In
chemistry
Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science that covers the Chemical element, elements that make up matter to the chemical compound, compounds made of atoms, molecules and ions ...
, an alkali (; from ar, القلوي, al-qaly, lit=ashes of the
saltwort) is a
basic
BASIC (Beginners' All-purpose Symbolic Instruction Code) is a family of general-purpose, high-level programming languages designed for ease of use. The original version was created by John G. Kemeny and Thomas E. Kurtz at Dartmouth College ...
,
ionic salt of an
alkali metal
The alkali metals consist of the chemical elements lithium (Li), sodium (Na), potassium (K),The symbols Na and K for sodium and potassium are derived from their Latin names, ''natrium'' and ''kalium''; these are still the origins of the names ...
or an
alkaline earth metal. An alkali can also be defined as a base that dissolves in
water. A solution of a soluble base has a
pH greater than 7.0. The
adjective alkaline, and less often, alkalescent, is commonly used in
English as a
synonym
A synonym is a word, morpheme, or phrase that means exactly or nearly the same as another word, morpheme, or phrase in a given language. For example, in the English language, the words ''begin'', ''start'', ''commence'', and ''initiate'' are all ...
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'' (or ''alkali''),
meaning ''the calcined ashes'' (see
calcination), 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
Calcium hydroxide (traditionally called slaked lime) is an inorganic compound with the chemical formula Ca( OH)2. It is a colorless crystal or white powder and is produced when quicklime (calcium oxide) is mixed or slaked with water. It has m ...
(''slaked lime''), a far more strongly basic substance known as ''caustic potash'' (
potassium hydroxide) 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 symbol K (from the German name ''Kalium''), which ultimately derived from alkali.
Common properties of alkalis and bases
Alkalis are all
Arrhenius bases, ones which form
hydroxide ions (OH
−) when dissolved in water. Common properties of alkaline
aqueous solution
An aqueous solution is a solution in which the solvent is water. It is mostly shown in chemical equations by appending (aq) to the relevant chemical formula. For example, a solution of table salt, or sodium chloride (NaCl), in water would be re ...
s include:
* Moderately concentrated solutions (over 10
−3 M) have a
pH of 10 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 substances 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.
Difference between alkali and base
The terms "base" and "alkali" are often used interchangeably, particularly outside the context of
chemistry
Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science that covers the Chemical element, elements that make up matter to the chemical compound, compounds made of atoms, molecules and ions ...
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.
* A basic salt of an alkali metal or alkaline earth metal (this includes Mg(OH)
2 (
magnesium hydroxide) but excludes NH
3 (
ammonia)).
* Any base that is soluble in water and forms hydroxide ions or the solution of a base in water. (This includes both Mg(OH)
2 and NH
3, which forms
NH4OH.)
The second subset of bases is also called an "
Arrhenius base".
Alkali salts
Alkali salts are soluble hydroxides of
alkali metals and
alkaline earth metals, of which common examples are:
*
Sodium hydroxide
Sodium hydroxide, also known as lye and caustic soda, is an inorganic compound with the formula NaOH. It is a white solid ionic compound consisting of sodium cations and hydroxide anions .
Sodium hydroxide is a highly caustic base and alkali ...
(NaOH) – often called "caustic soda"
*
Potassium hydroxide (KOH) – commonly called "caustic potash"
*
Lye – generic term for either of two previous salts or their mixture
*
Calcium hydroxide
Calcium hydroxide (traditionally called slaked lime) is an inorganic compound with the chemical formula Ca( OH)2. It is a colorless crystal or white powder and is produced when quicklime (calcium oxide) is mixed or slaked with water. It has m ...
(Ca(OH)
2) – saturated solution known as "
limewater
Limewater is the common name for a saturated aqueous solution of calcium hydroxide. Calcium hydroxide, Ca(OH)2, is sparsely soluble at room temperature in water (1.5 g/L at 25 °C). "Pure" (i.e. less than or fully saturated) limewater i ...
"
*
Magnesium hydroxide (Mg(OH)
2) – 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
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 (including vegetables like
cabbage
Cabbage, comprising several cultivars of ''Brassica oleracea'', is a leafy green, red (purple), or white (pale green) biennial plant grown as an annual vegetable crop for its dense-leaved heads. It is descended from the wild cabbage ( ''B.&nb ...
and fodder like
buffalo grass), most plants prefer a mildly acidic soil (with pHs between 6.0 and 6.8), and
alkaline soils can cause problems.
[
]
Alkali lakes
In alkali lakes (also called ''soda lakes''), evaporation concentrates the naturally occurring carbonate salts, giving rise to an alkalic and often saline lake.
Examples of alkali lakes:
* Alkali Lake, Lake County, Oregon
* Baldwin Lake, San Bernardino County, California
* Bear Lake[Davis, Jim and Milligan, Mark (2011)]
Why is Bear Lake so blue?
Public Information Series 96. Utah Geological Survey, Department of Natural Resources on the Utah–Idaho border
* Lake Magadi in Kenya
*Lake Turkana
Lake Turkana (), formerly known as Lake Rudolf, is a lake in the Kenyan Rift Valley, in northern Kenya, with its far northern end crossing into Ethiopia. It is the world's largest permanent desert lake and the world's largest alkaline lake. B ...
in Kenya
* Mono Lake, near Owens Valley in California
* Redberry Lake, Saskatchewan
* Summer Lake, Lake County, Oregon
*Tramping Lake, Saskatchewan
Tramping Lake ( 2016 population: ) is a village in the Canadian province of Saskatchewan within the Rural Municipality of Tramping Lake No. 380 and Census Division No. 13. The village gets its name from nearby Tramping Lake, which is a lake ...
See also
* Alkali metals
* Alkaline earth metals
* Base (chemistry)
References
{{Authority control
Inorganic chemistry