Since we want our hypothetical ontology to be a little more expressive, the Differentia phrase, "that results in the interconversion of chemical species", informs us that we also need to define a class called chemical species. The IUPAC definition for 'chemical species' is linked in the metadata as a related concept and indicates, "An ensemble of chemically identical molecular entities that can explore the same set of molecular energy levels on the time scale of the experiment. [. . . ] Except where the context requires otherwise, the term is taken to refer to a set of molecular entities containing isotopes in their natural abundance." The IUPAC definition for 'molecular entities' is further linked as a related term, "Any constitutionally or isotopically distinct atom, molecule, ion, ion pair, radical, radical ion, complex, conformer etc., identifiable as a separately distinguishable entity. Molecular entity is used in this Compendium as a general term for singular entities, irrespective of their nature, while chemical species stands for sets or ensembles of molecular entities." Following these definitions, we would actually need to define two classes, one representing the 'chemical species' and the other the 'molecular entity' concept. The Genus of both could be mapped to be equivalent with the BFO class material entity, allowing us to assert both to be a subclass thereof. For the 'molecular entity' concept, we could reuse the class molecular entity from the ontology for Chemical Entities of Biological Interest (CHEBI) that applies the IUPAC definition, although the IUPAC source is not credited (more on citing IUPAC terms from the Gold Book below). The chemical species class could then be defined accordingly as a material entity that is an "ensemble of chemically identical molecular entities that can explore the same set of molecular energy levels on the time scale of the experiment." From the Gold Book definitions of the terms 'reactant' and 'product' , we could derive the definition of classes that represent the essential roles inherent in the molecular entities participating in a chemical reaction. Where the reactant role would inhere in those molecular entities that get consumed in it, and the product role would inhere in those molecular entities that get formed by it. Together with the relations consumes, has role, and produces defined in the Relation Ontology (RO) , we could then define our chemical reaction class formally with an equivalence class axiom using Manchester syntax like this: process and ('consumes' some (('chemical species' or 'molecular entity') and ('has role' some 'reactant role')))