Final answer:
A Brønsted-Lowry base is defined as a proton acceptor in acid-base chemistry. This concept is part of the Brønsted-Lowry theory, and it is illustrated by ammonia's reaction with water where ammonia accepts a proton.
Step-by-step explanation:
A Brønsted-Lowry base is correctly defined as a proton acceptor. This definition is central to the Brønsted-Lowry theory of acids and bases, which was introduced in 1923. This theory was a significant step forward in the understanding of acid-base chemistry, as it expanded the definitions beyond those of the earlier Arrhenius theory. According to the Brønsted-Lowry theory, bases do not have to contain hydroxide ions to be considered as such; they simply need to accept a proton (H+) from another species. To illustrate this, consider the reaction of ammonia in water where ammonia acts as a Brønsted-Lowry base:
NH3(aq) + H2O(l) → NH4+(aq) + OH−(aq)
In this reaction, ammonia (NH3) accepts a proton from water, forming ammonium (NH4+) and a hydroxide ion (OH−). Therefore, the correct answer to the student's question is that a Brønsted-Lowry base accepts a proton.