Alkali

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In chemistry, an alkali (from Arabic: Al-Qaly القلي, القالي ) is a basic, ionic salt of an alkali metal or alkaline earth metal element. Some authors also define an alkali as a base that dissolves in water. A solution of a soluble base has a pH greater than 7. The adjective alkaline is commonly used in English as a synonym for base, especially for soluble bases. 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 are still among the more common bases.

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Etymology

The word "alkali" is derived from Arabic al qalīy = the calcined ashes, referring to the original source of alkaline substance. Ashes were used in conjunction with animal fat to produce soap, a process known as saponification.

Common properties

Alkalis are all Arrhenius bases, which form hydroxide ions (OH-) when dissolved in water. Common properties of alkaline aqueous solutions include:

  • Moderately concentrated solutions (over 10−3 M) have a pH of 7.1 or greater. This means that they will turn phenolphthalein from colorless to pink.
  • Concentrated solutions are caustic (causing chemical burns).
  • Alkaline solutions are slippery or soapy to the touch, due to the saponification of the fatty acids on the surface of the skin.
  • Alkalis are normally water soluble, although some like barium carbonate are only soluble when reacting with an acidic aqueous solution.
  • Acids and alkalis are measured on a pH scale
  • Turns Universal Indicator Blue-Dark Purple.

Difference between alkali and base

The terms "base" and "alkali" are often used interchangeably, particularly outside of a scientific context.

There are various definitions for alkali. Alkali is often defined as a subset of base. However, two subsets are commonly chosen.

  • A basic salt of an alkali metal or alkaline earth metal [1][2] (This includes Mg(OH)2 but excludes NH3.)
  • Any base that is water-soluble and forms hydroxide ions [3][4][5] or the solution of a base in water [6]. (This excludes Mg(OH)2 but includes NH3.)

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