Arrhenius Base
Definition and meaning of Arrhenius Base in chemistry.
An Arrhenius base is a substance that increases the concentration of hydroxide ions (OH<sup>-</sup>) when dissolved in water. This concept is a classic way to identify basic compounds, relying entirely on aqueous solutions to function.
In more detail
Introduced by Svante Arrhenius in 1884, the Arrhenius definition of a base is a cornerstone of classical acid-base chemistry. Under this theory, a base is any chemical compound that dissociates or ionizes in water to release hydroxide ions into the solution. This influx of hydroxide ions raises the overall pH of the mixture above 7, giving the solution its characteristic basic, or alkaline, properties.
The most common examples of Arrhenius bases are ionic compounds containing alkali metals or alkaline earth metals bonded to hydroxide groups. Sodium hydroxide and potassium hydroxide are classic examples. When these solid salts are placed in water, the polar water molecules surround and pull the ionic bonds apart.
The metal cations and the hydroxide anions separate and disperse evenly throughout the liquid, immediately and dramatically increasing the overall hydroxide concentration in the solvent. The hallmark of an Arrhenius acid-base reaction is neutralization. When an Arrhenius acid, which produces hydrogen ions, is mixed with an Arrhenius base, which produces hydroxide ions, the two ions rapidly combine.
This combination forms pure liquid water, while the remaining metal and nonmetal ions form an ionic salt. This predictable formation of water and salt is a key feature of Arrhenius chemistry. Like the Arrhenius acid definition, the Arrhenius base definition is fundamentally limited because it requires water as a solvent.
It also struggles to explain why compounds that do not contain a hydroxide group, such as ammonia, act as strong bases in water. To explain how ammonia makes a solution basic without directly releasing its own hydroxide ions, chemists rely on the more comprehensive Bronsted-Lowry theory.
Key facts
| Field | General Chemistry |
|---|---|
| Definition | Produces hydroxide ions in water |
| Solvent Requirement | Aqueous solution |
| Active Ion | Hydroxide (OH-) |
| Effect on pH | Raises pH above 7 |
| Neutralization Product | Water and a salt |
Sodium hydroxide acts as a strong Arrhenius base because it completely dissociates in water to release sodium ions and hydroxide ions.
Frequently asked questions
Why is ammonia not considered an Arrhenius base?
Ammonia (NH3) does not contain a hydroxide group to release into water. It makes water basic by stealing a proton from water, which is best explained by the Bronsted-Lowry theory.
What happens during a neutralization reaction?
The hydrogen ions from the acid and the hydroxide ions from the base combine to form liquid water, neutralizing the extreme pH levels.
Do all Arrhenius bases contain a metal?
Most common Arrhenius bases are metal hydroxides, but the strict definition only requires that the substance produces hydroxide ions when added to water.