The simple and direct answer is yes, strong bases dissociate completely in water. By definition, a strong base is a substance that undergoes virtually complete separation into its constituent ions when dissolved in water. This total breakdown distinguishes a strong base from its weaker counterparts, profoundly affecting the resulting solution’s properties.
Understanding Chemical Bases and the Dissociation Process
A chemical base is defined by its ability to increase the concentration of hydroxide ions (\(\text{OH}^-\)) when dissolved in an aqueous solution. These hydroxide ions are responsible for the characteristic properties of a base. Dissociation, or ionization, is the physical process where an ionic compound breaks apart into its component ions when surrounded by water molecules.
When a solid ionic compound is placed in water, the polar water molecules pull the crystal lattice apart, separating the positively charged cation from the negatively charged anion. For a base, this process results in the release of hydroxide ions (\(\text{OH}^-\)) into the solution. The term “strong” indicates that this separation reaction occurs nearly \(100\%\) of the time.
The Definitive Answer: Complete Ionization
Strong bases are categorized as strong electrolytes because they completely ionize in water, resulting in a high concentration of mobile ions. This complete breakdown means that for every molecule of the base added to the water, one or more hydroxide ions are released. The strength refers to the degree to which the base breaks apart when dissolved, not how concentrated the solution is.
The mechanism involves the ionic bonds of the base being entirely overcome by the attractive forces of the surrounding water molecules, a process called solvation. Once separated, the resulting cation and the hydroxide anion are stabilized by surrounding water molecules, preventing them from recombining. Because the forward reaction (dissociation) is overwhelmingly favored, the reaction is essentially irreversible and goes to completion.
This complete ionization leads to a high concentration of free hydroxide ions, which is the direct cause of the solution’s high alkalinity, or high pH. Since the original base compound has fully converted into solvated ions, it ceases to exist in its molecular form within the solution. This complete conversion is represented chemically with a single arrow pointing from the reactants to the products, signifying a reaction that runs to completion.
Key Examples and Chemical Equations
The most common strong bases are the soluble metal hydroxides formed by the alkali metals (Group 1) and the heavier alkaline earth metals (Group 2). Group 1 examples include lithium hydroxide (\(\text{LiOH}\)), sodium hydroxide (\(\text{NaOH}\)), and potassium hydroxide (\(\text{KOH}\)). Group 2 examples are calcium hydroxide (\(\text{Ca}(\text{OH})_2\)), strontium hydroxide (\(\text{Sr}(\text{OH})_2\)), and barium hydroxide (\(\text{Ba}(\text{OH})_2\)).
The complete dissociation of sodium hydroxide is shown with a single-arrow reaction:
$\(\text{NaOH}(\text{s}) \rightarrow \text{Na}^+(\text{aq}) + \text{OH}^-(\text{aq})\)$
For bases like barium hydroxide, which contain two hydroxide units, the dissociation releases two hydroxide ions per molecule:
$\(\text{Ba}(\text{OH})_2(\text{s}) \rightarrow \text{Ba}^{2+}(\text{aq}) + 2\text{OH}^-(\text{aq})\)$
The strength of these compounds is due to the nature of the ionic bond and the high stability of the resulting ions when surrounded by water.
The Difference Between Strong and Weak Bases
The distinction between strong and weak bases is based entirely on the degree of ionization in water. Unlike strong bases that fully dissociate, weak bases only partially ionize. A common example of a weak base is ammonia (\(\text{NH}_3\)).
When ammonia is added to water, it reacts with water molecules to produce hydroxide ions and ammonium ions (\(\text{NH}_4^+\)). This reaction does not go to completion, meaning most of the original ammonia remains unreacted in the solution. This partial reaction establishes a chemical equilibrium, where the forward and reverse reactions occur at the same rate.
The equilibrium state is represented using a double arrow (\(\rightleftharpoons\)) in the chemical equation:
$\(\text{NH}_3(\text{aq}) + \text{H}_2\text{O}(\text{l}) \rightleftharpoons \text{NH}_4^+(\text{aq}) + \text{OH}^-(\text{aq})\)$
Because only a small fraction of the weak base molecules react, the concentration of hydroxide ions produced is much lower than that of a strong base of equal concentration. This limited production of hydroxide ions is why solutions of weak bases are less alkaline and are considered weak electrolytes.