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9 The Phase Rule
It is essential in science for experiments to be repeated, tested, and verified before
being accepted into the body of theory.^1 To duplicate thermochemical experiments,
one must be able to duplicate the thermochemicalsystem, and to do that we must know
how many of its infinite number of physical properties—massm, energy, entropy, the
heat capacitiesCpandCV, refractive indices, and so on—must be specified.
A fundamental truth that we usually take for granted is that there is a certain
number of variables that completely describe any selected system. When we measure
some of them, we know all we can ever know about the system and we can, in
principle, calculate all the variables we haven’t measured from those that we have.
Which variables are they and how many of them are there? Must we know a substantial
portion of the infinite number of possibilities in order to define a thermochemical
system? If that were true, science as we know it would hardly be possible because we
could never really verify (or contradict) an experimental result by replication. This is
the problem that was solved by J. Willard Gibbs.9.1 COMPONENTS, PHASES, AND DEGREES OF FREEDOM
Gibbs distinguished between componentsCof a system, which arechemicallydis-
tinguishable, and the phasesPof a system, which arephysicallydistinct. Thus, the(^1) There are exceptions. The science of cosmology offers interesting contradictions because we would be
hard pressed to duplicate the creation of the universe.
Concise Physical Chemistry,by Donald W. Rogers
Copyright©C2011 John Wiley & Sons, Inc.
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