I often search for valid collocations using google with a specific query phrase. In case of the phrase in question, i.e. in/at an equilibrium I have a problem since both seem to be used almost equally often. So I am wondering whether there is any difference in meaning in phrases like:

  1. The molecular system is in an equilibrium.

  2. The molecular system is at an equilibrium.

  3. The molecular system is in equilibrium.

  4. The molecular system is at equilibrium.

Are the above phrases all correct?

2 Answers 2


From a physicist's perspective (statistical mechanics and thermodynamics), the two ways I use the word myself, and have seen it used around me, are:

  • in equilibrium with something
  • at equilibrium

The first focusses on the fact that equilibrium is reached within a given environment, while the second focusses on the system itself.


"Equilibrium" is always defined as a specific state.

"in" seems then more adequate.


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