A chemical mixture is a collection of molecules or atoms of different types. A mixture is distinguished from a pure substance, which has constant composition (is composed of a only one type of molecule or atom), and a unique set of physical properties (no matter how large or small a sample is observed). The properties of a mixture depend not only on the types of substances that compose it, but also on their relative amounts; the composition of a mixture is not constant. The separation of mixtures is big business, and separation science is a sub-division of chemistry.
Heterogeneous mixtures are non-uniform, with the components jumbled irregularly together; frequently the substances making up the mixture can be seen as bits of different textured or colored material. Homogeneous mixtures, on the other hand, look uniform. The molecules or atoms making up a homogeneous mixture are distributed evenly, all in the same phase. (Homogeneous mixtures can sometimes be mistaken for pure substances because of their uniform appearance.) Salt water solution is a homogeneous mixture, for example, but salt mixed with sand is a heterogeneous mixture.
Mixtures are separable into their component elements or compounds (at least theoretically) by purely physical processes. For example, if iron filings and sulfur powder are mixed together, they constitute a chemical mixture. This particular mixture is easy to separate by applying a magnet to draw out the iron-utilizing magnetic force is a physical process. If the iron filings and sulfur powder are heated, however, they undergo a chemical reaction to form a compound, iron sulfide, which does not respond to a magnet. Pure iron sulfide is homogeneous (uniform in appearance and properties), shows constant composition (a consistent ratio of iron to sulfur throughout any sample of it, large or small), consists of molecules all of one type, is no longer separable into two separate substances without another chemical reaction, and is thus no longer a mixture.