Amphoterism
From Wikipedia, the free encyclopedia
In chemistry, an amphoteric substance is one that can react as either an acid or base.[1] The word is derived from the Greek prefix ampho- (αμφί-) meaning "both". Many metals (such as zinc, tin, lead, aluminium, and beryllium) and most metalloids have amphoteric oxides. Other examples include amino acids and proteins, which have amine and carboxylic acid groups, and self-ionizable compounds such as water and ammonia.
[edit] Examples
Zinc oxide (ZnO) reacts differently depending on the pH of the solution:
In acids: ZnO + 2H+ → Zn2+ + H2O
In bases: ZnO + H2O + 2OH- → [Zn(OH)4]2-
This effect can be used to separate different cations, such as zinc from manganese.
There are many other examples of chemical compounds which are also amphoteric, for the simplest example water:
Base (proton acceptor): H2O + HCl → H3O+ + Cl−
Acid (proton donor): H2O + NH3 → NH4+ + OH−
- (It can do both at once: 2H2O → H3O+ + OH−)
Aluminium hydroxide is as well:
Base (neutralizing an acid): Al(OH)3 + 3HCl → AlCl3 + 3H2O
Acid (neutralizing a base): Al(OH)3 + NaOH → Na[Al(OH)4]
Some other examples include:
Some elements not mentioned that are able to form amphoteric oxides: Si, Ti, V, Fe, Co, Ge, Zr, Ag, Sn, Au[2]
