Alkali: Wikis

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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. Alkalis are best known for being bases that dissolve in water. Bases are compounds with 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. Since Brønsted-Lowry acid-base theory, the term alkali in chemistry is normally restricted to those salts containing alkali and alkaline earth metal elements.

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Etymology

The word "alkali" is derived from Arabin 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

Confusion between alkali and base

The terms "base" and "alkali" are often used interchangeably, particularly outside of a scientific context, but they do not have the same meaning. While all alkaline solutions are basic, not all bases are alkaline. The following are common mistakes:

  • The phrase "measuring the alkalinity of soil" is incorrect since the property measured is actually the pH (base property).
  • Calling bases that are not alkalis, such as ammonia, alkaline (ammonia is a base but not an alkali).

Also, not all salts formed by alkali metals are alkaline; this designation applies only to those salts that are basic. And while most electropositive metal oxides are basic, only the soluble alkali metal and alkaline earth metal oxides can be correctly called alkalis.

This definition of an alkali as a basic salt of an alkali metal or alkaline earth metal is the most common, based on dictionary definitions [1][2], however conflicting definitions of the term alkali do exist. These include:

  • Any base that is water-soluble and forms hydroxide ions [3][4]. In chemistry, this is more accurately called an Arrhenius base.
  • The solution of a base in water [5]. This would be an Arrhenius base in solution.
  • Name given by chemists to an important group of substances, the behaviour of bases the main alkali are 1,the hydroxides potassium 2,

Salts

Most basic salts are alkali salts, of which common examples are:

  • sodium hydroxide (often called "caustic soda")
  • potassium hydroxide (commonly called "caustic potash")
  • lye (generic term, for either of the previous two, or even for a mixture)
  • calcium carbonate (sometimes called "free lime")
  • magnesium hydroxide is an example of an atypical alkali since it has low solubility in water (although the dissolved portion is considered a strong base due to complete dissociation of its ions).

Alkaline soil

Soil with a pH value higher than 7.3 is normally referred to as alkaline. This soil property can occur naturally, due to the presence of alkali salts. Although some plants do prefer slightly basic soil (including vegetables like cabbage and fodder like buffalograss), most plants prefer a mildly acidic soil (pH between 6.0 and 6.8), and alkaline soils can cause problems.

Lakes

In alkali lakes (a type of salt lake), evaporation concentrates the naturally-occurring alkali salts, often forming a crust of mildly-basic salt across a large area.

Examples of alkali lakes:

References

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1911 encyclopedia

Up to date as of January 14, 2010
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Simple English

In chemistry, an alkali is an aqueous (from water) solution with a pH of more than seven. The word 'Alkali' comes from the Arabic 'qali' meaning 'from the ashes' since ashes mixed with water were used as cleaning products, and feel soapy.

An alkali is where a base is dissolved in water. Often it is the salt of an Alkali metal
An alkali is the opposite to an acid and can be neutralised (brought down to pH 7) by adding acid.

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Characteristics

  • It tastes soapy
  • It is corrosive (it can burn your skin away)
  • It is above pH The higher the number is over 7 on the pH scale the stronger the alkali is. Some Alkalies are:

Potassium, Soap, toothpaste, Household cleaners, baking soda, sodium hydroxide and more.

Strength of an Alkali

Someone can find out how strong or weak an alkali is by adding universal indicator to it. Some universal indicators can be poured into alkalis and some are soaked into paper, and the paper is touched to the alkali.

For example, soap and toothpaste are minor-strength alkalis. Lye is a strong alkali.

File:Sodium
Sodium Hydroixide

Uses of an Alkali

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