Basic Concepts

A thermodynamic system is made up of components and phases. A number of state variables define the properties and the relationships.

A component is a system-wide entity; sometimes it is specifically called a system component. A component has a unique name and some thermodynamic properties are associated with it, for example, its amount and activity or chemical potential. At equilibrium the activity or chemical potential of the components are the same in the whole system.

A phase is a system-wide entity, which has a composition expressed in the amounts of components, enthalpy content, a volume, and many other properties. The phase has constituents that may be different from the components. The constituents have a stoichiometry that can be expressed in terms of the components and possibly a charge. Condensed phases may have an internal structure like sub-lattices or clusters, and these clusters may be modeled as constituents.