From Wikipedia, the free encyclopedia.
In chemistry, an alkali is the solution of a base in water. Since a base is defined as a substance which yields or creates hydroxide ions when dissolved in water, an alkali is a solution of hydroxide anions and their corresponding positively charged cations.
1 Common properties of alkalis
2 Confusion between base and alkali
3 Alkali salts
4 Alkali soil
Common properties of alkalis
Common properties of alkalis include:
- Alkalis are aqueous solutions.
- Alkalis are bitter to taste (compared with acid solutions which are described as sour).
- Caustic (causing chemical burns).
- Slippery or soapy to the touch (due to the caustic reaction dissolving the surface of the skin and fingerprint).
- Alkalis have a pH greater than 7 and hence can be detected with litmus paper (litmus will turn blue on contact with an alkali).
Confusion between base and alkali
The terms base and alkali are often used interchangeably but many of the substances referred to as alkalis are more correctly bases. To clarify: an alkali is a type of base (one which has been dissolved in water) but the undissolved form is not an alkali. So while people often refer to common bases - like calcium carbonate or ammonia - as alkalis, strictly speaking, only when they are dissolved in water - in these cases becoming carbonate and ammonium ions respectively - can they be called alkalis. Due to the prevalent use of alkali in this manner though, it is difficult to fault it outside of carefully worded scientific prose.
Demonstrating the extent of the intrenched misuse of the term alkali, most basic salts are commonly referred to as alkali salts.
Common alkali salts include:
- sodium hydroxide (often called "caustic soda")
- potassium hydroxide (commonly called "potash")
- lye (normally a mix of the previous two)
- calcium carbonate (sometimes called "free lime")
Soil with a pH above 7.4 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 cabbage family vegetables and buffalograss), most plant prefer a mildly acidic soil (pH between 6.0 and 6.8), and high pH levels can cause a problem.
In alkali lakes (a type of salt lake), evaporation concentrates the naturally occuring alkali salts, often forming a crust of mildly basic salt across a large area.
- alkali metal
- alkaline earth metal