where there are a mixture of chemicals and phases, this is a useful concept. For example, if a container holds water and water vapor, there will be a chemical potential (which is negative) for the liquid, pushing water molecules into the vapor (evaporation) and a chemical potential for the vapor, pushing vapor molecules into the liquid (condensation). Only when these "forces" equilibrate is equilibrium obtained. See also Generalized coordinate and generalized force: analogous conjugate variable pairs found in classical mechanics. Intensive and extensive properties Bond graph References Further reading Thermodynamic properties