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Chemistry · Glossary

What is Amphoteric hydroxide?

Definition 11.16 University Chemistry — Year 1 · Chapter 11 — Precipitation and Solubility

A hydroxide is an amphoteric hydroxide if it dissolves both in acid, as the cation, and in excess base, as a hydroxo complex anion:

Zn(OH)X2(s)+2 HX3OX+→ZnX2++4 HX2O,Zn(OH)X2(s)+2 OHX−→Zn(OH)X4X2−.\ce{Zn(OH)2(s) + 2H3O+ -> Zn^2+ + 4H2O}, \qquad \ce{Zn(OH)2(s) + 2OH- -> Zn(OH)4^2-} .
Solubility of zinc hydroxide (left) and of aluminium hydroxide, gibbsite (right), against pH, two-species model (solid lines): slopes -2 and +2 for zinc, -3 and +1 for aluminium. For zinc the dashed curve includes all the hydroxo species, the uncharged Zn(OH)2(aq) setting a floor.
Solubility of zinc hydroxide (left) and of aluminium hydroxide, gibbsite (right), against pH, two-species model (solid lines): slopes −2-2 and +2+2 for zinc, −3-3 and +1+1 for aluminium. For zinc the dashed curve includes all the hydroxo species, the uncharged Zn(OH)X2(aq)\ce{Zn(OH)2(aq)} setting a floor.
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