A substance that releases hydroxyl ions in water is a base. Acids on the other hand release hydrogen ions when dissolved in water, while salts release sodium. To define acids and bases the Arrhenius definition is used by scientists---this describes what substances are produce by each type of solutions. Under the Arrhenius definition salt and water can be used to predict the result of the neutralization of an acid with a base.
In Chemistry, Bronsted-Lowry definition has superseded the Arrehenius definition.
