Chemical Compound

A compound is a substance composed of two or more elements chemically combined with each other. 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% oxygen and 11.1% hydrogen by weight. However, a mixture of hydrogen and oxygen gases can have any composition whatsoever.

Second, the elements that make up a compound lose their characteristic elemental properties when they become part of the compound, while the elements that make up a mixture retain those properties. In a mixture of iron and sulfur, for example, black iron granules and yellow sulfur crystals can often be recognized. Also, the iron can be extracted from the mixture by means of a magnet, or the sulfur can be dissolved out with carbon disulfide. One part of the compound is called iron(II) sulfide, however, both iron and sulfur lose these properties.

Third, the formation of a compound is typically accompanied by the evolution of light and heat, while no observable change is detectable in the making of a mixture. A mixture of iron and sulfur can be made simply by stirring the two elements together. But the compound iron(II) sulfide is produced only when the two elements are heated. Then, as they combine with each other, they give off a glow.