When discussing non-ideal gases, the first qualitative observation that is made is that specifying the internal parameters of a system does not uniquely determine an equilibrium state. The possible states are classified by stability and this gives rise to discussions on phase transitions, etc. My question is, what would happen if we fixed the external parameters in one of the metastable/unstable regions?

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For example, in the above picture, say we fixed the system at $V_k$ with the blue temperature instead of the associated pressure. Then could it exist without varying in this metastable state? The reason I am hesitating with this is we know the gas is made up of many packets all of which are at pressure/thermal equilibrium with each other so I'm a bit confused on how to reconcile the fact that the internal parts of the gas have internal constraints but the gas has a whole has external ones. Based on which is the case, the gas will end in a different state. What would happen in this situation?


No the gas will not stay in the unstable state. Instead you will get phase separation. Part of the gas will relax to the liquid phase at the appropriate temperature and pressure (point F) and part of it will go to the gas phase (point J).

Now at point F the specific volume of the gas is lower than at point H but at point J it is higher than at point H, so my having an appropriate fraction of the mixture in each phase we can have any volume we like in between the two.

  • $\begingroup$ So even if external parameters are fixed, it will act as though it is fixed by the corresponding internal parameter? $\endgroup$ – Aakash Lakshmanan Jul 9 '19 at 15:36

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