In the given definitions, what role does a base play?

• A. Proton acceptor
• B. Proton donor
• C. Electron donor
• D. Catalyst

Answer: (A)

Base in the Brønsted–Lowry sense is defined by its ability to accept a proton (H+) during a reaction. That makes the base the proton acceptor. In acid–base reactions, the acid donates the proton and the base takes it, forming a conjugate acid and conjugate base. For example, hydroxide accepts a proton to become water, and ammonia accepts a proton to become ammonium. This is why proton acceptance is the key role. The idea of a proton donor describes what an acid does, not what the base does; a catalyst speeds up reactions but isn’t defined by taking up a proton; and while a Lewis base relates to electron donation, the Brønsted–Lowry base is specifically about proton transfer.