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  Home > JCE Print > Journal of Chemical Education > Issues > 1999  > February  >
In the Classroom
Description of Regions in Two-Component Phase Diagrams
Robert M. Rosenberg
Department of Chemistry, Northwestern University, Evanston, IL 60208-3113
Cover
February 1999
Vol. 76 No. 2
p. 223
Abstract
This paper is concerned with helping students in an undergraduate physical chemistry course to interpret various geometric regions in reduced two-component phase diagrams in which either temperature or pressure is constant. This aim is accomplished by correlating the dimensionality of a geometric region with the degrees of freedom of the system obtained from the phase rule. If the system contains one phase at equilibrium, the number of degrees of freedom is 2, and the system is represented by an area. If the system contains two phases at equilibrium, the number of degrees of freedom is 1, and the system is represented by curves. If the system contains three phases at equilibrium, the number of degrees of freedom is 0, and the system is represented by points. The crucial argument in distinguishing between "curves", which represent the functional relationship between temperature or pressure and composition at equilibrium, and other curved lines in the diagram is that a curve must be continuous and have a continuous first derivative in order to be a well-behaved function. The approach is particularly useful with phase diagrams that contain regions with single solid phases of variable composition, either solid solutions or nonstoichiometric compounds.
More Information
*  Citation
 Rosenberg, Robert M. J. Chem. Educ. 1999 76 223.
*  Keywords
 Physical Chemistry; Equilibrium; Phase Transitions / Diagrams; Thermodynamics
*  History
 Created:
Last Updated:		 June 15, 1999
June 22, 2005
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