substance

Substance is a term used in science, philosophy, and everyday language with several closely related meanings. In chemistry and physics, a substance is a form of matter that has a definite composition and set of properties that do not vary from sample to sample under the same conditions. By this definition, substances stand in contrast to mixtures, which consist of two or more substances physically intermingled without fixed proportions.

Pure substances are classified as elements or compounds. Elements consist of a single type of atom and cannot be broken into simpler substances by chemical means. Compounds consist of two or more elements bonded in fixed proportions and have properties distinct from their constituent elements. Substances can be homogeneous (uniform composition throughout) or heterogeneous (containing distinct phases).

In chemical terms, the identity of a substance is described by its chemical formula and structure, and its properties include melting point, boiling point, density, and reactivity. Substances undergo chemical reactions to form new substances, and their behavior under conditions such as temperature and pressure helps determine their classification and applications.

In philosophy, substance refers to the underlying reality that persists through change, often contrasted with properties or events. In pharmacology and medicine, a substance can denote any chemical, including drugs, toxins, or reactants. Regulatory usage distinguishes substances for safety, labeling, and regulatory oversight, while everyday usage often treats substances as any material with definable properties.