Aluminum hydroxide amphoteric character

Describe the successive observations that are made when NaOH solution is added gradually to an AlCU solution until an excess of the base is present. Likewise describe observations when HC1 solution is added gradually to an Na3A103 solution until the acid is in excess. Write fully ionized intersecting equations corresponding to each observation, and explain how the amphoteric character of aluminum hydroxide is manifested in this experiment. 

A weakly acidic behavior is typical of the sdanol groups of silica as well as of the B—OH groups of boria-containing materials. Considering that aluminum is at the border between metal and semimetal character and that aluminum hydroxides are typically amphoteric, both bonding concepts may apply. In other words, the properties of bulk alumina are better explained in terms of ionicity of the Al —bonds, whereas the Al—OH groups at the surface may have a more covalent character. 

The amphoteric character of aluminum hydroxide is a reflection of the partial non-metallic character of aluminum. Table 22.5 lists the major uses of aluminum compounds. 

The Group lllA elements clearly show the trend of increasing metallic character in going down any column of elements in the periodic table. Boron, at the top of Group lllA, is a metalloid, and its chemistry is typical of a nonmetal. The compound B(OH)3 is actually acidic (boric acid). The rest of the elements (aluminum, gallium, indium, and thallium) are metals, but their hydroxides change from amphoteric (acidic and basic) for aluminum and gallium to basic for indium and thallium. 

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