- Arrhenius concept: acids produce H+ in solution, bases produce OH- ion (but what about NH3?)
- Bronsted-Lowry: An acid is a proton (H+) donor. A base is a proton acceptor
- Lewis Acids: electron-deficient molecules (+)
- Lewis Base: Electron-rich molecule (-)
- Conjugate base: what an acid becomes after it donates a proton
- Conjugate acid: what a base becomes after it receives a proton