Arrhenius acid-base theory

The Arrhenius acid-base theory is the first modern scientific definition of acids and bases. It was discovered by and named after Svante Arrhenius, and was part of why he won the Nobel Prize in Chemistry in 1903.

The Arrhenius theory has been mostly replaced by the Brønsted–Lowry and Lewis theories in modern chemistry, which use more general definitions of acid based on hydrogen ions and electrons. It is still used in education to introduce the ideas of acids and bases.

Definitions

An Arrhenius acid is a chemical that increases the amount of hydronium ions in a mixture. An Arrhenius base is a chemical that increases the amount of hydroxide ions.

An Arrhenius acid-base reaction starts with an Arrhenius acid and an Arrhenius base, and ends with water and a salt. Hydrochloric acid and sodium hydroxide are a classic example of an Arrhenius acid and base, and undergo the reaction

HCl + NaOH → NaCl + H₂O

to make table salt and water. These reactions are also known as neutralization reactions, because they take an acid and a base and make a neutral salt solution.

Weaknesses

The Arrhenius theory only describes chemicals that can dissolve in water, and only how they behave when in water. Acids and bases can act differently in different solvents like ammonia or oil. Some acids and bases do not dissolve in water at all, like fatty acids and calcium carbonate from antacids.

Ammonia does not contain a hydroxide ion or react with water to make hydroxide, but still acts like a base in experiments, such as changing the color of litmus paper.

The theory also cannot explain amphoteric chemicals like sodium bicarbonate that can act like either an acid or a base.