conjugate acid–base pair

Think of a glove turned inside out: it is still the same glove, just missing one thing or wearing one extra. In acid–base chemistry, every acid has a partner that is exactly itself minus one proton, and every base has a partner that is itself plus one proton. Those two are a conjugate acid–base pair — the before-and-after of a single proton handover.

Precisely, when an acid HA gives up its proton, what is left, A-, is its conjugate base; when a base B picks up a proton to become BH+, that BH+ is its conjugate acid. The two differ by just one H+. They always come paired in any Brønsted–Lowry reaction, because the proton an acid releases has to be caught by a base, creating two new conjugate species on the product side.

This pairing matters because it predicts strength: the stronger an acid, the weaker its conjugate base, and vice versa. A strong acid like HCl leaves behind a feeble conjugate base (Cl-) that has no urge to grab the proton back. The idea also powers buffers, where an acid and its conjugate base coexist on purpose to steady the pH.