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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 chemical 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 7.0. The adjective alkaline is commonly, and alkalescent less often, used in English as a synonym for basic, 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 they are still among the most common bases.
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 (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.
Alkalis are all Arrhenius bases, ones which form hydroxide ions (OH-) when dissolved in water. Common properties of alkaline aqueous solutions include:
There are various definitions for the concept of an alkali. Alkalis are sometimes defined as a subset of the bases. However, two subsets are commonly chosen.
The second subset of bases is also called an "Arrhenius base".
Soils with pH values higher than 7.3 are usually defined as being alkaline. These soils 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[disambiguation needed]), most plants prefer a mildly acidic soil (with pHs between 6.0 and 6.8), and alkaline soils can cause problems.
Examples of alkali lakes: