In chemistry, an alkali is a water-based solution that contains dissolved base and has a pH greater than 7. Such solutions contain hydroxide ions (OH−) or other species that make the solution alkaline.
Etymology
The word originates from the Arabic term qali, which referred to plant ashes used for cleaning. Historically, those ash solutions produced alkaline liquids useful for washing and soap production.
Chemical meaning
Technically an alkali is any base that is soluble in water. In practice, strong alkalis are often metal hydroxides. When a base dissolves, it increases the concentration of OH− in the solution and raises the pH.
Common examples
- Sodium hydroxide (NaOH) and potassium hydroxide (KOH) — hydroxides of the alkali metals.
- Calcium hydroxide (Ca(OH)2) — a less soluble hydroxide from the alkaline earth metals that still produces an alkaline solution.
- Aqueous ammonia (NH3 in water) — a molecular base that forms an alkaline solution.
Relationship to salts
Neutralization of an alkali by an acid produces water and a salt. Many soluble salts, when dissolved, can also give alkaline solutions if the corresponding acid is weak and the salt hydrolyzes to produce OH−.
Uses and safety
Alkalis are widely used in industries and households: they are important in soap and detergent manufacture (saponification), paper and textile processing, water treatment, and chemical synthesis. Concentrated alkalis are corrosive and can cause chemical burns; appropriate protective measures are required when handling them.