What does the chemical potential μ of a pure substance represent?

The chemical potential μ of a pure substance is defined as the change in the Gibbs free energy of the system when an infinitesimal amount of the substance is added, holding temperature and pressure constant. This means that the chemical potential essentially represents the molar Gibbs energy of the substance, reflecting how the free energy changes with the number of particles in the system.

The significance of the chemical potential lies in its role in determining how substances behave in chemical reactions and phase transitions. A higher chemical potential indicates a greater driving force for the substance to participate in reactions or to change phases, which is essential in understanding thermodynamic equilibria.

In relation to other options, thermal energy relates to the kinetic energy of molecules and their motion, while potential energy usually refers to the energy stored due to an object's position in a field. Neither of these concepts encompasses the specific notion of chemical potential as it applies to the Gibbs energy of a substance. Thus, the correct choice focuses on the relationship between chemical potential and the molar Gibbs energy.