A compound is said to be amphoteric if it can behave both as an acid and as a base. For example, with strong acids aluminum hydroxide acts as a base, Al(OH)₃, forming aluminum salts. With strong bases it behaves as an acid, H₃AlO₃, forming salts containing the ion AlO₃^3-. The formation of amphoteric hydroxides is a characteristic of metalloids.

 

Compounds are also classified as amphoteric if they contain both basic and acidic functional groups. Amino acids, for example, contain both an amino group (–NH₂) and a carboxyl group (–COOH) in their structure. The amino group can act as a base and accept a proton, while the carboxyl group can act as an acid and donate a proton. This allows amino acids to form peptide bonds, which are essential for the formation of proteins.

 

Amphoteric substances can be used as buffers to maintain a constant pH in a solution. A buffer is a solution that resists changes in pH when an acid or base is added to it. This is because the amphoteric substance in the buffer can act as either an acid or a base to neutralize any changes in pH. One example of a buffer system is the bicarbonate buffer system in the human body. This system helps to maintain a constant pH in the blood. Carbon dioxide in the blood reacts with water to form carbonic acid, which can then dissociate into bicarbonate ions and hydrogen ions. The bicarbonate ions can act as a base and accept hydrogen ions, while the carbonic acid can act as an acid and donate hydrogen ions. This helps to keep the pH of the blood within a narrow range.