2. Experimental determination of thermodynamic quantities for vapor phases

Thermodynamics is concerned only with the macroscopic states of matter whose properties can be perceived experimentally. Vapors are defined as the form in which the gaseous phase of a compound appears at temperatures below a so-called "critical" temperature. Above this temperature, the properties of the liquid are similar to those of the gaseous phase. In particular, the refractive index can no longer be used to separate the liquid from the gas, so there is no longer any notion of gas but of fluid. The gas phase is thermodynamically characterized by its chemical constituents (atoms and/or molecules), its pressure and its absolute temperature (in K).