ABSTRACT
There are numerous definitions whose meaning one must understand and which must be used repeatedly in discussing one, two, three, and four component systems. This chapter introduces those which are involved in the Phase Rule itself and a few others which are needed to describe some common events which occur in simple one-component systems. The fundamental concept upon which the phase rule, and the phase diagram is based, is the state of thermodynamic equilibrium. The components of a system are the smallest number of independently variable chemical constituents necessary and sufficient to express the composition of each phase present in any state of equilibrium. In ceramic systems, special mention should be made of the influence of variable valence elements on the selection of the components of a system. Construction of phase diagrams is based on the assumption that the system under consideration is at equilibrium.
