Explain the difference between a double-salt and a complex, giving an example in each case.

Double salts exist in solid state but dissociate into constituent ions on dissolution in water. They lose their identity in aqueous solution. Mohr's salt $FeS{O}_4\cdot (N{H}_4{)}_{2}S{O}_4\cdot 6H_{2}O$ is a double salt. The aqueous solution of this salt gives the test of $F{e}^{2+}$, $N{H}_4^{+}$ and $S{O}_4^{2-}$ ions.
On the other hand complexes ( or coordination compounds) retain their identity in aqueous solutions and show properties which are different from those of constituent ions. For example, ferrocyanide ion $\left[Fe\right(CN{)}_6{]}^{4-}$ is a complex ion and it retains its identity in solution. It does not dissociates to give $F{e}^{2+}$ ions and $C{N}^{-}$ ions. Hence, the aqueous solution of , ferrocyanide ion $\left[Fe\right(CN{)}_6{]}^{4-}$ will not give test of $F{e}^{2+}$ ions and $C{N}^{-}$ ions.