The experiment P2.4.3.1 investigates the behavior of sulfur hexafluoride (SF6) close to the critical temperature. The critical temperature of this substance is TC = 318.7 K and the critical pressure is pc = 37.6 bar. The substance is enclosed in a pressure chamber designed so that hot water or steam can flow through the mantle. The dissolution of the phase boundary between liquid and gas while heating the substance, and its restoration during cooling, are observed in projection on the wall. As the system approaches the critical point, the substance scatters short-wave light particularly intensively; the entire contents of the pressure chamber appears red-brown. This critical opalescence is due to the variations in density, which increase significantly as the system approaches the critical point.
Note: The dissolution of the phase boundary during heating can be observed best when the pressure chamber is heated as slowly as possible using a circulation thermostat.
