Alkali
Alkalis are water-soluble bases with pH above 7.0.
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In chemistry, an alkali is a hydroxide of an alkali metal (Group I s-block elements) or an alkaline earth metal (Group II s-block elements). It can also be defined more broadly as any base that dissolves in water, producing a solution with a pH greater than 7.0. The term "alkaline" (and less commonly "alkalescent") is used synonymously with "basic," especially for water-soluble bases, because alkalis were the first bases known to fit the Arrhenius definition of a base—forming hydroxide ions (OH⁻) when dissolved—and they remain among the most common bases. The word "alkali" comes from Arabic, referring to the ashes of burnt Salicornia, a saltwort that was the original source of alkaline substances.
A water extract of burned plant ashes, called potash and composed mainly of potassium carbonate, was mildly basic. When heated with calcium hydroxide (slaked lime), it produced caustic potash (potassium hydroxide), a much stronger base used since antiquity with animal fats to make soft soap through saponification. Plant potash gave its name to the element potassium, whose chemical symbol K derives from the German name for alkali.
Alkaline aqueous solutions share common properties: moderately concentrated solutions (over 10⁻³ M) have a pH of 10 or higher and turn phenolphthalein pink; concentrated solutions are caustic, causing chemical burns; and they feel slippery or soapy due to saponification of skin fats. Alkalis are normally water-soluble, though some, like barium carbonate, dissolve only when reacting with an acidic solution. While "base" and "alkali" are often used interchangeably, alkalis are typically defined as a subset of bases. Common definitions include basic salts of alkali or alkaline earth metals (e.g., magnesium hydroxide, but not ammonia) or any water-soluble base that forms hydroxide ions (including ammonia, which forms ammonium hydroxide).
Notable alkali salts include sodium hydroxide (caustic soda), potassium hydroxide (caustic potash), lye (a mixture of these), calcium hydroxide (limewater), and magnesium hydroxide, which is atypical due to its low solubility. Alkaline soils have pH values above 7.3, often from natural alkali salts, and can challenge plants that prefer mildly acidic conditions. Alkali lakes, or soda lakes, form when evaporation concentrates carbonate salts, creating alkaline, saline environments.
Quick Facts
- Field
- Chemistry
- Etymology
- From Arabic al-qalīy, meaning 'the calcined ashes' of burnt Salicornia
Facts from the source article.
Lore & Background
The word alkali derives from Arabic al-qalīy, referring to calcined ashes of Salicornia, a saltwort. A water extract of burned plant ashes, called potash and composed mostly of potassium carbonate, was mildly basic. Heating this with calcium hydroxide (slaked lime) produced caustic potash (potassium hydroxide), a far stronger base used since antiquity with animal fats to make soft soap via saponification. Plant potash gave its name to the element potassium, whose symbol K comes from German Kalium, ultimately from alkali.
In chemistry, an alkali is a hydroxide of an alkali metal (Group I s-block) or an alkaline earth metal (Group II s-block). It 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 adjectives alkaline and, less often, alkalescent are used as synonyms for basic, especially for water-soluble bases.
Alkalis are Arrhenius bases, forming hydroxide ions (OH⁻) when dissolved. Moderately concentrated solutions (over 10⁻³ M) have a pH of 10 or greater and turn phenolphthalein pink. Concentrated solutions are caustic, causing chemical burns, and feel slippery or soapy due to saponification of skin fats.
Alkalis are normally water-soluble, though some like barium carbonate require an acidic solution. Common alkali salts include sodium hydroxide (caustic soda), potassium hydroxide (caustic potash), lye (a mixture of the two), calcium hydroxide (limewater), and magnesium hydroxide, which has low solubility but is a strong base when dissolved. Alkaline soils have pH above 7.3, often from natural alkali salts. Alkali lakes, or soda lakes, form when evaporation concentrates carbonate salts, creating alkaline, often saline conditions; examples include Mono Lake in California and Lake Magadi in Kenya.
Reader's Guide
Alkalis are a subset of bases, commonly defined as either a basic salt of an alkali metal or alkaline earth metal, or any base soluble in water that forms hydroxide ions. Their solutions have pH of 10 or greater at moderate concentrations, turn phenolphthalein pink, are caustic, and feel slippery due to saponification of skin fats. Common alkali salts include sodium hydroxide (caustic soda), potassium hydroxide (caustic potash), lye, calcium hydroxide (limewater), and magnesium hydroxide (an atypical alkali due to low solubility).
Alkaline soils have pH above 7.3 and can occur naturally from alkali salts. Alkali lakes, also called soda lakes, form when evaporation concentrates carbonate salts, creating alkaline and often saline conditions. Examples include Mono Lake in California and Lake Magadi in Kenya.
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Sources
Compiled from Wikipedia and the sources listed below. Text from Wikipedia is available under CC BY-SA 4.0; this entry is adapted from it.
- Wikipedia: Alkali (CC BY-SA 4.0).
- Word definitions: the Codexery glossary, each quoted from its Wikipedia article.
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