ABSTRACT
This chapter outlines the modern classification, emphasize metastable states and the resulting hysteresis for first order phase transitions, metastable states being the raison detre. The existence of latent heat implies that conversion from phase-1 to phase-2 will proceed over a finite time even after the control parameter has reached its critical value (TC). Phase coexistence at the phase transition point implies that a first order phase transition must proceed by nucleation and growth of one phase in the matrix of the other. In the Ehrenfest classification, there was no clear conclusion about the possibility of supercooling or superheating across second order or higher order transitions. In the modern classification, phase coexistence at TC and the existence of latent heat emerge as accepted characteristics distinguishing first order phase transitions from continuous ransitions. The chapter discusses hysteresis with control variables other than temperature, in the understanding of first order transitions.
