This document discusses chemical equilibrium. It defines equilibrium as a state where the macroscopic properties of a system (such as pressure, volume, temperature) have stopped changing, even though microscopic reactions continue to occur. For a system to be at equilibrium, the rates of the forward and reverse reactions must be equal. The document provides examples to illustrate these concepts, such as the equilibrium between NO2 and N2O4 gases. It emphasizes that regardless of whether a system starts with only reactants or only products, the equilibrium concentrations or pressures will be the same once equilibrium is reached.