Editing Le Chatelier's principle (section)

===Thermodynamic equilibrium processes===
Le Chatelier's principle refers to states of [[thermodynamic equilibrium]]. The latter are [[Mechanical equilibrium|stable]] against perturbations that satisfy certain criteria; this is essential to the definition of thermodynamic equilibrium.

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It states that changes in the [[temperature]], [[pressure]], [[volume]], or [[concentration]] of a system will result in predictable and opposing changes in the system in order to achieve a new [[Thermodynamic equilibrium|equilibrium state]].

For this, a state of thermodynamic equilibrium is most conveniently described through a [[Fundamental thermodynamic relation|fundamental relation]] that specifies a cardinal function of state, of the energy kind, or of the entropy kind, as a function of state variables chosen to fit the thermodynamic operations through which a perturbation is to be applied.<ref>Münster, A. (1970), pp. 173–174.</ref><ref>[[Herbert Callen|Callen, H.B.]] (1960/1985), Chapter 8, pp. 203–214.</ref><ref>Bailyn, M. (1994), Chapter 8, Part A, pp. 312–319.</ref>

In theory and, nearly, in some practical scenarios, a body can be in a stationary state with zero macroscopic flows and rates of chemical reaction (for example, when no suitable catalyst is present), yet not in thermodynamic equilibrium, because it is metastable or unstable; then Le Chatelier's principle does not necessarily apply.