What term describes a substance that can act as either an acid or a base?

The term that describes a substance capable of acting as either an acid or a base is "amphoteric." An amphoteric substance can donate protons (H⁺) in conditions where it is acting as an acid and can accept protons when it behaves as a base. This dual ability is crucial in various chemical reactions and in maintaining pH balance in biological systems.

Examples of amphoteric substances include water, bicarbonate, and certain amino acids. These substances can react differently depending on the environment they are in, showcasing their unique ability to function in both roles. Understanding this concept is fundamental in analytical chemistry, as it plays a role in titrations and buffer solutions among other applications.