amphoterism
/ am-FOT-er-izm /
Some substances refuse to commit to being an acid or a base — they will act as either one, depending on who they meet. Aluminium hydroxide is the textbook case: pour acid on it and it dissolves like a base; pour strong alkali on it and it ALSO dissolves, this time behaving as an acid. A species that can react as both an acid and a base is called amphoteric (from the Greek amphoteros, 'both'). Amphoterism is most famous among the oxides and hydroxides of metals near the metal-nonmetal dividing line.
Watch aluminium hydroxide, Al(OH)3, do both jobs. With acid it acts as a base: Al(OH)3 + 3 H+ gives Al3+ + 3 H2O. With strong base it acts as an acid, dissolving to form the aluminate ion: Al(OH)3 + OH- gives [Al(OH)4]-. The same dual behavior appears for ZnO, BeO, Cr2O3, SnO2, PbO, and Sb2O3. The pattern is periodic and predictable: metallic-element oxides (Na2O, MgO, CaO) are basic, nonmetal oxides (CO2, SO3, P4O10) are acidic, and the in-between elements along the diagonal staircase (Be, Al, Ga, Zn, Sn, Pb, Sb) give amphoteric oxides — they straddle the boundary, so they can swing both ways. Acidity of an oxide generally rises across a period and with the oxidation state of the element (CrO is basic, Cr2O3 is amphoteric, CrO3 is acidic).
Amphoterism is practically useful and conceptually clarifying. It is the basis of separating aluminium from iron in ore processing (the Bayer process dissolves amphoteric Al2O3 in hot NaOH as aluminate while basic Fe2O3 stays behind), it shows why some metal hydroxides redissolve in excess base (a classic qualitative-analysis clue), and it neatly demonstrates that acid and base are roles set by context, not fixed identities. Water itself is amphoteric (amphiprotic) in the Bronsted sense, as are species like HCO3- and HSO4-, which can both donate and accept a proton.
Aluminium hydroxide is amphoteric: with acid, Al(OH)3 + 3 H+ gives Al3+ + 3 H2O (acting as a base); with strong alkali, Al(OH)3 + OH- gives [Al(OH)4]- (acting as an acid). It dissolves at both ends of the pH scale but is least soluble around neutral pH.
One hydroxide, two roles: base toward acid, acid toward base — amphoterism in a single substance.
Do not confuse amphoteric (acts as acid OR base) with amphiprotic (specifically can donate AND accept a proton, like H2O or HCO3-). All amphiprotic species are amphoteric, but Lewis-amphoteric oxides like Al2O3 need not involve a proton at all.