The ability of a chemical to behave as both an acid and a base is called amphoterism, and this type of substance is known as an amphoteric substance. Generally, such a substance acts as an acid in the presence of a base, and it acts as a base in the presence of an acid.
The simplest example of an amphoteric substance is water. In the presence of an acid, it behaves as a base (proton acceptor); in the presence of a base, it behaves as an acid (proton donor). In particular, when hydrochloric acid (HCl) is dissolved in water, water acts as a base. The chemical reaction can be written as:
In its reaction with ammonia (NH3, a weak base), water acts as an acid, as follows:
Zinc oxide (ZnO) is another amphoteric substance. Its behavior as an acid or base depends on the pH of the solution. In an acidic solution, it reacts as a base; in a basic solution, it reacts as an acid. The chemical reactions can be written as:
A third example of an amphoteric substance is aluminum hydroxide (Al(OH)3). It can react with hydrochloric acid or sodium hydroxide (NaOH), represented as follows:
Likewise, beryllium hydroxide (Be(OH)2) is amphoteric:
Many amphoteric substances are also described as amphiprotic—that is, they can donate or accept a proton, thus acting as acids or bases. Water, amino acids, hydrogen carbonate (bicarbonate) ions, and hydrogen sulfate ions are examples of amphiprotic species. Because they can donate a proton, all amphiprotic substances contain a hydrogen atom.
It should be noted that all amphoteric substances are not amphiprotic. For instance, if a substance can accept or donate a pair of electrons, (thus acting as a Lewis acid or base), it would be amphoteric but not amphiprotic.
A common example of an amphiprotic substance is the hydrogen carbonate ion. It can accept a proton, thus acting as a base; or it can donate a proton, thus acting as an acid. Its reactions with water can be written as follows:
All links retrieved October 2, 2012.
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