What is an activity coefficient and when is it needed?

Activity coefficient (gamma) corrects for non-ideal liquid mixture behavior: a = gamma * x, where a is activity and x is mole fraction. For ideal solutions, gamma = 1. Non-ideal behavior arises from molecular size differences and interaction forces. Activity coefficients are used in modified Raoult's law: y*P = gamma*x*P_sat. Values can be >1 (positive deviation, less stable mixture) or <1 (negative deviation, more stable). Needed for polar, hydrogen-bonding, or dissimilar molecules. Calculated from models like NRTL, UNIQUAC, or Wilson.