Many theories and concepts exist regarding the classification of acids and bases. In earlier times, substances that tasted sour were considered as acids (for example-citric acid, acetic acid, oxalic acid), and substances that tasted bitter were considered as bases. However, this proved to be wrong later on when scientists were unable to explain the concept of acids and bases in a solvent other than water or aqueous solution.
However, since then many theories have come up explaining the acidic behaviour of acids and the basic properties of hydroxides in terms of their ability to yield hydrogen and hydroxide ions in solutions, such as the Arrhenius theory, Lewis Acid-Base theory, Lowry-Bronsted Acid-Base theory, Pearson’s concept and many other theories.
The Arrhenius theory was proposed by Swedish chemist Svante Arrhenius in 1884. Svante August Arrhenius was popularly known for his work in electrolytic dissociation and also for the famous Arrhenius equation. He was also awarded the Nobel prize in 1903. This theory was used to classify acids and bases on the basis of the type of ions they dissociate on dissolving in water. If a compound releases H+ after dissociation it is categorised as acid and if a compound releases OH– after dissociation it is categorised as base.
According to Arrhenius theory, an acid is a substance that dissociates to give hydrogen ions when dissolved in water. For example: When hydrogen chloride gas dissolves in water, it gives hydrogen ions,
According to Arrhenius, base is a substance that dissolves to give hydroxide ions when dissolved in water.
For example: When sodium hydroxide dissolves in water it releases hydroxide ion
The release of hydroxide ion increases the concentration in the solution and makes the solution basic.
The various advantages of Arrhenius theory are that it provides knowledge about:
When an equal amount of acid and base reacts it forms a salt, which is neither acid nor base.
Hydrogen does not exist freely as hydrogen ions (H+) in water. It exists as an aqueous cation known as the hydronium ion (H3O+). It is a type of oxonium ion produced by the protonation of water. Its conjugate base is water.
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Arrhenius theory is only applicable when a substance is dissolved in water. It considers only those substances as acids, which release hydrogen ions in aqueous solutions and only those substances as bases, which releases hydroxide ions in aqueous solutions. The theory fails to explain the exception of neutralisation. The Arrhenius concept was the very first inference that was drawn, and thus it had several flaws to speak of as it was thought of so long ago when other notions with regard to chemistry had not yet come into play. Since not much was known, Arrhenius based his ideas on water, rather, on protons alone, disregarding electrons, unlike Lewis in later years.
As the world around us evolves, so do natural phenomena, so do the reasons behind them, or in other words, science. Ideas, thought processes, and inferences alter and shift with the ages, and we find new theories because the previous ones failed to explain certain aspects of the phenomena. The first theories often cause a stir among the populace, in fact, Arrhenius was a Nobel laureate, despite the Bronsted Lowry and Lewis theory being more feasible as the former theory was considered too specific in 1923.
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