Editing Phase rule (section)

=== Aqueous solution of 4 kinds of salts ===
Consider an aqueous solution containing sodium chloride (NaCl), potassium chloride (KCl), sodium bromide (NaBr), and potassium bromide (KBr), in equilibrium with their respective solid phases. Each salt, in solid form, is a different phase, because each possesses a distinct crystal structure and composition. The aqueous solution itself is another phase, because it forms a homogeneous liquid phase separate from the solid salts, with its own distinct composition and physical properties. Thus we have P = 5 phases. 

There are 6 elements present (H, O, Na, K, Cl, Br), but we have 2 constraints:

* The stoichiometry of water: n(H) = 2n(O).
* Charge balance in the solution: n(Na) + n(K) = n(Cl) + n(Br).

giving C = 6 - 2 = 4 components. The Gibbs phase rule states that F = 1. So, for example, if we plot the P-T phase diagram of the system, there is only one line at which all phases coexist. Any deviation from the line would either cause one of the salts to completely dissolve or one of the ions to completely precipitate from the solution.