A base is a substance that accepts protons (\(\text{H}^+\)) or releases hydroxide ions (\(\text{OH}^-\)) when dissolved in an aqueous solution. This action makes a solution “basic” or alkaline. Bases vary significantly in strength, which refers to the extent they perform this action in water. A strong base generates the maximum possible concentration of hydroxide ions in a solution. Identifying a strong base requires understanding its fundamental chemistry in water, recognizing common examples, and confirming its properties through laboratory measurement.
Defining Strength Through Complete Dissociation
The fundamental characteristic defining a strong base is its complete, or near-complete, dissociation in an aqueous solution. When a strong base, such as a metal hydroxide, is added to water, it breaks apart 100% into its constituent ions, producing the maximum concentration of hydroxide ions (\(\text{OH}^-\)) possible for that initial concentration. This process is a one-way reaction, meaning the ions do not recombine to form the original molecule, unlike the reaction of a weak base.
For a strong base, this reaction is not an equilibrium; virtually all the base molecules are converted to ions. This complete dissociation means the concentration of hydroxide ions in the final solution is directly equal to the concentration of the dissolved strong base, or a multiple if the base releases more than one \(\text{OH}^-\) per molecule.
Chemists quantify base strength using the Base Dissociation Constant (\(K_b\)). This constant measures the extent of dissociation. Strong bases have a very large \(K_b\) value, often so large that it is not typically measured because the reaction goes to completion. In contrast, a weak base only partially dissociates, establishing an equilibrium and resulting in a much smaller, measurable \(K_b\) value.
Recognizing Common Strong Bases
The most practical method for identifying a strong base is recognizing the specific compounds that chemically guarantee complete dissociation. Most common strong bases are ionic compounds containing a metal cation and one or more hydroxide (\(\text{OH}^-\)) anions. These are known as Arrhenius bases.
The strongest and most common examples are the hydroxides of the Group 1 alkali metals. These compounds are highly soluble in water and dissociate fully upon dissolution, releasing a single hydroxide ion per formula unit.
The Group 1 strong bases include:
- Lithium hydroxide (\(\text{LiOH}\))
- Sodium hydroxide (\(\text{NaOH}\))
- Potassium hydroxide (\(\text{KOH}\))
- Rubidium hydroxide (\(\text{RbOH}\))
- Cesium hydroxide (\(\text{CsOH}\))
Another set of strong bases comes from the heavier Group 2 alkaline earth metals: calcium hydroxide (\(\text{Ca}(\text{OH})_2\)), strontium hydroxide (\(\text{Sr}(\text{OH})_2\)), and barium hydroxide (\(\text{Ba}(\text{OH})_2\)). Although these Group 2 hydroxides are not as soluble as the Group 1 hydroxides, the portion that dissolves dissociates completely. Base strength generally increases moving down a group on the periodic table because the metal-hydroxide bond weakens, making it easier to release \(\text{OH}^-\).
Some non-hydroxide compounds also act as strong bases because they react with water to produce hydroxide ions. For instance, highly reactive ionic metal oxides, such as sodium oxide (\(\text{Na}_2\text{O}\)), react completely with water to form a metal hydroxide. However, the common list of Group 1 and heavier Group 2 hydroxides serves as the reliable identifier for a strong base.
Laboratory Methods for Confirmation
In a laboratory setting, a base’s strength can be confirmed through direct and indirect measurements of the resulting hydroxide ion concentration. These methods include measuring the \(\text{pH}\), electrical conductivity, and analyzing titration curves.
pH Measurement
The simplest method involves using a \(\text{pH}\) meter. Strong bases produce a solution with a very high \(\text{pH}\), typically in the range of 12 to 14, depending on the concentration. This high \(\text{pH}\) reading provides direct evidence of the high concentration of \(\text{OH}^-\) ions resulting from complete dissociation.
Electrical Conductivity
Another measurable property is the solution’s electrical conductivity. Because a strong base dissociates completely into mobile ions, a solution of a strong base is an excellent conductor of electricity. A conductivity meter will show a high reading, significantly greater than the conductivity of a solution containing an equivalent concentration of a weak base, which only partially ionizes.
Titration Curve Analysis
A more advanced confirmation method involves performing a titration and analyzing the resulting titration curve. When a strong base is titrated with a strong acid, the plot of \(\text{pH}\) versus the volume of added acid shows a distinct, steep, and symmetrical \(\text{pH}\) jump around the equivalence point. This sharp change is characteristic of the complete reaction between a strong base and a strong acid, distinguishing it from the gradual \(\text{pH}\) change observed when titrating a weak base.