Alkali

From Encyclopedia Jr, free information reference for Kids

For other uses of the word "alkali" see Alkali (disambiguation).

In chemistry, an alkali (from Arabic: al-qaly) is a basic, ionic salt of an alkali metal or alkali earth metal element. Alkalis are best known for being bases (compounds with pH greater than 7) that dissolve in water. The adjective alkaline is commonly used in English as a synonym for basic, especially for soluble bases. This broad use of the term is likely because alkalis were the first bases known to obey the Arrhenius definition of a base and are still among the more common bases. Since Brønsted-Lowry acid-base theory, the term alkali in chemistry is normally restricted to those salts containing alkali and alkali earth metal elements.

[edit] 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:

• All alkalis have a pH greater than seven and hence can be detected with litmus paper (litmus will turn blue on contact with an alkali).
• Most alkalis have a pH of 10 or greater. This means that they will turn phenolphthalein from colorless to pink.
• Caustic (causing chemical burns).
• Alkaline solutions are slippery or soapy to the touch (due to the caustic reaction dissolving the surface of the skin and fingerprint).
• Alkalis normally form aqueous solutions (although some like barium carbonate are only soluble when reacting with an acidic aqueous solution).

[edit] 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; this designation applies only to those salts which are basic.

While most electropositive metal oxides are basic, only the soluble alkali metal and alkali earth metal oxides can be correctly called alkalis.

This definition of an alkali as a basic salt of an alkali metal or alkali earth metal does appear to be the most common, based on dictionary definitions [1][2], however conflicting definitions of the term alkali do exist. These include:

• Any base that is water soluble [3][4]. This is more accurately called an Arrhenius base.
• The solution of a base in water [5].

[edit] Alkali salts

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")
• magnesium hydroxide is an example of an atypical alkali: it is a weak base (cannot be detected by phenolphthalein) and it has low solubility in water.

[edit] Alkaline soil

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.

[edit] Etymology

The word "alkali" is derived from Arabic al qalīy = "the calcined ashes", referring to the original source of alkaline substance. Ashes were used in conjunction with animal fat to produce soap, a process known as saponification.