Buffers — holding an equilibrium in mind while perturbing it

A buffer is a mixture of a weak acid and its conjugate base (or weak base and its conjugate acid) in comparable amounts, which resists pH change because added H⁺ is mopped up by the base component and added OH⁻ by the acid component. Conceptually hard because it requires holding an equilibrium in mind while perturbing it, and because the quantitative treatment (pH = pKa + log([base]/[acid])) is a rearranged equilibrium expression whose derivation students often skip and therefore misapply — especially when the buffer is made by partial neutralisation rather than by mixing two named substances.