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HE PREVIOUS CHAPTER introduced some of the concepts of equilib-

rium. This chapter and the next will expand on those concepts as we

apply them to certain types of chemical systems. Here, we focus on the sim-

plest of systems, those that consist of a single chemical component. It may

seem strange that we would spend much effort on such simple systems, but

there is a reason. The ideas we develop using simple systems apply to more

complicated systems. The more thoroughly the basic concepts are developed,

the more easily they can be applied to real systems.

6.1 Synopsis

Very few kinds of equilibria can be considered for single-component systems,

but they provide the basis for our understanding of the equilibria of multi-

component systems. First, we will define component and phase. We will use

some of the mathematics from the previous chapter to derive new expressions

that we can use to understand the equilibria of single-component systems. For

such simple systems, graphical methods of illustrating these equilibria—phase

diagrams—are useful. We will explore some simple examples of phase dia-

grams and discuss the information that they provide. Finally, we will introduce

a simplifying equation called the Gibbs phase rule, which is useful for multi-

component systems as well.

6.2 A Single-Component System

Suppose you have a system you want to describe thermodynamically. How do

you do it? Perhaps most important in your description is what’s in the system;

that is, the components of the system. For our purposes, a componentis de-

fined as a unique chemical substance that has definite properties. For example,

a system composed of pure UF 6 has a single chemical component: uranium

hexafluoride. Granted, UF 6 is composed of two elements, uranium and fluo-

rine, but each element lost its individual identity when the compound UF 6 was

formed. The phrase “chemically homogeneous” can be used to describe single-

component systems.

6.1 Synopsis

6.2 A Single-Component System

6.3 Phase Transitions

6.4 The Clapeyron Equation

6.5 The Clausius-Clapeyron
Equation

6.6 Phase Diagrams and the
Phase Rule

6.7 Natural Variables and
Chemical Potential

6.8 Summary

Equilibria in

Single-Component

Systems