What is a substance that can behave as an acid or a base?
These substances are called amphiprotic. Water, amino acids, hydrogen carbonate ions, and hydrogen sulfate ions are common examples.
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Technically, a substance that can behave as an acid or a base is amphoteric.
A substance is amphoteric (from Greek amphoteros = "each of two") if it can act as an acid or a base.
For example, aluminum hydroxide is amphoteric because it can act as a base and neutralize strong acids.
It can also act as an acid and neutralize strong bases:
A substance is amphiprotic (Greek "both" + "proton") if it is able both to donate and accept a proton (i.e. to act as a Brønsted-Lowry acid or base).
One example of an amphiprotic compound is the hydrogen sulfate ion,
It can act as an acid by donating a proton:
It can also act as a base by accepting a proton:
NOTE: All amphiprotic substances are also amphoteric, but not all amphoteric substances are amphiprotic.
Nevertheless, many chemists ignore the distinction and use the term "amphoteric" for all cases.