A substance composed of two or more elements that do not vary in composition from sample to sample, and that have fixed and definite physical properties, such as density and refractive index. The elements in compounds cannot be separated by simple physical or mechanical means, but only by chemical treatment. When compounds are formed from their elements, heat is generated or absorbed. These properties distinguish them from mixtures.
For example, if iron filings and sulfur powder are mixed together, the two elements can be separated, either by removing the iron filings with a magnet or by dissolving the sulfur in an appropriate solvent, and the individual particles of each element are distinguishable under a magnifying glass. If iron filings and sulfur are heated together, a chemical reaction takes place and a new substance is formed, iron sulfide, with properties totally different from iron or sulfur. See also: Density; Element (chemistry)
Most chemical compounds are formed in fixed and definite proportions by weight from their elements, and they obey the laws of chemical combination. However, a number of solid compounds, known as nonstoichiometric compounds, exhibit departures from the law of definite proportions. See also: Definite composition, law of; Equivalent weight; Inorganic chemistry; Multiple proportions, law of; Nonstoichiometric compounds; Organic chemistry