Final answer:
In a Brønsted-Lowry acid-base reaction, the original base becomes a conjugate acid in the reverse reaction because it has accepted a proton (H+).
Step-by-step explanation:
In a Brønsted-Lowry acid-base reaction, the original base becomes a conjugate acid in the reverse reaction. This transformation happens because the base has accepted a proton (H+), thereby gaining a hydrogen ion. For example, when ammonia (NH3) acts as a base and accepts a proton, it becomes an ammonium ion (NH4+), which is the conjugate acid of ammonia.
Conversely, when an acid donates a proton, it forms its conjugate base. The species remains after losing the proton from the acid and can potentially accept a proton in the reverse reaction. This concept illustrates the dynamic nature of acid-base reactions, showing how substances can transform between acting as acids and bases.