Definition of Activity coefficient

An Activity coefficient is a factor used in thermodynamics to account for deviations from ideal behaviour in a mixture of chemical substances. In an ideal mixture the interactions between each pair of chemical species are the same; more formally, the enthalpy of mixing is zero. Ideal behavior is represented, for example, by Raoult's law, Henry's law and the law of mass action. In each of these examples, a property is related to the concentration or partial pressure of the substances in a mixture. Deviations from ideality are accommodated by replacing concentration with activity. Activity is the product of concentration and activity coefficient. For gases partial pressure may be replaced by fugacity, which is the product of partial pressure and a fugacity coefficient.