ABSTRACT
INTRODUCTION TO THE PHASE RULE Bancroft has stated that the two expressions describing in a qualitative manner all states and changes of equilibrium are the Phase Rule and the Theorem of Le Chatelier (1) . One of these principles describes the possibilities that might exist among substances in equilibrium, and the other describes how such equilibrium systems would react to an imposed stress. These changes may entail alterations in chemical composition, but could just as well involve transitions in the physical state. There is no doubt that thermodynamics is the most powerful tool for the characterization of such equilibria. Consider the situation presented by elemental sulfur, which can be obtained in either a rhombic or monoclinic crystalline state. Each of these forms melts at a different temperature, and is stable under certain welldefi ned environmental conditions. An understanding of this system would entail knowing under what conditions these two forms could equilibrate with liquid sulfur (either singly or together), and what would be the conditions under which the two could equilibrate with each other in the absence of a liquid phase.
