A comprehensive understanding of planetary bodies requires that we study how changes in physical conditions give rise to chemical phenomena. Physical conditions may be determined, for example, by the intensive variables P and T, where possible P–T combinations are in turn determined by the nature of heat sources and heat transfer mechanisms (Chapters 2 and 3). Chemical phenomena are transformations that entail redistribution of matter among and within phases. Some examples are: mineral transformations and melting in solid planets, changes in the relative amounts of molecular species that make up a gas or a supercritical fluid phase, and changes in the ionic constituents in an electrolyte solution such as seawater. The study of phenomena such as these is based on a mathematical description of chemical equilibrium, even in those cases in which departures from equilibrium cannot be ignored (Chapter 12). In this chapter we lay the foundations for the study of chemical equilibrium, including a comprehensive discussion of the use of composition as a thermodynamic variable. The principles and mathematical formalisms that we develop here are general, but important differences in implementation for different types of systems exist. These are dealt with in subsequent chapters.

Chemical equilibrium

Fundamental concepts

We begin by distinguishing between homogeneous and heterogenous systems. A homogeneous system consists of a single phase.