When bases dissolve in water they release what type of negative ion?

A base generally releases a hydroxide ion (OH-) when dissolved in water.

There are exceptions, such as ammonia NH3, which acts as a base but does not produce OH - ions. There are three definitions of acids and bases (Arrhenius, Bronsted-Lowry, and Lewis) and each one looks at acid/base characteristics differently. OH - donation is the Arrhenius definition.