Introduction

In this chapter, we develop some fundamental understanding of the nature of phase transitions by examining two well-studied examples: the vapor-to-liquid transition of fluids and the paramagnetic-to-ferromagnetic transition in magnetic materials. Here, our focus is on the experimentally observed features of these two transitions and how to interpret and navigate the many phase diagrams that describe them. The theoretical interpretation will be tackled later in Chapter 17. We will find that, in general, a phase transition is accompanied by some change in the amount of order as when, for example, liquid water freezes into crystalline ice. Moreover, we can describe this amount of ordering quantitatively by introducing an appropriate order parameter, whose value changes significantly only during the transition. Based upon the manner in which the order parameter changes, we can distinguish two different types of phase transitions: those of first order for which the order parameter changes discontinuously and those of second order for which it changes continuously. Second-order transitions are possible for both the vapor-to-liquid transition and the paramagnetic-to-ferromagnetic transition and are of interest due to the way in which many properties diverge near the transition in a similar, power law manner.