Alkali
Alkalis are water-soluble bases with pH above 7.0.
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.
- field
- Chemistry
- known_for
- Alkalis are Arrhenius bases that form hydroxide ions in water; they are caustic, slippery to the touch, and have pH greater than 7.0
- etymology
- From Arabic al-qalīy, meaning 'the calcined ashes' of burnt Salicornia
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.
Did You Know?
- Alkalis are all Arrhenius bases, forming hydroxide ions when dissolved in water.
- Concentrated alkaline solutions are caustic and cause chemical burns.
- The word alkali comes from Arabic al-qalīy, meaning 'the calcined ashes' of burnt Salicornia.
- Alkaline soils are defined as having pH values higher than 7.3.
Frequently Asked Questions
What is an alkali?
An alkali is a water-soluble base, most commonly the hydroxide of an alkali metal or alkaline earth metal. In aqueous solution it releases hydroxide ions, which is what qualifies it under the Arrhenius definition of a base.
How does an alkali differ from a generic base?
Every alkali is a base, but the reverse is not true because many bases are insoluble in water. The defining trait of an alkali is that it must dissolve to form a solution that generates hydroxide ions.
Why do alkalis feel slippery on the skin?
Hydroxide ions in the solution react with the natural oils on your skin, producing a soap-like residue that creates that characteristic slick, caustic feel. This is essentially the same chemistry behind saponification.
Where does the word 'alkali' come from?
It traces back to the Arabic term al-qalīy, which described the calcined ashes left after burning Salicornia plants. Those ash residues were among the earliest substances people recognized for their basic, water-soluble properties.
What pH range do alkalis occupy?
Any alkali solution will read above 7.0 on the standard pH scale. The stronger the alkali, the farther the value climbs toward 14.
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