Equilibrium

Summary: A state in a reversible reaction where the rates of the forward and backward reactions are equal, and the concentrations of reactants and products remain constant. Tags: igcse chemistry definition reactions Created: 2026-07-14 Last Updated: 2026-07-14

In chemistry, equilibrium refers to a state of a reversible reaction in a closed system where the rate of the forward reaction equals the rate of the backward reaction, so the concentrations of all reactants and products remain constant over time. This is a dynamic equilibrium — not a static one — because both forward and reverse reactions continue to occur at the molecular level even though no macroscopic change is observed in properties such as colour, concentration, or pressure. The position of equilibrium can be shifted by changing temperature, pressure (for gaseous systems), or concentration of the reactants or products, as summarised by Le Chatelier’s Principle: the system adjusts to counteract any imposed change. For example, in the Haber process (N₂ + 3H₂ ⇌ 2NH₃), increasing the pressure shifts equilibrium to the ammonia side (fewer gas molecules), while increasing the temperature shifts it towards the reactants because the forward reaction is exothermic. IGCSE exam questions frequently ask students to predict and explain how changes in conditions affect the yield of a reversible reaction using Le Chatelier’s Principle, and to distinguish carefully between the rate of reaction and the position of equilibrium — a catalyst, for instance, speeds up both forward and reverse rates equally but does not alter the equilibrium position.

Key term — IGCSE Chemistry (0620/0971) glossary.