amphoterism [key], in chemistry, the property of certain substances of acting either as acids or as bases depending on the reaction in which they are involved. Many hydroxide compounds are amphoteric. For example, aluminum hydroxide, Al(OH)3, reacts as a base with common acids to form salts, e.g., with sulfuric acid, H2SO4, to form aluminum sulfate, Al2(SO4)3. It reacts as an acid with strong bases to form aluminates, e.g., with sodium hydroxide, NaOH, to form sodium aluminate, Na[Al(OH)4(OH2)2]. Organic molecules that contain both acidic (e.g., carboxyl) and basic (e.g., amino) functional groups are usually amphoteric.
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