Activity (chemistry)

The activity of an ion or other solute is its effective concentration for the purposes of chemical reactions and other mass action[?]. Activity differs from concentration, because of the affinity of solutes for the solvent molecules around them in solution. To some extent this affinity restrains solutes from engaging in other interactions (such as with reactants). Because their affinities differ, so do solutes' activity coefficients. These describe the constant of proportionality between a solute's concentration and its activity in a given solvent (typically assumed to be water).

Compare: fugacity[?]