In chemistry, an alkali (from Arabic: al-qaly 'the calcined ashes') is a specific type of base, formed as a carbonate, hydroxide or other ionic salt of an alkali metal or alkali earth metal element. The word alkali or the adjective alkaline are frequently used to refer to all bases, since most common bases are alkalis, although strictly speaking this is inaccurate. The original production source of alkaline substances, were ashes used in conjunction with animal fat to produce soap, a process known as saponification.
Common properties of alkalis
Alkalis are all Arrhenius bases and share many properties with other chemicals in this group (Arrhenius bases form hydroxide ions when dissolved in water). Common properties of alkaline solutions include:
- Alkalis all form 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 seven and hence can be detected with litmus paper (litmus will turn blue on contact with an alkali).
- Another common test for alkalis is the use of phenolphthalein since it turns from colourless to pink when the pH moves from eight to ten (making it suitable for detecting all but the most dilute solutions of alkalis).
Confusion between base and alkali
The terms 'base' and 'alkali' are often used interchangeably, since most common bases are alkalis. It is common to speak of 'measuring the alkalinity of soil' when what is actually meant is the measurement of the pH (base property). Similarly, bases which are not alkalis, such as ammonia, are sometimes erroneously referred to as alkaline.
Note that not all or even most salts formed by alkali metals are alkaline or even basic.
Most basic salts are alkali salts, of which common examples are:
- sodium hydroxide (often called "caustic soda")
- potassium hydroxide (commonly called "potash")
- lye (generic term, for either of the previous two, or even for a mixture)
- 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 plants 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 occurring alkali salts, often forming a crust of mildly basic salt across a large area.
Examples of alkali lakes:
- Redberry Lake, Saskatchewan, Canada.
- Tramping Lake, Saskatchewan, Canada.