Hydration may be either acid or base catalyzed. With base catalysis, the hydroxyl group carries out a faster nucleophilic addition step than normal water. System pH is maintained as the base regenerates when the tetrahedral intermediate abstracts a proton from water in the final step of hydration. Acid catalysis, on the other hand, begins with protonation of the carbonyl oxygen, activating the carbonyl group for nucleophilic attack by water.