01Brønsted–Lowry
An acid donates H⁺; a base accepts H⁺. After the transfer, each becomes its conjugate.
Water is amphoteric: it can act as either.
02Notebook box
Conjugate pairs differ by exactly one H⁺.
Worked example: NH₃ + H₂O ⇌ NH₄⁺ + OH⁻: base NH₃, acid H₂O, conjugate acid NH₄⁺, conjugate base OH⁻.