The effective concentration of a species in a solution can be expressed precisely in terms of its activity. Activity considers the effect of electrolytes present in the vicinity of the species of interest and depends on the ionic strength of the solution. The activity of a species is expressed as the product of molar concentration and the activity coefficient of the species.
The thermodynamic equilibrium constant is more accurately defined in terms of activity rather than concentration. Activity is formally defined in terms of the chemical potential, also known as the partial molar Gibbs energy. The chemical potential of a system is the Gibbs energy change of the system per mole of a species in question, given that the temperature, pressure, and the number of moles of other species are held constant. For a chemical reaction to take place spontaneously, the chemical potential of the products must be less than that of the reactants. If the number of moles of other species varies in the solution—i.e., a change in the composition of the solution—the chemical potential of the solution changes, and so does the activity.
