Editing Activity coefficient (section)

{{Short description|Value accounting for thermodynamic non-ideality of mixtures}}

In [[thermodynamics]], an '''activity coefficient''' is a factor used to account for deviation of a [[mixture]] of [[chemical substance]]s from ideal behaviour.<ref>{{GoldBookRef|title=Activity coefficient|file=A00116}}</ref> In an [[ideal mixture]], the microscopic interactions between each pair of [[chemical species]] are the same (or macroscopically equivalent, the [[enthalpy change of solution]] and volume variation in mixing is zero) and, as a result, properties of the mixtures can be expressed directly in terms of simple [[concentration]]s or [[partial pressure]]s of the substances present e.g. [[Raoult's law]]. Deviations from ideality are accommodated by modifying the concentration by an ''activity coefficient''. Analogously, expressions involving gases can be adjusted for non-ideality by scaling partial pressures by a [[fugacity]] coefficient.

The concept of activity coefficient is closely linked to that of [[activity (chemistry)|activity in chemistry]].