A two step process can be used to experimentally determine the ratio of the heat capacities. The thermodynamic expressions used to derive the heat capacity ratio apply only to the part of the gas that remains in the carboy after the stopper is replaced since molar volumes and molar heat capacities were used. Physically, the reversibility of the expansion can be justdied as follows. One can imagine an invisible surface separating the gas that remains within the carboy and the gas that escapes when the stopper is removed. The gas below this surface expands in an approximately reversible way against the surface.
Work is done as the upper gas is pushed out.
The change is approximately adiabatic only because it is rapid. No appreciable amount of heat is transferred between the reservoir and the gas in the carboy during the short period of the expansion. After the stopper is replaced, the temperature is seen to increase as equilibrium is reestablished. The lab is equipped with several apparati as shown above, each filled with a different sample gas. Student teams shall rotate through the stations so that each team may obtain data on all the sample gases.