Inchemistry, analkali (/ˈælkəlaɪ/ⓘ; from theArabic wordal-qāly,القالِي) is abasicsalt of analkali metal or analkaline earth metal. An alkali can also be defined as a base that dissolves inwater. A solution of a soluble base has apH greater than 7.0. Theadjectivealkaline, and less often,alkalescent, is commonly used inEnglish as asynonym 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 theArrhenius definition of a base, and they are still among the most common bases.
Etymology
The wordalkali is derived from Arabical qalīy (oralkali),[1] meaning'the calcined ashes' (seecalcination), referring to the original source of alkaline substances. A water-extract of burned plant ashes, calledpotash and composed mostly ofpotassium carbonate, was mildly basic. After heating this substance withcalcium hydroxide (slaked lime), a far more strongly basic substance known ascaustic potash (potassium hydroxide) was produced. Caustic potash was traditionally used in conjunction with animal fats to produce softsoaps, one of the caustic processes that rendered soaps from fats in the process ofsaponification, one known since antiquity. Plant potash lent the name to the elementpotassium, which was first derived from caustic potash, and also gave potassium its chemical symbolK (from the German nameKalium), which ultimately derived from alkali.
Alkaline solutions are slippery or soapy to the touch, due to thesaponification of the fatty substances on the surface of the skin.
Alkalis are normallywater-soluble, although some likebarium 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 ofchemistry andchemical 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[2] (this includes Mg(OH)2 (magnesium hydroxide) but excludes NH3 (ammonia)).
Any base that is soluble in water and forms hydroxide ions[3][4] or the solution of a base in water.[5] (This includes both Mg(OH)2 and NH3, which formsNH4OH.)
The second subset of bases is also called an "Arrhenius base".
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)
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 likecabbage and fodder likebuffalo grass), most plants prefer mildly acidic soil (with pHs between 6.0 and 6.8), andalkaline soils can cause problems.[1]
Alkali lakes
Inalkali lakes (also calledsoda lakes), evaporation concentrates the naturally occurring carbonate salts, giving rise to an alkalic and often saline lake.
