Since this article later refers to calculations performed on PubChem queries, "compound" in this article refers to a chemical that fulfils the definition of a compound according to PubChem with a unique PubChem Compound Identifier (CID). A mixture, according to IUPAC, is a "portion of matter consisting of two or more chemical substances called constituents" . Mixtures can be simple (e.g., xylene is a mixture of three isomers, o-xylene, m-xylene and p-xylene) or complex (e.g., C9-C14 alcohols, or mineral oils). The latter are often referred to as "substances of Unknown or Variable composition, Complex reaction products or Biological origin" (UVCBs) .