Compound, Chemical (Encyclopedia of Science)
A chemical compound is a substance composed of two or more elements chemically combined with each other. Compounds are one of three general forms of matter. The other two are elements and mixtures. Historically, the distinction between compounds and mixtures was often unclear. Today, however, the two can be distinguished from each other on the basis of three primary criteria.
First, compounds have constant and definite compositions, while mixtures may exist in virtually any proportion. A sample of water always consists of 88.9 percent oxygen and 11.1 percent hydrogen. It makes no difference whether the water comes from Lake Michigan, the Grand River, or a cloud in the sky. Its composition is always the same.
By comparison, a mixture of hydrogen and oxygen gases can have any composition whatsoever. You can make a mixture of 90 percent hydrogen and 10 percent oxygen; 75 percent hydrogen and 25 percent oxygen; 50 percent hydrogen and 50 percent oxygen; or any other combination.
(The entire section is 1533 words.)
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