Identification of variant and invariant properties in the thermodynamics of mixtures: tests for models and computer codes

The consequences of the assumption of mixture molar volume invariance, within thermodynamic models for mixtures, are analyzed. The variant or invariant character, resulting from the assumption of the invariance of the mixture molar volume, is established from exact thermodynamics for each thermodynamic property. Recommendations for testing computer codes corresponding to invariant-molar-volume thermodynamic models are given. A method to demonstrate the invariance of composition-dependent expressions is proposed, together with a simple test to recognize whether the dilution effect is present. A discussion about the experimental evidence supporting the validity of the assumption of mixture molar volume invariance is included.