Why is aluminum hydroxide amphoteric hydroxide


In general, the so-called aluminum hydroxide actually refers to the hydrate of aluminum oxide. For example, the white colloidal precipitate obtained by adding ammonia or alkali to the aluminum salt solution has an uncertain water content and uneven composition, which is collectively referred to as hydrated alumina.

True aluminum hydroxide can only be obtained by passing CO2 through solutions in aluminate solutions (containing Al(OH)4-ions). Crystalline aluminum hydroxide is different from hydrated aluminum oxide. It is difficult to dissolve in acid and does not dehydrate when heated to 373K. It can be converted into aluminum hydroxide (AlO(OH)) when heated at 573K for 2h.

Aluminum hydroxide is amphoteric hydroxide. Because of its existence of two kinds of ionization forms, is a weak acid, can have acid chemical formula H3AlO3, is a weak base, can have basic chemical formula Al(OH)3. Aluminum hydroxide has amphoteric, can react with acid and can react with alkali. The acidity of aluminum hydroxide lies in that it is a Lewis acid that can add OH-, thus reflecting the alkaline Al(OH)3. Due to the existence of two kinds of ionization, two kinds of salts can be produced:

Aluminum salt: Al3 AlCl3,KAl(SO4)2-12H2O (alum). Their aqueous solutions are acidic due to the hydrolysis of Al3. AgNO3 and dilute nitric acid are added dropwise respectively to produce white precipitate. The light yellow precipitate is Br-; The yellow precipitate is I-.

2. The metaaluminates, AlO2- NaAlO2,KAlO2. Their aqueous solutions are alkaline: AlO2- 2H2O → Al(OH)3 OH-When the two types of salts are mixed, a double hydrolysis reaction occurs to generate Al(OH)3 Al3 3 AlO2- 6H2O = = 4Al(OH)3 ↓.

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