This document discusses acids and bases according to several theories. It defines Arrhenius acids and bases as substances that increase the concentration of H+ or OH- ions in aqueous solutions. Brønsted-Lowry acids are proton donors and bases are proton acceptors. Lewis acids accept electron pairs to form bonds while bases donate electron pairs. Strong acids and bases ionize completely in water. Conjugate acid-base pairs are formed in acid-base reactions. Amphoteric substances can act as both acids and bases.