Substances

In science, a substance is a form of matter with a definite composition and set of properties. Substances are contrasted with mixtures, which combine several materials without a fixed composition.

Pure substances include elements, which consist of only one type of atom, and compounds, which are substances formed from two or more elements in fixed ratios and through chemical bonds. Pure substances have definite melting and boiling points and cannot be separated into other substances by physical means.

Mixtures are combinations of two or more substances that retain their own properties. They may be homogeneous (uniform composition, such as a solution) or heterogeneous (not uniform, such as sand and water). Substances in mixtures can usually be separated by physical methods such as filtration, evaporation, or distillation.

Properties of substances are studied as physical properties (such as density, color, melting point) and chemical properties (such as reactivity). Identification often uses measurements, spectroscopy, chromatography, and qualitative chemical tests.

Examples include water (a compound of hydrogen and oxygen), table salt (sodium chloride, a compound), and oxygen (an element). Substances play central roles in science, industry, medicine, and regulation, where purity, stability, and hazard classification are routinely considered.