What condition defines a system in equilibrium?

A system in equilibrium is characterized by the absence of net flux, meaning that there are no net changes occurring within the system over time. This condition signifies that all macroscopic properties of the system—such as temperature, pressure, and concentration—remain constant, indicating that the system is stable and balanced. When there is no flux present, it implies that there is no active transfer of energy or matter occurring, which would disturb the equilibrium.

In contrast, active energy transfer would indicate a change in the system, as energy would be moving in or out, disrupting the stability required for equilibrium. Constant pressure and variable temperature could occur in equilibrium under specific conditions, but they are not definitive characteristics of equilibrium on their own because a system can still operate with changes in pressure or temperature while being in equilibrium concerning other properties. Therefore, the presence of no flux is what most clearly defines a system in true equilibrium.