Here's why:

* Hydroxide ions (OH-) are the defining characteristic of bases. They are responsible for the characteristic properties of bases, like their ability to neutralize acids and turn red litmus paper blue.

* When hydroxide ions dissolve in water, they increase the concentration of OH- ions in the solution. This makes the solution more basic.

* The pH of a basic solution is greater than 7.

Example: Sodium hydroxide (NaOH) is a strong base. When it dissolves in water, it dissociates into sodium ions (Na+) and hydroxide ions (OH-):

NaOH (s) → Na+ (aq) + OH- (aq)

The increased concentration of OH- ions makes the solution basic.