ABSTRACT
For phase diagrams of mixtures of different chemical species, the phase rule can be used to determine the number of degrees of freedom, F, in the system (see Topic D4). This is given by:
F=C−P+2
where C is the number of components and P is the number of phases present. The number of components is the number of independent chemical species in the system. This is usually equal to the number of different chemical substances present; for example, a mixture of benzene and water would have two components. However, in a few cases, new chemical species are formed by reaction. An example is the ionization of a weak acid (see Topic C3):
In this case, although there appears to be four chemical species, there are two equations linking them; the equilibrium constant expression (see Topic C1) and an equation maintaining the overall electroneutrality of the system, which equates the number of cations and ions to maintain no overall charge. This means that in reality there are only two components. Generally C=S−R, where C is the number of components, S is the number of chemical species present and R is the number of different equations linking them. For systems involving substances that ionize, the number of components is generally equal to the number of chemical species present without the complication of ionization. This is because ionization produces the same number of extra chemical species as equations linking them and increases both S and R by the same amount.
