What is a characteristic feature of amphoteric substances?

Amphoteric substances are characterized by their ability to both donate and accept protons, which means they can act as either acids or bases depending on the circumstances. This dual behavior allows them to react with both acids and bases, making them versatile within chemical reactions. For instance, a common example of an amphoteric substance is water, which can donate a proton to form hydroxide ions (acting as an acid) or accept a proton to form hydronium ions (acting as a base).

The other options do not accurately describe the behavior of amphoteric substances. For instance, substances that only donate protons are classified strictly as acids, rather than amphoteric. Not reacting with acids or only reacting with bases also does not capture the essence of amphotericity, as these conditions would limit the substance's ability to act in both roles. Thus, the defining feature of amphoteric substances lies in their capability to engage in reactions as both acids and bases by transferring protons in either direction.