Vapor Pressure Plus: An Experiment for Studying Phase Equilibria in Water, with Observation of Supercooling, Spontaneous Freezing, and the Triple Point

Abstract

Liquid−vapor, solid−vapor, and solid−liquid−vapor equilibria are studied for the pure substance water, using modern equipment that includes specially fabricated glass cells. Samples are evaporatively frozen initially, during which they typically supercool to −5 to −10 °C before spontaneously freezing. Vacuum pumping lowers the temperature to ∼−20 °C, and temperature and pressure are recorded on warm-up, first for sublimation, then vaporization, with a pronounced halt at the triple point. Analysis of the data by the integrated Clausius−Clapeyron equation typically yields ΔsubHm and ΔvapHm values within ∼2% (1 kJ/mol) of literature, from which ΔfusHm can also be estimated with fair precision. The experiment is designed for the undergraduate physical chemistry teaching laboratory; aspects of it are also appropriate for demonstrations in general and physical chemistry courses.