In chemistry, an alkali is a basic, ionic salt of an alkali metal or alkaline earth metalchemical element. An alkali also can be defined as a base that dissolves in water. A solution of a soluble base has a pHgreater than 7.0. The adjectivealkaline is commonly, and alkalescent less often, used in English as a synonym for basic, especially for bases soluble in water. This broad use of the term is likely to have come about because alkalis were the first bases known to obey the Arrhenius definition of a base, and they are still among the most common bases.
Etymology
The word "alkali" is derived from Arabic al qalīy, meaning the calcined ashes, referring to the original source of alkaline substances. A water-extract of burned plant ashes, called potash and composed mostly of potassium carbonate, was mildly basic. After heating this substance with calcium hydroxide, a far more strongly basic substance known as caustic potash was produced. Caustic potash was traditionally used in conjunction with animal fats to produce soft soaps, one of the caustic processes that rendered soaps from fats in the process of saponification, one known since antiquity. Plant potash lent the name to the element potassium, which was first derived from caustic potash, and also gave potassium its chemical symbolK, which ultimately derived from alkali.
Common properties of alkalis and bases
Alkalis are all Arrhenius bases, ones which form hydroxide ions when dissolved in water. Common properties of alkaline aqueous solutions include:
Moderately concentrated solutions have a pH of 7.1 or greater. This means that they will turn phenolphthalein from colorless to pink.
Concentrated solutions are caustic.
Alkaline solutions are slippery or soapy to the touch, due to the saponification of the fatty substances on the surface of the skin.
The terms "base" and "alkali" are often used interchangeably, particularly outside the context of chemistry and chemical engineering. There are various more specific definitions for the concept of an alkali. Alkalis are usually defined as a subset of the bases. One of two subsets is commonly chosen.
Magnesium hydroxide - an atypical alkali since it has low solubility in water
Alkaline soil
Soils with pH values that are higher than 7.3 are usually defined as being alkaline. These soils can occur naturally, due to the presence of alkali salts. Although many plants do prefer slightly basic soil, most plants prefer a mildly acidic soil, and alkaline soils can cause problems.
Alkali lakes
In alkali lakes, evaporation concentrates the naturally occurring carbonate salts, giving rise to an alkalic and often saline lake. Examples of alkali lakes:
