What term describes a substance that can act as both an acid and a base?

The term that describes a substance capable of acting as both an acid and a base is "amphoteric." This characteristic is key in chemistry because it allows amphoteric substances to participate in a variety of chemical reactions by donating protons (acting as an acid) or accepting protons (acting as a base), depending on the pH of the environment they are in.

For instance, water is a classic example of an amphoteric substance since it can react with strong acids, like hydrochloric acid, to form hydronium ions and can also react with strong bases, such as sodium hydroxide, to form hydroxide ions. This dual behavior is important in many biochemical processes and helps maintain pH balance in various systems, including biological ones.

In contrast, the other terms refer to specific characteristics: a "neutral" substance has a pH of 7 and does not exhibit acidic or basic properties; "alkaline" substances are predominantly basic (having a pH greater than 7); while "acidic" substances have a pH less than 7 and primarily exhibit acidic properties. Therefore, these terms do not encompass the dual action described by the term "amphoteric."