conjugate acid-base pair
/ KON-juh-git AS-id BAYS PAIR /
Think of a coat with a single button. Buttoned, it is one thing; unbuttoned, it is the same coat in a different state — the only difference is that one button. A conjugate acid-base pair is two chemical species that differ by exactly one hydrogen ion, like a molecule with and without its single proton.
In the Bronsted-Lowry picture, an acid is anything that donates a hydrogen ion and a base is anything that accepts one. When an acid gives up its hydrogen ion, what remains is its conjugate base; when a base picks up a hydrogen ion, the result is its conjugate acid. So acetic acid and the acetate ion form one pair, ammonium and ammonia form another — each member is the other minus or plus one proton.
Pairs matter because every acid-base reaction is really a hand-off of a hydrogen ion from one pair to another, and because buffers are simply a pair kept together on purpose. The honest caveat is a tidy seesaw rule: the stronger an acid is at giving up its proton, the weaker its conjugate base is at taking one back, so a strong acid has a feeble conjugate base, and vice versa.
When ammonia dissolves in water it grabs a hydrogen ion to become ammonium; ammonia is the base and ammonium is its conjugate acid, the two differing by exactly one proton.
Add one proton to a base and you get its conjugate acid.
Water is its own kind of chameleon here: it acts as the conjugate base of the hydronium ion and the conjugate acid of the hydroxide ion, which is why it appears on both sides of so many acid-base equations.