Acid (chemistry)

Article

August 19, 2022

An acid is a term from chemistry, the definition of which has been tightened several times. Acids are the counterparts of the bases. Acids and bases react with each other in a process called neutralization. An aqueous solution is called acidic when its acidity (pH) is less than 7. The human taste can recognize many acidic substances. Citric acid and acetic acid are well-known examples of substances that give a sour taste. In high concentrations, acids can be harmful, especially if the pH is below 1. Base metals such as iron, zinc and magnesium dissolve in acids such as concentrated hydrochloric acid, releasing hydrogen gas. In concentrated hydrochloric acid, nitric acid or aqua regia also nobler metals dissolve, but this is only possible because these substances are not only strong acids but also strong oxidizers and the dissolution has therefore not so much to do with their acidic character.

Historical development

The concept of acid has undergone significant development in the history of chemistry. Four stages can be distinguished. The 'primal definition' (of Lavoisier, among others) was based on the idea that oxygen was needed to form an acid and saw an acid as an acidic oxide dissolved in water. The definition of an acid as a substance that splits off hydrogen ions in water comes from Arrhenius. Later, Arrhenius' definition was refined and made more general to a proton donor - also in conditions other than aqueous solutions - by Brønsted and Lowry. Gilbert Lewis then formulated an even broader definition of an acid, which also includes substances that do not contain hydrogen: a Lewis acid is an acceptor of an electron pair.

The oxygen theory

As early as the 17th century, people were aware that there was a contradiction between acids and bases. Sour fruits, acidification of wine to vinegar and a growing number of inorganic acids were known. Lye, a fairly strong base, was obtained by leaching (removing the soluble parts with water) of wood ash. Litmus, an indicator that changed color depending on acidity, was also known early on. Boyle wrote about this in 1680. Its color could be changed at will by adding more acid or more lye. The background to this was less clear. With the discovery of oxygen and its role in the combustion and formation of oxides, the theory of acids and bases began to take on more solid forms. Initially, it was thought that the element oxygen was needed to form an acid or a base. (Hence the name of the element). Oxides of elements were classified as base-forming and acid-forming oxides. Lavoisier was an important figure in this development. He formulated a theory in 1787 that every acid was a combination of a basic principle and oxygen. In part, this classification was certainly successful. When oxides are dissolved in water, one group (eg sulfur dioxide SO2) does indeed form an acidic solution, the other group (eg calcium oxide CaO) forms a basic solution. Although Lavoisier lived nearly a century before the discovery of the periodic table, the acidic and basic character of oxides reflects this system very well. The oxides on the left, such as sodium or calcium, are very basic, while those on the right, such as sulfur or chlorine, are very acid-forming. It later turned out that there was another factor at play. If an element has several oxides such as SO3 and SO2, the highest oxide is always the most acidic and the lowest is the most basic. In addition to acidic and basic oxides, amphoteric oxides were also known, such as aluminum oxide, which acts with strong acids as base and with strong bases as acid. However, it soon became apparent that there were also acidic, but oxygen-free substances, a good example of which is hydrochloric acid. This strong acid was known as spirit of salt and Davy tone