ABSTRACT
Beyond the systematic existence of the equilibrium constants commanding chemical reactions which are the mathematical expressions of this law, a very important point concerning the mass action law is the fact that it is expressed in terms of activities. A puzzling point concerning the concept of activity lies in the arbitrary character of the choice of the concentration scale to which the chemical activity of a species is related. It is important to mention that any kind of concentration scale can be chosen. The true invariant is the change in the Gibbs function between two thermodynamic states and not the equilibrium constant. The standard Gibbs energy changes and the thermodynamic equilibrium values are not invariant. There exists a kind of spontaneous compensation between the values of the standard reaction Gibbs energy and those of the activities in order that the Gibbs function change should be a constant, whatever the choice of standard states is.
