Partially-Miscible Liquids Phase Diagrams: Summary

The answers to the ConcepTests are given below and will open in a separate window. 
Key points from this module:
  • Two single components (A, B) may be only partially miscible over some concentration and temperature range so that two phases co-exist in equilibrium, and each phase contains both components.
  • If two liquid phases of a binary system (A, B) are in equilibrium, one phase will be enriched in component A and one will be enriched in component B.
  • At a given pressure, two partially-miscible liquid phases for a binary system will be in equilibrium with a vapor phase at only one temperature.
  • For two phases at equilibrium in a binary system (A, B), the fugacity of component A will be the same in each phase, and the fugacity of component B will be the same in each phase.
  • Liquid-phase separation occurs when attractive interactions between unlike molecules is less than the average attraction between like molecules.
  • Liquid-phase separation occurs when a system has positive deviations from Raoult’s law.
  • The lever rule can only be used when two phases (not three phases) are present.
From studying this module, you should now be able to:
  • Given temperature and composition, use phase diagrams for partially miscible liquids of binary systems to identify which phases are present.
  • Identify regions where liquid phase separation occurs on a phase diagram.
  • When two phases are present, determine the amount of each phase for a given point on a phase diagram 


Prepared by John L. Falconer, Department of Chemical and Biological Engineering, University of Colorado Boulder