I'm trying to find an answer to a question in one of my thermodynamics textbooks. (Undergraduate chemical engineering is the context).

It is a conceptual question, asking, "Do all systems in equilibrium continue to be in equilibrium if they are instantly isolated from their surroundings?" What does everyone think?

I've consulted a few authoritative sources, and the answer becomes confusing and conceptual and even philosophical. I figured there might be a very straightforward answer.

Thank you.


2 Answers 2


Certainly, an isolated system at equilibrium will remain so when isolated. This example is a bit of a tautology though. But the implications are important, because we next have to consider non-isolated systems.

An open system allows mass flow but may have an insulated boundary. A closed system allows no mass flow but may have a diathermal boundary. Finally, a system may allow mass and heat flow (as well as flow of other work).

While these systems can be in steady state, can they be in true equilibrium? See the discussion here. The answer is no. They can be at steady state, a quasi-equilibrium condition.

The instant a non-isolated system under steady state conditions is isolated, its steady state is lost. The once non-isolated system that is now isolated can then move to its true equilibrium.

But, though a non-isolated system that is suddenly isolated can move its state, will it move its state? Think about a refrigerator that is losing heat and being cooled to make up. Allow a bit of air flow leaking in and out at various places around the doorway. The inside is at a steady state temperature and pressure (and chemical potential and ...). Now, isolate that system. No heat or mass flow is allowed at all. The result is, the inside of the refrigerator will remain at the temperature (and pressure and chemical potential) that it had at the instant it is isolated. Why would it change?

OK, we have taken out chemical reactions from this consideration (oh no, we forgot that we left both yogurt and mold spores inside). When a chemical reaction is occurring in a steady state (non-isolated) system, it may not be at its true chemical equilibrium within the confines of the system. We only need to realize that we purge mold spores (and yogurt) in/out of the refrigerator over a steady period to see the difference. Once the system is isolated, the mold can grow freely to a new equilibrium state.

So, systems that are not isolated to start and are at steady state but include chemical reactions may not remain at the state they have when they are instantly isolated. They can change internally because the chemical reaction itself was only at a quasi-equilibrium state, not a true (isolated case) equilibrium state.

What about non-isolated systems that act as isolated systems because they have no flows across their boundaries. If they are at equilibrium, they remain so when isolated.

In summary, the connection that is to be made is that equilibrium is a state only of isolated systems or systems that behave exactly as such to start. The question itself is therefore somewhat a tautological one.

  • $\begingroup$ Mr. Weimer: Excellent points, many thanks to you. I appreciate the upfront clarifications near the top, and the examples below. This was very clear! It was especially important to note that, for open (mass and heat flow) and closed (heat flow) systems that do not behave as isolated systems (they have flow), they may attain steady-state status but this is only quasi-equilibrium. $\endgroup$ Apr 10, 2019 at 0:45
  • $\begingroup$ Also, in paragraph 5, you said "loosing" but meant losing! Not to be pedantic, but worth a mention! Many thanks again. $\endgroup$ Apr 10, 2019 at 0:48
  • $\begingroup$ Fixed mis-spelling. Thanks. $\endgroup$ Apr 10, 2019 at 3:17

Equilibrium is not the same as either maximal entropy or constant temperature. All it really means is you can't set up a heat engine inside the region and extract work from it. It is possible for the system to be interacting with the environment in such a way that the environment is determining the equilibrium point, and equilibrium is then lost when the environment is changed.

Thermal equilibrium just means the temperature is uniform throughout a region at a given instant. But it does not mean the temperature is constant. An insulated body that is being slowly heated uniformly throughout can be in thermal equilibrium during the process.

Black body radiation provides a simple example of how the environment can change an equilibrium. In a vacuum, a body subject to uniform surface radiation will eventually reach thermal equilibrium. Suddenly remove the radiation source and the equilibrium is destroyed. The surface will begin to cool through black body radiation.

  • $\begingroup$ Mr. Sweet: I appreciate the clarification, though I'm having some trouble with this, "you can't set up a heat engine inside the region..." I'll give it a re-read. I'm not able to connect it to the original question of isolating a system in equilibrium. Thanks though! $\endgroup$ Apr 10, 2019 at 0:51
  • $\begingroup$ Equilibrium and steady state are not the same. A system that interacts with the environment is not isolated. Such a system may be at steady state; it is however not necessarily at its equilibrium. When a parameter of a system is changing with time, the system is by definition not at equilibrium. Just because two objects are in thermal or mechanical or chemical equilibrium does not mean that either one of them are of themselves at thermal or ... equilibrium. It only means, the process is reversible. Reversible processes that cause changes only exist in theory not in practice. $\endgroup$ Apr 12, 2019 at 13:04

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