The Ideal Gas Law and the Superheated Region

Last post we talked about using the ideal gas law on the right side of the steam dome and found that it could be used as a decent approximation for very low pressures, even when the vapor is about to start condensing into liquid.  Today we’ll look at the accuracy of the ideal gas approximation out in the superheated region.

The Ideal Gas Law and the Steam Dome

The ideal gas law describes the approximate behavior of gases at temperatures that are high relative to the critical temperature and at pressures that are low relative to the critical pressure.  This figure shows the approximate region where the ideal gas law applies best. 

Normally, one would use steam tables to determine properties close to the saturation region, and only use the ideal gas approximation far away, as shown in the figure. Today we’ll look at the error introduced by the ideal gas approximation as we get close to the saturation region.