What type of substance can exhibit both acidic and basic properties?

The correct answer, which is amphoteric, refers to substances that can act as either an acid or a base depending on the circumstances. This dual behavior arises from the ability of amphoteric substances to either donate protons (H⁺ ions) when acting as acids or accept protons when functioning as bases. A common example of an amphoteric substance is water (H₂O), which can donate a proton to become hydroxide (OH⁻) or accept a proton to form hydronium (H₃O⁺).

In contrast, neutral substances do not exhibit acidic or basic properties; they have a balanced concentration of hydrogen and hydroxide ions. Alkaline substances are specifically basic and do not possess acidic properties, while acidic substances exclusively donate protons and do not accept them. Thus, amphoteric substances are unique in their ability to showcase both behaviors, making them essential in various chemical reactions and equilibria.