Ligands added to a solution containing a
complex ion may react with it and
replace the original ligand as shown below.
| Zn(H2O)42+(aq) | + | 4NH3(aq) |  | Zn(NH3)42+(aq) | + | 4H2O(l) |
| | | | | colourless | | |
Insoluble compounds may dissolve in the presence of a ligand if the metal ion in the insoluble compound forms a complex ion containing that ligand.
Consider the dissolving of Zn(OH)
2 and Al(OH)
3 in the presence of excess OH
– and the dissolving of Zn(OH)
2 and Cu(OH)
2 in the presence of excess NH
3.
| Zn(OH)2(s) | + | 2OH–(aq) | | Zn(OH)42–(aq) |
| | | | | colourless |
| Al(OH)3(s) | + | OH–(aq) | | Al(OH)4–(aq) |
| | | | | colourless |
| Zn(OH)2(s) | + | 4NH3(aq) | | Zn(NH3)42+(aq) | + | 2OH–(aq) |
| | | | | colourless | | |
| Cu(OH)2(s) | + | 4NH3(aq) | | Cu(NH3)42+(aq) | + | 2OH–(aq) |
| | | | | blue | | |
The dissolving is due to the conversion of the insoluble compound to a soluble compound that exists as ions in solution. The cation associated with the hydroxo complex anions is Na+.
Observations based on colour and the formation of precipitates which may or may not redissolve when excess reagent is added are the basis of "qualitative analysis" - a procedure that allows identification of unknown ions in solution.