Barotropic phenomenon, consists in the fact that in liquid — liquid systems (liquid — gas or gas — gas), at high pressures and certain temperatures, the coexisting phases change places: the phase that lies above (in the gravitational field) and is the less dense under ordinary conditions becomes the heavier one and settles downwards. The barotropic phenomenon occurs because, as the pressure rises, the specific volumes of the phases, previously different, become equal; the phase containing the greater amount of the component with the larger molar mass becomes heavier and sinks in the other phase.
The barotropic phenomenon was first observed by the Dutch physicist H. Kamerlingh Onnes in the system hydrogen (liquid) — helium (gas): at a temperature of 20.1 K and a pressure of 49 atm the gas phase sank below the liquid one. In the region of equilibrium in the gas — liquid system the barotropic phenomenon has been detected in the systems ammonia — nitrogen (at a temperature of 180 K and a pressure of 1800 atm), ammonia — nitrogen-hydrogen (at pressures of 3500—3700 atm and a temperature of 170 K) and others. In ternary systems, in the case of a three-phase equilibrium with two liquid phases, the barotropic phenomenon is observed between them (the systems methanol — toluene, acetone — aniline and others).