The pH scale measures how acidic or basic a water-based solution is. Substances with a pH greater than 7 are considered basic, or alkaline, and they increase the concentration of hydroxide ions (\(\text{OH}^-\)) when dissolved in water. The behavior of these substances in solution determines their chemical category. Bases are categorized based on the extent to which they generate these hydroxide ions in an aqueous environment.
What Defines a Strong Base
The classification of a base as “strong” is based on a rigorous chemical definition centered on its behavior in water. A strong base is any compound that undergoes complete ionization or dissociation when dissolved in an aqueous solution. This means that virtually every molecule of the base breaks apart into its constituent ions, releasing all its available hydroxide ions into the water.
This complete separation of ions is represented by a single-arrow chemical equation, indicating the reaction proceeds fully in one direction. The strength is determined only by the degree of this dissociation, not by the total concentration of the solution itself. Group 1 metal hydroxides, such as sodium hydroxide (\(\text{NaOH}\)) and potassium hydroxide (\(\text{KOH}\)), are classic examples of compounds that exhibit this complete dissociation, leading to a high concentration of \(\text{OH}^-\) ions.
Classification of Barium Hydroxide
Barium hydroxide, with the chemical formula \(\text{Ba}(\text{OH})_2\), is classified as a strong base because it adheres to the principle of complete dissociation. When solid barium hydroxide is introduced to water, it fully breaks down into its component ions.
The dissociation reaction is a clear illustration of this complete ionization: \(\text{Ba}(\text{OH})_2 \rightarrow \text{Ba}^{2+} + 2\text{OH}^-\). The use of a single arrow in the equation signifies that the reaction goes to completion, with no undissociated \(\text{Ba}(\text{OH})_2\) molecules remaining in the solution. This process is highly effective because barium is a Group 2 alkaline earth metal.
A notable factor contributing to its strength is the stoichiometry of the dissociation. Each molecule of \(\text{Ba}(\text{OH})_2\) yields two hydroxide ions (\(\text{OH}^-\)) for every one barium ion (\(\text{Ba}^{2+}\)) released. This dual release of hydroxide ions means that a given amount of barium hydroxide contributes twice the amount of \(\text{OH}^-\) to the solution compared to a monoprotic base like \(\text{NaOH}\).
Comparing Barium Hydroxide to Other Bases
The strength of barium hydroxide is best understood when contrasted with other types of bases, particularly weak bases like ammonia (\(\text{NH}_3\)). Ammonia is a weak base because it only partially ionizes in water, establishing an equilibrium symbolized by a double-arrow chemical equation. Only a small fraction of ammonia molecules react with water to produce \(\text{OH}^-\), resulting in a much lower hydroxide ion concentration than that produced by \(\text{Ba}(\text{OH})_2\).
It is important to separate the concept of a base’s strength from its solubility. Barium hydroxide is less soluble in water compared to Group 1 hydroxides like \(\text{NaOH}\), but this difference does not affect its classification. The portion of \(\text{Ba}(\text{OH})_2\) that dissolves in water dissociates completely into its ions. This complete ionization of the dissolved material is the sole criterion for its strong designation.