What indicates that a reversible reaction is at equilibrium?

In a reversible reaction that has reached equilibrium, the concentrations of both reactants and products remain constant over time, even though the forward and reverse reactions continue to occur. This means that the rate at which the reactants convert into products is equal to the rate at which the products convert back into reactants.

This dynamic balance results in no net change in the concentrations of the reactants and products, which is a hallmark of chemical equilibrium. Therefore, the observation that the concentrations of reactants and products are unchanged is crucial in identifying that the system is at equilibrium.

The other choices do not accurately describe the state of a system at equilibrium. For example, if no product formation occurs, it suggests the reaction may not have started or has completely gone to completion, which contradicts the concept of equilibrium. Similarly, stating that the forward reaction is complete or that all reactants have been consumed implies that the reaction has progressed to a point where equilibrium is not established, as an equilibrium state necessitates a balance between the forward and reverse processes.