Chemical equilibrium is characterized by the condition where the concentrations of reactants and products remain constant over time. This does not mean that the reactions have stopped; rather, it indicates that the rate at which the reactants are converted to products is equal to the rate at which the products are converted back to reactants. As a result, although there may still be ongoing reactions occurring in both directions, their effects cancel each other out, maintaining constant concentration levels.
This state of equilibrium is dynamic, meaning molecules are continuously reacting, but the overall concentrations do not change. Therefore, this characteristic of constant concentrations defines the essence of chemical equilibrium and is fundamental in understanding how reactions behave over time.
In contrast, the other options present scenarios that do not accurately reflect equilibrium conditions. For instance, if the forward reaction proceeds faster than the reverse, it implies changes in concentrations, moving away from equilibrium. The presence of only products in significant quantities suggests that equilibrium has not been reached, as reactants would still be present. Lastly, the notion that the reaction has stopped completely indicates a lack of movement or change, which contradicts the essential dynamic nature of equilibrium.