By definition, an amphiprotic substance is a substance that is capable of both donating and accepting a proton (hydrogen ion). This means that an amphiprotic substance possesses the capability to act as either an acid or a base depending on the other substance it is reacting with.

Amphiprotic substances are either weak acids or weak bases, and are therefore subject to react in either direction. The stronger acid or base that they react with will push them in a certain direction. This means that when an amphiprotic substance reacts with a stronger acid, it will act as a base, and when it reacts with a stronger base, it will act as an acid.

The most common example of an amphiprotic substance is water. Water can accept a proton and act as a base, but it can also donate a proton to a base and act as an acid.

However, water is not the only amphiprotic substance.

As we know, when polyprotic acids ionise in solution they become progressively weaker, even to the point where they will act like bases. Ions such as hydrogen sulfate, hydrogen carbonate, dihydrogen phosphate, and hydrogen phosphate are all considered to be amphiprotic substances because they can act as either an acid or base depending on what they are reacting with.