equilibrium

Equilibrium is the state a reversible reaction reaches when its forward and reverse rates are equal, and it is written over a double arrow, as in N₂ + 3 H₂ ⇌ 2 NH₃. Three things about it are worth holding: the reaction has not stopped, only settled — both directions still run, at the same rate — the amounts of reactant and product are not equal, and the position can sit well to either side. What the amounts settle at shifts when conditions change, and the shift always partly undoes the disturbance: adding a reactant or removing a product pushes toward products, raising the temperature moves the position toward the endothermic side, and a catalyst brings the same position about sooner without changing where it lands. The carbonic-acid equilibrium in blood, CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻, is the one a clinical reader meets: rapid breathing removes CO₂ and raises the pH.