Glossary of Terms for Phase Equilibria Diagrams
- Variance
"The number of intensive variables which can be altered independently and arbitrarily without
bringing about the disappearance of a phase or the formation of a new one is called the number
of degrees of freedom of a system." Intensive variables are those
which are independent of mass, such as pressure, temperature, and composition.
The number of degrees of freedom of a system may also be defined as the "number of variable factors,
temperature, pressure, and concentration of the components, which must be arbitrarily fixed in
order that the condition of the system may be perfectly defined."
A system is termed invariant, mono-variant, bi-variant, tri-variant, and so on, according to
whether it possesses, respectively, 0, 1, 2, 3, etc., degrees of freedom.
Reference: Levin, E.M., McMurdie, H.F., and Hall, F.P., Phase Diagrams
for Ceramists: Volume 1, The American Ceramic Society, Columbus, Ohio, p. 8, 1956.
Note: The reference for the definition of Degrees of Freedom (or Variance) includes
the following additional sources.
MacDougall, F.H., Thermodynamics and Chemistry, John Wiley and Sons, New York, NY, 1939.
Findlay, A., Campbell, A.N., and Smith, N.O., The Phase Rule and Its Applications, Ninth Edition,
Dover Publications, Inc., New York, NY, 1951.
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