The 180° Rule at Triple Points: A Consideration Based on a Modified Butterfly Model

Abstract

The disposition of phase boundaries at triple and multiple points in phase diagrams of single and multicomponent systems is governed by a class of rules called collectively “The 180 Rule” in the literature of physics. These rules state that in a phase diagram with thermodynamically proper independent variables no phase occupies more than 180° of angle at a triple point. For simple one component systems (such as pure substances) described thermodynamically by two independent intensive variables, say temperature T and pressure p, the 180° rule is usually stated in the following form1 : The metastable extension of each two-phase equilibrium curve beyond the triple point must lie between the other two stable curves (see Fig. 1).KeywordsInternal EnergyBifurcation PointTriple PointThermodynamic PotentialThermodynamic SystemThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.