The word alkaline has an unique etymology, for it is derived from the Arabic word, al qaliy, which refers to the calcined ashes that were combined with animal fat to make soap. Today, alkaline is often defined as being the opposite of acidic, also called basic. However, scientifically speaking, alkaline has a much narrower definition, for it refers to substances derived from two columns or groups on the Periodic Chart and the various salts and compounds that can be formed from these elements. This article will be mainly concerned with the scientific definition of alkaline.
The Periodic Chart
The Periodic Chart is a chart of all of the elements that occur in nature (in recent years this chart has grown to also include a few man-made elements such as plutonium). At first glimpse the layout of the chart may appear random, but in fact the layout is far from random, for each vertical column contains a series of related elements. On the far right of the Periodic Chart one will find the elements lithium, sodium, potassium, rubidium, caesium and francium. These are the alkali elements. The next line over consists of the elements beryllium, magnesium, calcium, strontium, barium and radium, which form the group of elements that are known as alkaline earth metals.
The alkali group contains two very common elements, sodium and potassium. These elements are almost never found in their pure state, but they are common in salts and various minerals that naturally occur in the soil. Thus soil that has a high content of calcium or potassium is called alkaline soil. One way to test for alkaline soil is to measure the PH content of the soil. Soil that reads higher than 7.3 ( 7 is neutral on the PH scale) is considered alkaline because a high PH reading in soil is almost always due to the presence of a compound that contains an alkali or alkali metallic element. However, not every compound that has a PH reading higher than 7 contains an alkaline element.
Alkali Earth Metals
Next to the alkali metal elements on the Periodic Chart are a row of elements referred to as the alkali earth elements.Calcium and magnesium are the two most common elements in this group, but the group also includes beryllium, strontium, barium and radium. One trait that this shares with alkaline metals is that both groups are highly reactive and so they are almost always found in nature as a pure element. This high reactivity is caused by their molecular structure.
Salts are the product of a reaction between an acid and a base. Alkali elements react readily with halogens to form several types of salt, including table salt, which contains the elements sodium and chlorine. However, when this reaction occurs these elements are never present in their pure form, but rather they occur naturally joined with other elements in chemical combinations called compounds. Salts are compounds that can be found in nature.
Around the globe there are sometimes found very salty lakes, which are also referred to as alkali lakes. One of these lakes is formed when the evaporation rate becomes very high and the naturally occurring alkaline salts become highly concentrated. As a result these lakes often have a crusty layer of salt that borders the lake.