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A Demonstration of the Continuous Phase (Second-Order) Transition of a Binary Liquid System in the Region around Its Critical Point

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Michael R. Johnson

Department of Chemistry, Virginia Tech, Blacksburg, VA 24061

Vladimir M. Petrusevski

Institute of Chemistry, Faculty of Natural Sciences and Mathematics, Sts. Cyril and Methodius University, Arhimedova 5, 1000 Skopje, Macedonia

J. Chem. Educ., 2006, 83 (7), p 1014

DOI: 10.1021/ed083p1014

Publication Date (Web): July 1, 2006

Section:

History, Education, and Documentation

Abstract

In most general chemistry and introductory physical chemistry classes, critical point is defined as that temperature–pressure point on a phase diagram where the liquid–gas interface disappears, a phenomenon that generally occurs at relatively high temperatures or high pressures. Two examples are: water, with a critical point at 647 K (critical temperature: T_c) and 215 bar (critical pressure: P_c); and carbon dioxide, with a critical point of 304 K (T_c) and 73 bar (P_c). In colloid chemistry, there is another critical point, which occurs at a specific temperature and mole fraction in a specific binary liquid system where immiscible or partially miscible species become completely miscible. These phenomena of colloidal systems occur at atmospheric pressure and over a temperature range which, for some systems, includes room temperature (~300 K). The purpose of this demonstration is to show this critical-temperature phase-transition phenomenon for a specific binary liquid system, where T_c is ~300 K.