According to the Brønsted-Lowry concept, an acid is a proton ($\text{H}^+$) donor and a base is a proton acceptor. Species that can both donate and accept a proton are called amphoteric or amphiprotic.

• $\text{HCO}_3^-$ can act as an acid by donating $\text{H}^+$ to form $\text{CO}_3^{2-}$ and as a base by accepting $\text{H}^+$ to form $\text{H}_2\text{CO}_3$.
• $\text{NH}_3$ can act as an acid by donating $\text{H}^+$ to form $\text{NH}_2^-$ and as a base by accepting $\text{H}^+$ to form $\text{NH}_4^+$.
• $\text{HSO}_4^-$ can act as an acid by donating $\text{H}^+$ to form $\text{SO}_4^{2-}$ and as a base by accepting $\text{H}^+$ to form $\text{H}_2\text{SO}_4$.
• However, $\text{HCl}$ can only donate a proton to form its conjugate base $\text{Cl}^-$. It does not have the tendency to accept an additional proton under normal conditions, therefore it acts exclusively as a Brønsted acid.