The volume changes on mixing of the two binary mixtures acetic acid – water and acetic acid – chloroform and of the ternary mixture acetic acid – chloroform – water have been determined. In the system acetic acid – water a volume contraction occurs, showing a minimum excess molar volume at 50 mole%. Partial molal volume determinations suggest the formation of a mixed association of one molecule of water with one molecule of acetic acid. In the system acetic acid – chloroform the excess molar volume function is positive over the complete concentration range and this suggests the probability of a positive deviation from Raoult's law.The refractivities of the two binary systems show that the true molecular volumes of acetic acid, water, and chloroform are additive on mixing.Studies of the vapor–liquid equilibria show that in all three constituent binary systems there is positive deviation from Raoult's law but if the dimerization of acetic acid in the vapor phase is taken into account the vapor pressure behavior of the system acetic acid – chloroform is virtually ideal. The vapor pressures in the ternary system show strong positive deviations from Raoult's law over the complete concentration range. This shows that the line of pseudo-ideality in terms of zero excess molar volume does not really represent ideal behavior. Zero excess molar volume is caused by the counteracting effects of water and of chloroform molecules on acetic acid molecules.In the two binary systems investigated the experimental data satisfy the Gibbs–Duhem–Margules equation. This proves that treatment for dimerization of acetic acid in the vapor phase is not merely permissible but necessary.
