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 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
concentrations or
partial pressures 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.