A mixture of substances
B,
C, ... is called an ideal mixture when

,

, ... or

,

where

and

are the
relative activity and the
activity coefficient, respectively, of a substance
B in a liquid or solid mixture.
Source:
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
Cite as:
IUPAC. Compendium of Chemical Terminology, 2nd ed. (the "Gold Book"). Compiled by
A. D. McNaught and A. Wilkinson. Blackwell Scientific Publications, Oxford (1997).
XML on-line corrected version: http://goldbook.iupac.org (2006-) created by M. Nic,
J. Jirat, B. Kosata; updates compiled by A. Jenkins. ISBN 0-9678550-9-8.
https://doi.org/10.1351/goldbook.