Which statement best describes amphiprotic substances?

• A. A substance that can act as acid or base in different reactions
• B. A substance that appears to act as Bronsted-Lowry acid in some reactions and a Bronsted-Lowry base in other reactions
• C. A substance that fully ionizes in water
• D. A substance that donates electrons

Answer: (B)

The main concept here is that amphiprotic substances can act as proton donors or proton acceptors depending on the reaction context. In Brønsted-Lowry terms, acids donate protons and bases accept protons, so an amphiprotic species can do both in different reactions. A classic example is the bicarbonate ion: it can donate a proton to become carbonate (acid behavior) or accept a proton to become carbonic acid (base behavior). This dual ability is what defines amphiprotic substances. The description that captures this dual Brønsted-Lowry behavior most clearly is that the substance acts as an acid in some reactions and as a base in others. The other options describe related but different ideas (full ionization, electron donation, or a more general acid/base role) and don’t pinpoint the proton-transfer flexibility as precisely.