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Chemistry: Pure Substances

Pure Substances

If you're a salesperson, you know the word "pure" is a great way to sell something. When consumers think of the word "pure," they imagine something that's perfect in every possible way. Have cockroaches been crawling in the potato salad? Of course not—it's pure!

To a chemist, the word pure has a somewhat different connotation. When we say something is pure, we mean that only one substance is present in the material, and that it is completely uniform in composition. By uniform composition, we mean that if you take a sample from one part of a material and compare it with a sample from another part of the material, both samples will be identical in every way. Things that have a uniform composition are said to be "homogeneous."

Pure Substance #1: Elements

Before you read any further, I want you to come up with a definition of the word element. Since you probably already know what an element is, this should be a really easy task.

Molecular Meanings

An element is a substance that cannot be chemically decomposed into simpler substances. The periodic table is a list of all the known elements.

Just like with the word "atom," "element" is a surprisingly difficult term to define. You probably have an internal idea of what an element is, and you probably already know that a list of all the elements can be found on the periodic table, but putting it in words is often difficult.

For chemical purposes, an element can be defined as a substance that cannot be chemically decomposed into simpler substances. You may already be aware that nuclear reactions are able to break elements apart into even simpler particles, but these particles are unimportant for most chemical purposes.

Pure Substance #2: Compounds

Molecular Meanings

Compounds are pure substances made up of two or more elements in defined proportions. By using chemical reactions, compounds can be broken into their constituent elements, though this is usually difficult.

The other type of pure substance is a chemical compound. Compounds are pure substances made up of two or more elements in defined proportions. Unlike elements, compounds can be broken into simpler parts using chemical reactions. These pieces, of course, are just the elements that make up the compounds. For example, it is possible to convert sodium chloride back into pure sodium and pure chlorine using a process called electrolysis.

You can typically figure out whether a material is an element or a chemical compound by looking at its name or formula. The names of many chemical compounds contain two words (e.g., "sodium chloride," "magnesium sulfate") and the symbols of chemical compounds contain more than one atomic symbol (e.g., "NaCl," "MgSO4").

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Excerpted from The Complete Idiot's Guide to Chemistry © 2003 by Ian Guch. All rights reserved including the right of reproduction in whole or in part in any form. Used by arrangement with Alpha Books, a member of Penguin Group (USA) Inc.

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