Aluminium hydroxide amphoteric behaviour

This behaviour distinguishes beryllium hydroxide from the other hydroxides of Group II which are not amphoteric this amphoterism is also shown by aluminium hydroxide in Group III, and it has been discussed more fully in Chapter 2, where we saw it as characteristic of small ions of high charge, i.e. Be2+ and Al3+. 

Sodium hydroxide dissolves in water to give OH (aq) ions, the strongest base which can exist in an aqueous system. Chloric(VII) acid (perchloric acid) dissociates practically completely in dilute solution to give H30+(aq) ions, which represent the strongest acid which can exist in an aqueous system. The amphoteric behaviour of aluminium is noticed in a series of hydrated salts containing the Al3 + ion and in compounds such as NaA102, which contains the aluminate(III) ion, AIO2-. 

Other amphoteric hydroxides are lead hydroxide (Pb(OH)2) and aluminium hydroxide (Al(OH)3). We can use this sort of behaviour to help identify metal cations, as their hydroxides are soluble in strong bases. 

With the aid of two equations, show how aluminium hydroxide exhibits amphoteric behaviour. 

An understanding of the chemical behaviour of the element can aid in the choice of appropriate techniques and methods, the application of which would not disrupt the interaction of the element with associated constituents. For example, in the study of aluminium some relevant information may include its amphoteric nature, its ability to form predominantly ionic complexes, its tendency to form hydroxides, and the stability of aluminium complexes formed with biological ligands. It is clear that in order to maintain the ionic interactions the pH, ionic strength and, of lesser importance, the ionic composition of the medium used for sample preparation should be similar to that found in vivo. In addition, highly charged surfaces should not come into contact with the sample. 

The insoluble oxides of aluminium and silicon show their acidic nature by reacting and dissolving in an alkaline solution, such as hot, concentrated sodium hydroxide solution, forming a soluble salt. This behaviour is typical of a covalently bonded oxide. However, aluminium oxide also reacts and dissolves in acidic solutions, forming a soluble salt - behaviour typical of a basic metal oxide with ionic bonding. This dual nature provides evidence that the chemical bonding in aluminium oxide is not purely ionic nor purely covalent, hence its amphoteric nature. 

---