The state reached by a reversible reaction when forward and reverse reaction rates are equal.

• A. Equilibrium
• B. Phase
• C. Phase Diagram
• D. Critical Point

Answer: (A)

In a reversible reaction, two opposite processes occur: reactants convert to products and products convert back to reactants. The system reaches equilibrium when the rate of the forward reaction equals the rate of the reverse reaction. At that moment, the concentrations of reactants and products become constant over time, even though both directions are still happening — this is dynamic equilibrium. The equality of rates defines equilibrium, and the position of that equilibrium depends on temperature (and, for gases, pressure), since these factors change how fast each direction proceeds. The equilibrium constant expresses the ratio of product to reactant concentrations at that temperature and remains fixed unless the temperature changes. The other terms mentioned describe different concepts (phases, phase diagrams, or critical points) and do not describe this balance of rates.