A mixture of substances B
, ... is called an ideal mixture when aB = xB, aC = xC, ... or fB = 1, fC = 1 where aB and fB are the relative activity and the activity coefficient, respectively, of a substance B
in a liquid or solid mixture.
PAC, 1994, 66, 533. 'Standard quantities in chemical thermodynamics. Fugacities, activities and equilibrium constants for pure and mixed phases (IUPAC Recommendations 1994)' on page 543 (https://doi.org/10.1351/pac199466030533)