55Equilibrium
Although the concept of equilibrium is widely used across sciences, in the context of chemistry, it
arises whenever a number of different states of the chemical composition are possible, as for
example, in a mixture of several chemical compounds that can react with one another, or when a
substance can be present in more than one kind of phase.
A system of chemical substances at equilibrium, even though having an unchanging composition,
is most often not static; molecules of the substances continue to react with one another thus giving
rise to a dynamic equilibrium. Thus the concept describes the state in which the parameters such
as chemical composition remain unchanged over time.