It's not (*) concentration, but diffusion occurs along gradients in this quantity.
Along with temperature and volume, this quantity is held constant in the grand canonical ensemble. This quantity for a component in solution is equal to a standard value of this quantity plus the product of the gas constant, temperature, and the natural log of the activity. The Gibbs phase rule can be derived by noting that this quantity is equal in all phases. It's not (*) concentration, but diffusion occurs along gradients in this quantity. When temperature and pressure are constant, this quantity is defined as the derivative of the Gibbs free energy with respect to the number of particles. For 10 points, name this thermodynamic quantity which represents the change in energy when a particle is added to the system, commonly symbolized mu.
Along with temperature and volume, this quantity is held constant in the grand canonical ensemble.
chemical potential [do not accept or prompt on "potential," prompt on "mu" before read]