In chemistry, an alkali ( /ˈælkəlaɪ/; 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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The word "alkali" is derived from Arabic al qalīy = 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 (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, 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, which ultimately derives from alkali.
Alkalis are all Arrhenius bases, which form hydroxide ions (OH-) when dissolved in water. Common properties of alkaline aqueous solutions include:
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.
The second subset of base is also called an Arrhenius base.
Most basic salts are alkali salts, of which common examples are:
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.
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: