Final answer:
The concentration of a solvent is considered constant during the progress of a reaction, as their concentration is significantly higher and thus remains largely unchanged.
Step-by-step explanation:
The concentration of a reactant that is considered constant during the progress of a reaction is typically that of a solvent. For reactions carried out in solution, the solvent's concentration is not included in the equilibrium constant expression even when it is part of the balanced chemical equation. This is because the solvent's concentration is much greater than the concentrations of the reactants or products, and remains largely unchanged throughout the reaction. An example of such a solvent is water with a concentration of about 55.5 M. Therefore, in equilibrium reactions, the concentration of the solvent is essentially constant and seen as a pure liquid. As a reaction proceeds toward equilibrium, the concentrations of reactants decrease, while the concentrations of products increase until the equilibrium is reached where the reactant and product concentrations, including the equilibrium constant (Qc), remain constant.