The Solubility of Amphoteric Metal Hydroxides

Many metal hydroxides are insoluble or only very slightly soluble in pH-neutral water. For example, Al(OH)3 has K p = 2 X 10 , which means that if you put Al(OH)3 in water, the vast majority of it will settle to the bottom as an undissolved solid. All metal hydroxides, however, have a basic anion (OH ) and therefore become more soluble in acidic solutions (see the previous subsection and Section 16.5). The metal hydroxides become more soluble because they can act as a base and react with H30 (a ). For example, Al(OH)3 dissolves in acid according to the reaction  

Interestingly, some metal hydroxides can also act as acids— they are amphoteric. The ability of an amphoteric metal hydroxide to act as an acid inaeases its solubility in basic solution. For example, Al(OH)3(5 ) dissolves in basic solution according to the reaction  

A1(0H)3 is soluble at high pH and soluble at low pH but insoluble in a pH-neutral solution. 

We can observe the whole range of the pH-dependent solubility behavior of Al by considering a hydrated aluminum ion in solution, beginning at an acidic pH. We know from Section 15.8 that Al in solution is inherently acidic because it complexes with water to form Al(H20)5 (a ). The complex ion then acts as an acid by losing a proton from one of the complexed water molecules according to the reaction  

Addition of base to the solution drives the reaction to the right and continues to remove protons from complexed water molecules  

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