What does “equilibrium” mean in chemistry?

In chemistry, "equilibrium" refers to a state in a chemical reaction where the concentrations of reactants and products remain constant over time. This does not mean that the reaction has stopped; rather, it indicates that the rate of the forward reaction is equal to the rate of the reverse reaction. Consequently, the concentration of each species involved in the reaction remains unchanged even though the chemical processes are still occurring.

This concept is crucial in understanding dynamic systems in chemistry, where a reaction can be reversible and can reach a balance point. The correct answer emphasizes the importance of constant concentrations as an indicator of equilibrium, distinguishing it from notions of change or imbalance.

In contrast, the other choices highlight misconceptions about equilibrium. Some suggest that equilibrium involves imbalances or constant changes in concentrations, which contradicts the fundamental idea of a stable state. One choice even implies a consideration of mass balance, which, while relevant, focuses more on conservation rather than the equilibrium state specific to concentration constancy.