(1) Chemical equilibrium is a condition in which a reaction and its opposite or reverse reaction occur at the same rate, resulting in a constant concentration of reactants; for example, ammonia synthesis is at equilibrium when ammonia molecules form and decomposes at equal velocities (N2 + 3H2 ←→ 2NH3).(2) Physical equilibrium is exhibited when two or more phases of a system are changing at the same rate so that the net change in the system is zero. An example is the liquid-to-vapor–vapor-to-liquid interchange in an enclosed system, which reaches equilibrium when the number of molecules leaving the liquid is equal to the number entering it.