Thursday, September 25, 2008

Amphoteric substance

In chemistry, an amphoteric substance is one that can react as either an acid or base. 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.


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:

* Beryllium hydroxide
o with Acid: Be(OH)2 + 2HCl → BeCl2 + 2H2O
o with Base: Be(OH)2 + 2NaOH → Na2Be(OH)4

* Lead oxide
o with acid: PbO + 2HCl → PbCl2 + H2O
o with base: PbO + Ca(OH)2 +H2O → Ca2+[Pb(OH)4]2-

* Zinc oxide
o with acid: ZnO + 2HCl → ZnCl2 + H2O
o with base: ZnO + 2NaOH + H2O → Na22+[Zn(OH)4]2-

Some elements not mentioned that are able to form amphoteric oxides: Si, Ti, V, Fe, Co, Ge, Zr, Ag, Sn, Au


In chemistry and physical sciences, a substance is described as amphiprotic if it can both donate or accept a proton, thus acting either like an acid or a base (according to Brønsted-Lowry theory of acids and bases: acids are proton donors and bases are proton acceptors. In Lewis theory of acids and bases; acids are electron pair acceptors and bases are electron pair donors). Water, amino acids, hydrogen carbonate ions and hydrogen sulfate ions are common examples of amphiprotic species. Since they can donate a proton, all amphiprotic substances contain a hydrogen atom. Also, since they can act like an acid or a base, they are amphoteric.

An amphoteric substance, however, is not necessarily amphiprotic. For example it may be a base and a Lewis acid, but not a Brønsted acid.


A common example of an amphiprotic substance is the hydrogen carbonate ion, which can act as a base:

HCO3- + H2O → H2CO3 + OH-

Or as an acid:

HCO3- + H2O → CO32- + H3O+

Thus, it can effectively accept or donate a proton.

Water is the most common example:

Basic: H2O + HCl → H3O+ + Cl-

Acidic: H2O + NH3 → NH4+ + OH-

From Wikipedia, the free encyclopedia

A Lewis acid is a chemical compound, A, that can accept a pair of electrons from a Lewis base, B, that acts as an electron-pair donor, forming an adduct, AB.

A + :B → A—B

Gilbert N. Lewis proposed this definition, which is based on chemical bonding theory, in 1923[1] Brønsted-Lowry acid-base theory was published in the same year. The two theories are distinct but complementary to each other as a Lewis base is also a Brønsted-Lowry base, but a Lewis acid need not be a Brønsted-Lowry acid.