mixture, in chemistry, a physical combination of two or more pure substances (i.e., elements or compounds). A mixture is distinguished from a compound, which is formed by the chemical combination of two or more pure substances in a fixed, definite proportion. The components of a mixture retain their own chemical properties and may be present in any proportion. For example, iron filings may be mixed with powdered sulfur in any proportion, and even if very fine iron powder is carefully mixed with powdered sulfur, the two components are easily separated by means of a magnet; the magnet will draw out the iron from the mixture. However, if seven parts by weight of iron filings or powder are mixed with four parts by weight of powdered sulfur and the mixture is heated to a red glow (e.g., in a test tube, using a Bunsen burner), the iron and sulfur react to form the compound iron sulfide; they are chemically combined and are not readily separated. The iron sulfide is not attracted by a magnet. Mixtures are often classified as homogeneous or heterogeneous. Solutions and colloids are homogeneous mixtures. The components of a homogeneous mixture are too intimately combined to be distinguished from one another by visual observation. A suspension is a heterogeneous mixture. The particles in a heterogeneous mixture are coarse enough to be distinguished by visual observation. Alloys are mixtures of metals and may be either homogeneous or heterogeneous. The components of a mixture usually can be separated by physical means such as distillation, evaporation, precipitation, filtration, solvent extraction, or chromatography.