Aluminium oxide amphoteric behaviour

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. 

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