P − Vplots for two gases during adiabatic processes are shown in Fig. Plots 1 and 2 should correspond respectively to
He and Ar
For an adiabatic process, = constant, Differentiating, we have
Since at any instant PV = constant, , i.e. the slope of P − V curve is proportional to
. Now, for a diatomic gas,
is than that of for a monoatomic gas for which
. Therefore, the slope of the P − V curve is less for a diatomic gas than for a monoatomic gas. Hence curve 1 corresponds to diatomic gas and curve 2 to monoatomic gas