Some substances can only give off hydrogen ions but not accept them. Other substances can only accept hydrogen ions but not give them off. However, there are substances that can do both, functioning as acids and bases, but only as one at a time. These substances are called amphoteric, and have the flexibility to give out or accept hydrogen ions.
One of the most common amphoteric substances out there is water. The pair of chemical equations that show that water is amphoteric is very important:
$$H_2O + H^{+} \rightarrow H_3O^{+}$$ $$H_2O \rightarrow H^{+} + OH^{-}$$
The first reaction shows that water is a base, and the second reaction shows that water is an acid. Thus by definition it is amphoteric. These two reactions occur spontaneously back and forth in water and in aqueous solutions.
Many acids can have one or more hydrogen ions removed from them to get an amphoteric substance. Thus the applications of these chemicals are more varied. Amphoteric substances can be identified by repeatedly removing hydrogen ions from an acid or by repeatedly adding hydrogen ions to a base.
Example 1: \(HNO_2\) is an acid. Remove one hydrogen ion from it:
$$HNO_2 \rightarrow H^{+} + NO_2^{-}$$
\(NO_2^{-}\) is not amphoteric because it is not an acid--it has no more hydrogen ions, let alone more hydrogen ions than can be removed.
Acids that can donate multiple hydrogen ions form a chain of chemical reactions with one or more amphoteric substances forming during the process. The next two examples illustrate what such chains look like.
Example 2: \(H_2O_2\) molecules can give up a maximum of two hydrogen ions. This is the reaction for getting rid of the first one:
$$H_2O_2 \rightarrow HO_2^{-} + H^{+}$$
Here is the second one:
$$HO_2^{-} \rightarrow O_2^{2-} + H^{+}$$
Take a closer look at \(HO_2^{-}\). This is an amphoteric substance, because it got rid of a hydrogen ion, and it can accept one. The first chemical equation shown can be reversed:
$$HO_2^{-} + H^{+} \rightarrow H_2O_2$$
Since the \(HO_2^{-}\) is accepting a hydrogen ion, it can function as an acid and as a base, and thus it is amphoteric.
Example 3: \(PO_4^{3-}\) is a base that can accept up to three hydrogen ions. Here are three chemical reactions that show the hydrogen ions being added one at a time:
$$PO_4^{3-} + H^{+} \rightarrow HPO_4^{2-}$$ $$HPO_4^{2-} + H^{+} \rightarrow H_2PO_4^{-}$$ $$H_2PO_4^{-} + H^{+} \rightarrow H_3PO_4$$
To identify the amphoteric substances in this set of reactions, write all the reactions in reverse order:
$$H_3PO_4 \rightarrow H^{+} + H_2PO_4^{-}$$ $$H_2PO_4^{-} \rightarrow H^{+} + HPO_4^{2-}$$ $$HPO_4^{2-} \rightarrow H^{+} + PO_4^{3-}$$
Within these six reactions, both \(HPO_4^{2-}\) and \(H_2PO_4^{-}\) are found to be both giving off and accepting hydrogen ions, so these are the two amphoteric substances.
Amphoteric substances can be identified by attempting to remove or add hydrogen ions to the molecules. They are the "transitional phases" between acids and bases, and are thus very common in acid-base chemistry experiments, including those pertaining to neutralization reactions.