What ionizes in water to produce excess hydroxyl ions?

The correct answer is a base because when a base is dissolved in water, it dissociates and ionizes to produce hydroxyl ions (OH⁻). This increase in hydroxyl ions raises the pH of the solution, making it more alkaline. In the context of chemistry, bases are substances that, when in solution, can accept protons or donate electron pairs. Common examples of bases include sodium hydroxide (NaOH) and potassium hydroxide (KOH), both of which significantly increase the concentration of hydroxyl ions when they dissolve in water.

In contrast, an acid would release hydrogen ions (H⁺) in a solution, thereby decreasing the pH and increasing the acidity of the solution but would not contribute to an excess of hydroxyl ions. Chlorine, when dissolved in water, typically forms hydrochloric acid or hypochlorous acid, which would decrease the pH further. Similarly, carbon dioxide dissolving in water forms carbonic acid, also leading to the production of hydrogen ions, which would not increase hydroxyl ions. Thus, the distinctive characteristic of a base is its ability to produce excess hydroxyl ions in a solution, making it the correct choice.