Exploring Phase Diagrams Using Supercritical Fluids. An Undergraduate Physical Chemistry Laboratory

Abstract

We present a physical chemistry laboratory in which students explore phase behavior using subcritical and supercritical fluids. In this experiment, the pressure-dependent dielectric constant of carbon dioxide (CO2) is measured at four temperatures spanning the critical point. For subcritical CO2, the dielectric constant undergoes a striking increase with pressure during condensation, demonstrating the existence of a phase transition. In contrast, the dielectric constant of supercritical CO2 undergoes a systematic increase with pressure demonstrating the absence of a phase transition in the supercritical region. The experiment provides physical chemistry students with a variety of experiences. From a practical standpoint, it allows students to directly use and handle supercritical fluids. From a pedagogical standpoint, the laboratory allows students to develop a cohesive picture of subcritical and supercritical behavior, thereby providing a physical picture of the phenomena schematically depicted in a phas...