The goal of this section is to predict what will happen to our element when it is subjected to changes in those variables that we can manipulate, usually temperature and pressure. For example, what happens when we heat a sample of pure sulfur? It will probably melt at some point. What happens when we subject carbon to very high pressures? We predict that diamond will form. We seek quantitative explanations of these phenomena and an ability to predict under what conditions they will occur.
Consider a single component, A. A component is a chemical constituent (element
or compound) that has a specified composition. For simplicity, we will assume that
component A is an element, but we will see in subsequent sections that it can be
anything that does not undergo a chemical change, including compounds. But for now, we have an element, A, that can exist in two phases, which we will designate ? and ?. A phase is defined as a homogeneous portion of a system that has uniform physical and chemical properties