The International Union of Pure and Applied Chemistry (IUPAC) defines a chemical substance as "matter of constant composition best characterized by the entities (molecules, formula, units, atoms) it is composed of. Physical properties such as density, refractive index, electric conductivity, melting point etc. characterize the chemical substance" . The term "substance" in this article is used in this context. Databases also refer to chemical entities as compounds and substances, but the context may be different. PubChem, for instance, define these as "a substance is a chemical sample description provided by a single source and a compound is a normalized chemical structure representation found in one or more contributed substances" .