The pH scale measures the acidity or alkalinity of a solution. A diluted aqueous solution with a pH value of less than 7 is described as acidic, with a pH value of 7 as neutral and with a pH value of more than 7 as basic or alkaline.

Bases are chemical compounds that can absorb protons from an acid to form a water molecule. A base is therefore the counterpart to an acid and is able to neutralize it.

In chemistry, bases (from the ancient Greek βάσις basis) are narrowly defined as compounds that are able to form hydroxide ions (OH-) in aqueous solution and thus increase the pH value of a solution. Hydroxide ions are chemical compounds that can absorb protons from an acid to form a water molecule. A base is therefore the counterpart to an acid and is able to neutralize it.

In addition, there are further definitions of the base concept of various acid-base concepts for much broader areas of chemical reactions that can go beyond those of hydroxide ions in water. The concepts according to Lewis (Lewis base and Lewis acid) and that according to Pearson, who speaks of hard and soft acids and bases (HSAB concept), are particularly significant.

The term "alkali" was first used for all bases in the 18th century. It is assumed that this was due to the growing popularity of Otto Tachenius' medical theory in the 17th century.

A number of alkalis were known in alchemy, such as lime (CaCO3, CaO and Ca(OH)2), soda, soda ash, potash and ammonia. Until the beginning of the 18th century, however, no precise distinction was made between soda and potash.

For centuries, acids and alkalis have been seen as opposites, but this is not the case.

The original definition of an acid was any substance that dissolves in water and produces hydrogen ions. The term "alkali" wasn't coined until the 18th century, but early chemists considered alkalis to be substances that could neutralize acids - which is actually a pretty accurate description!

Due to their exothermic reactions, alkalis were also associated with fire or "fire material".

In 1923, Johannes Nicolaus Brønsted presented his model. It has become widely accepted and has proven to be very useful, especially in analytical chemistry. According to his theory, bases and acids interact in a proton transfer reaction. In this process, bases absorb protons from acids. The model presented by Gilbert Newton Lewis, also in 1923, is helpful when considering reaction processes in organic chemistry and complex chemistry and goes beyond the usual definitions. For this reason, Lewis bases and Lewis acids are preferred. Many compounds that are normally referred to as acids are not acids according to this model. The concept of hard and soft acids and bases (HSAB concept) was developed in 1963 by Ralph G. Pearson and expands the view of reactions in organic and complex chemistry.

According to this concept, HSAB interactions are characterized by a charge transfer between donor (H) and acceptor (A) atoms of the reactants. For example, when an electron-rich donor atom interacts with an electron-poor acceptor atom, a polar covalent bond forms between them, which can be either directional or non-directional, depending on whether there is a single shared electron pair or more than one shared electron pair between the donor and acceptor atoms.

Properties of bases

• Many bases are soluble in water (e.g. sodium hydroxide, ammonia), but not all (e.g. aluminum hydroxide)
• They are corrosive and have a destructive effect on organic substances.
• They form soaps and glycerine from oils and fats.
• There are strong and weak bases.
• Bases can be diluted with water, but their effect is significantly weaker depending on the dilution.
• The basic solutions lead to a reddening of phenolphthalein and color red litmus paper blue.
• The "counterparts" of the bases (base solution = alkali) are the acids (see illustration). They can neutralize bases. Acids are also corrosive and attack many other substances that do not necessarily react with bases.
• Clothing, skin and eyes are at risk on contact. Make sure to wear protective goggles, as chemical burns can always occur.