Chemistry classifies substances based on their behavior in a solution, particularly how they interact with water. Bases, sometimes called alkaline substances, play a role in many chemical processes, from industrial manufacturing to biological functions. Understanding the specific particles bases release when they dissolve is fundamental to explaining their properties and how they influence the chemistry of a mixture.
The Defining Ion of Bases
When a base dissolves in water, it undergoes dissociation, which liberates a specific, negatively charged ion into the solution. Under the Arrhenius definition, a substance is classified as a base if it increases the concentration of the hydroxide ion, represented as OH-, when dissolved in water. This hydroxide ion is composed of one oxygen atom and one hydrogen atom, carrying an overall negative charge. The release of this ion is the defining chemical event that makes a substance behave as a base.
Many common bases, such as sodium hydroxide (NaOH), are ionic compounds that already contain the hydroxide group in their structure. When solid sodium hydroxide is introduced to water, the ionic bonds between the sodium cation (Na+) and the hydroxide anion (OH-) break apart. This dissociation releases the pre-formed hydroxide ions directly into the water, raising their concentration. Other bases, like ammonia (NH3), do not contain OH- but react with water molecules to produce hydroxide ions, still meeting the Arrhenius definition.
Bases Versus Alkalis
The terms “base” and “alkali” are often used interchangeably, but a technical distinction exists based on the physical property of solubility in water. A base is any substance that can react with an acid to neutralize it, typically by accepting a proton or increasing the hydroxide ion concentration. This broad category includes many substances that do not dissolve well in water, such as many metal oxides and hydroxides. An alkali is a specific type of base that is readily soluble in water. This means that all alkalis are bases, but not all bases are classified as alkalis. Common examples include the hydroxides of the Group 1 alkali metals, such as potassium hydroxide (KOH).
How the Hydroxide Ion Controls pH
The increase in hydroxide ion concentration directly determines the alkalinity of a solution, which is measured using the pH scale. The pH scale is a logarithmic measure that indicates the concentration of hydrogen ions (H+) in a solution. Pure water naturally maintains a balance between H+ and OH- ions, resulting in a neutral pH of 7. When a base is added to water, the released OH- ions react with the H+ ions in a process known as neutralization, forming stable water molecules (H2O). This reaction is summarized as H+ + OH- → H2O.
By consuming the hydrogen ions, the base effectively lowers their concentration in the solution. Since pH is inversely related to the H+ concentration, a decrease in H+ causes the pH value to rise above 7, resulting in a basic solution. The higher the concentration of released hydroxide ions, the further the pH rises toward the maximum value of 14.