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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.

Etymology[link]

The word "alkali" is derived from Arabic al qalīy (or alkali),^[1] 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, 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 (cf German Kalium), which ultimately derives from alkali.

Common properties[link]

Alkalis are all Arrhenius bases, which form hydroxide ions (OH^-) when dissolved in water. Common properties of alkaline aqueous solutions include:

• Moderately concentrated solutions (over 10⁻³ M) have a pH of 7.1 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.
• Acids and alkalis are measured on a pH scale
• Turns Universal Indicator Blue-Dark Purple.

Difference between alkali and base[link]

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.

• A basic salt of an alkali metal or alkaline earth metal^[2] (This includes Mg(OH)₂ but excludes NH₃.)
• Any base that is water-soluble and forms hydroxide ions^[3][4][5] or the solution of a base in water.^[6] (This excludes Mg(OH)₂ but includes NH₃.)

The second subset of base is also called an Arrhenius base.

Salts[link]

Most basic salts are alkali salts, of which common examples are:

• sodium hydroxide (often called "caustic soda")
• potassium hydroxide (commonly called "caustic potash")
• lye (generic term, for either of the previous two, or even for a mixture)
• calcium carbonate (sometimes called "free lime" or "limewater")
• magnesium hydroxide is an example of 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[link]

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.

Alkali lakes[link]

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:

• Alkali Lake, British Columbia, Canada Referenced in feature movies X-Men II and III
• Alkali Lake (Oregon), Oregon, United States
• Baldwin Lake (San Bernardino County, California)
• Mono lake, California, United States
• Redberry Lake, Saskatchewan, Canada
• Summer Lake, Oregon, United States
• Tramping Lake, Saskatchewan, Canada
• Lake Magadi, Kenya
• Lake Turkana, Kenya (the largest alkali lake in the world)

See also[link]

• Base (chemistry)
• Alkali metal

References[link]