Arrhenius definition: An acid is a substance that dissociates in water to
produce H+
ions (protons), and a base is a substance that dissociates in
water to produce OH–
ions (hydroxide); an acid-base reaction involves
the reaction of a proton with the hydroxide ion to form water
2. Brønsted–Lowry definition: An acid is any substance that can donate a
proton, and a base is any substance that can accept a proton; acid-base
reactions involve two conjugate acid-base pairs and the transfer of a
proton from one substance (the acid) to another (the base)
3. Lewis definition: A Lewis acid is an electron-pair acceptor, and a Lewis
base is an electron-pair donor