What role does hydroxide ion (OH-) play in solutions?

The hydroxide ion (OH-) is a significant indicator of the presence of alkalis in a solution. When hydroxide ions are present, they contribute to the basic (alkaline) properties of the solution. In aqueous solutions, a higher concentration of hydroxide ions leads to a higher pH, reflecting the basic nature of the solution.

In contrast, other choices do not accurately represent the function of hydroxide ions. For instance, hydroxide ions do not indicate acidity; rather, they are associated with basicity. Therefore, the presence of hydroxide ions corresponds to an alkaline environment, which is crucial in distinguishing alkali solutions from acidic or neutral ones.