* True: Bases do form hydroxide ions (OH-) when dissolved in water. This is the defining characteristic of Arrhenius bases.

* Not entirely true: There are other definitions of bases that extend beyond just hydroxide ion formation. For example, the Brønsted-Lowry definition describes bases as proton (H+) acceptors. This means a base can also form hydroxide ions by reacting with water, but it doesn't have to.

In summary:

* Arrhenius bases: Form hydroxide ions (OH-) in water.

* Brønsted-Lowry bases: Accept protons (H+), which can include reacting with water to form hydroxide ions.

It's important to understand both definitions for a complete picture of base chemistry.