In chemistry, an alkali (; from the Arabic word , القالِي) is a basic salt of an alkali metal 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 language 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 (or
alkali),
meaning (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 (
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
, 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 al
kali.
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
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 burn).
-
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 solubility, 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 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.
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 (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 (Ca(OH)2) – saturated solution known as "limewater"
-
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 and fodder like
buffalo grass), most plants prefer mildly acidic soil (with pHs between 6.0 and 6.8), and
can cause problems.
Alkali lakes
In
Soda lake (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 in Kenya
-
Mono Lake, near Owens Valley in California
-
Redberry Lake, Saskatchewan
-
Summer Lake, Lake County, Oregon
-
Tramping Lake, Saskatchewan
See also
-
Alkali manufacture
-
Alkali metals
-
Alkaline earth metals
-
Alkaline magma series
-
Base (chemistry)
