical properties of a substance. Pure substances have the potential to form predictable products from chemical reactions. The reason for this reliability comes from the absence of impurities that could start other reactions that might interfere with the reaction that is being studied by a chemist. Therefore, chemists often use substances of high purity when conducting chemical research.

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