What defines a conjugate acid/base pair?

A conjugate acid-base pair is defined by the difference in their composition due to the gain or loss of a proton (H⁺ ion). When an acid donates a proton, it transforms into its conjugate base, and when a base accepts a proton, it becomes its conjugate acid. This relationship highlights the fundamental concept of acids and bases as outlined in the Brønsted-Lowry theory, where acids are proton donors and bases are proton acceptors.

For example, when hydrochloric acid (HCl) donates a proton, it forms chloride ion (Cl⁻), making HCl and Cl⁻ a conjugate acid-base pair. Similarly, if ammonia (NH₃) gains a proton, it becomes ammonium ion (NH₄⁺), forming a pair with NH₃. The critical aspect here is the change in proton count, distinguishing a conjugate acid from its base and vice versa.

The other options do not accurately represent the definition of a conjugate acid-base pair, as they either refer to characteristics unrelated to proton transfer or to specific states or types of compounds that do not encompass the general principle of conjugate acids and bases.