Alkali
Alkalis are water-soluble bases with pH above 7.0.
An alkali is a hydroxide of an alkali metal or alkaline earth metal, or a base that dissolves in water. Alkalis were among the first bases known to obey the Arrhenius definition of a base and remain among the most common bases.
- 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 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.
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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