Double salt and coordination complex

Combining two or more separately stable compounds in a fixed stoichiometric ratio can give rise to two structurally different kinds of substance: a double salt or a coordination complex, and telling them apart comes down to what happens when each is dissolved in water. A double salt dissociates COMPLETELY in water into its simple constituent ions, with no complex ion surviving intact; for example, Mohr's salt, FeSO4(NH4)2SO4.6H2O, dissociates fully into Fe2+, 2NH4+ and 2SO4(2-) ions, and carnallite, KCl.MgCl2.6H2O, dissociates fully into K+, Mg2+ and 3Cl- ions -- in both cases every ion present in the solid ends up as a free, simple aqueous ion. A coordination complex, by contrast, dissociates in water to give AT LEAST ONE complex ion that remains fully intact rather than breaking apart further; K4[Fe(CN)6] is the standing example, dissociating into 4 free K+ counter ions plus one intact Fe(CN)6 complex ion -- the six CN- ligands stay bonded to iron rather than separating into six free CN- ions. This water-solubility behaviour (whether or not simple metal-ion or f …