At a phase transition two or more different phases may coexist, such as vapour and liquid. Phase transitions can be classified according to their order. A phase transition is of first order if going from one phase to the other involves a discontinuous change in entropy, and, thus, a finite amount of latent heat; higher-order phase transitions do not involve latent heat but exhibit other types of discontinuities. This chapter investigates the necessary conditions for the coexistence of phases, and how phases are represented in a phase diagram. The order of a phase transition is defined with the help of the Ehrenfest classification. The chapter discusses the Clausius–Clapeyron relation which, for a first-order phase transition, relates the discontinuous changes in entropy and volume. Finally, this chapter considers the Ising ferromagnet as a simple model which exhibits a second-order phase transition. It also introduces the notion of an order parameter.
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