Why are amino acids amphoteric?

• An amino acid behaves as an acid in an alkaline medium because it contains an Acidic functional group$\left(\mathrm{COOH}\right)$ in its molecule.
• An amino acid behaves as a Base in an acidic medium because it contains a basic functional group $\left({\mathrm{NH}}_2\right)$in its molecule.

Zwitter ion

• The term zwitterion is originally derived from the German word Zwitter which can be roughly translated as hybrid or hermaphrodite.
• A zwitterion is an ion that contains two functional groups.
• In simple terms, it is an ion possessing both positive and negative electrical charges.
• The zwitterions are mostly electrically neutral.

Hence they behave as acids in an alkaline medium and bases in an acidic medium. So, amino acids are amphoteric.