Q.What is the difference between a double salt and a complex ? Give an example.

Combining two or more separately stable compounds in a fixed stoichiometric ratio can produce either a double salt or a coordination complex, and the way to tell them apart is entirely behavioural -- what happens on dissolving in water, not the formula on paper. A double salt dissociates COMPLETELY into its simple constituent ions in solution, with no complex ion surviving intact at all: Mohr's salt, FeSO4(NH4)2SO4.6H2O, dissociates fully into Fe2+, 2NH4+ and 2SO4(2-), and carnallite, KCl.MgCl2.6H2O, dissociates fully into K+, Mg2+ and 3Cl-; in both cases the solution gives the ordinary chemical tests for every one of those simple, separated ions. A coordination complex, in contrast, dissociates in water to give AT LEAST ONE complex ion that stays intact rather than breaking apart further -- K4[Fe(CN)6] dissociates into four free K+ counter ions plus one fully intact [Fe(CN)6]4- complex ion, and the solution does NOT give the ordinary chemical test for free Fe ions or free CN- ions, because they remain locked together inside that surviving coordination sphere. This same dissolved-behaviour test -- does the metal ion (or a supposedly simple ligand) still give its ordinary free-ion chemical test in solution, or not -- is exactly the experimental evidence Werner used to argue that a genuine, intact coordination sphere really exists as a distinct structural unit, rather than the metal and ligands simply being loosely associated the way a double salt's ions are.