Chemical Equilibrium
Definition
The state where forward and reverse reactions run at equal speed, so concentrations stop changing: a busy stalemate, not a stopped reaction.
Technical definition
Chemical equilibrium is the dynamic state in which forward and reverse reaction rates are equal, so macroscopic concentrations remain constant. It is characterized by the equilibrium constant K and can be approached from either direction.
Example
In a sealed bottle, water evaporates and condenses at equal rates. The water level never changes, yet molecules constantly move both ways.
Common confusion
Equilibrium does NOT mean equal concentrations. At equilibrium the ratio of concentrations is fixed by K, and products may vastly outnumber reactants or vice versa.