Before introducing the Gibbs free energy in my thermodynamics class I asked the students to say which variables they thought were generally the "easiest" to control in chemistry and physics experiments: volume and temperature, volume and energy, pressure and temperature, ...., or all?
Many students thought volume and temperature. (Maybe because what they mostly learn about is gases!).
I think the "correct" answer is pressure and temperature because
* these are environmental not system variables
* it is very hard to control the volume of a solid.
Am I right?
I now realise this is a rather subtle point and worth getting students to think about.
Appreciating it makes students think about experiments rather than mathematics and helps motivate why the Gibbs free energy is actually the most useful thermodynamic function.