Fugacities a Summary

The fugacity is a property of systems and of system constituents that was invented in order to facilitate the evaluation of f dG for gaseous constituents. However, the fact that its only practical use is for gases does not change the fact that in principle it is a property of all system constituents. Each constituent fugacity is therefore a system property or state variable, whether measurable or not. (Our insistence on this point is connected with our development of the concept of activities). 

Let us summarize our development of the concept of the fugacity, /. Starting with the definition 

One implication of this is that the fugacity of i is the same in any two states or phases that are in mutual equilibrium. Another implication is that the fugacity, although conceived for use with gases, is actually applicable in principle to any constituent of any system, solid, liquid, or gas. 

Of course, for most constituents of liquids and especially of solids the usefulness of (11.17) would seem to be very limited, because the fugacities are far too small to measure, and are unknown. However, even in systems where constituent fugacities are unknown, the ratio of a constituent fugacity to its fugacity in some other state is quite often a measurable and useful quantity called the activity. 

Rewriting (11.17) so that state is any (unsuperscripted) state and state is a reference state designated by superscript °, we have 

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