At chemical equilibrium, what is always true of the concentrations of reactants and products?

At chemical equilibrium, the concentrations of reactants and products remain constant over time. This constancy occurs because, at equilibrium, the rates of the forward and reverse reactions are equal. As a result, the amounts of reactants being converted into products and the products being converted back to reactants cease to change, leading to a steady state of concentrations.

It is important to note that this does not imply that the concentrations of reactants and products are equal; they can vary greatly depending on the specific reaction and the conditions involved. Additionally, it is not the case that either the reactant concentration or the product concentration is zero at equilibrium. In a typical reversible reaction, there will usually be some amount of both reactants and products present. Therefore, the only universally true statement is that their concentrations are constant.