Alkali, an inorganic chemical compound that functions as a base (the opposite of an acid). Although alkali is sometimes used as a synonym for base, it is generally regarded as a more limited term, with several meanings. In chemistry, the term is applied to the hydroxides and carbonates of the alkali metals (lithium, sodium, potassium, rubidium, and cesium) and of ammonia. It is also applied to the hydroxides and oxides of the alkaline-earth metals (beryllium, magnesium, calcium, strontium, and barium).

In industry, the term alkali usually refers to soda ash (sodium carbonate), caustic soda (sodium hydroxide), or baking soda (sodium bicarbonate), but it may also refer to lime, potassium hydroxide, or other bases. In soil chemistry, soil that is basic is called alkaline soil. Large areas of the western United States and other parts of the world are barren because they are too alkaline.

Alkalies have chemical properties that are typical of all bases. Nearly all alkalies are solids at ordinary temperatures, and are soluble in water. In concentrated form, they can cause chemical burns.

Alkalies react with fats and oils to produce soaps. The qualities of the soap depend to a large extent on the particular alkali used. For example, potassium hydroxide is used to make highly soluble liquid soaps and shampoos, while sodium hydroxide is used to make less soluble bar soaps. Pioneer women made soap by cooking beef tallow with lye, an alkaline solution that they obtained by soaking wood ashes in water.

Industrial Alkalies

Soda ash, caustic soda, and baking soda have a wide range of applications. Soda ash is used in the manufacture of glass, chemicals, soap, and cleaning solutions. Caustic soda is used in the production of chemicals, paper, aluminum, petroleum products, textiles, and soap. Baking soda is used in fire extinguishers, baking powder, remedies for stomach upset, and cleaning solutions.

Most of the soda ash commercially produced in the United States is obtained from trona, a flaky, chalk-white mineral that is almost pure sodium carbonate.

Soda ash is also produced synthetically, primarily by a chemical process called the Solvay process. In this process, a saturated solution of common salt is treated with ammonia and with carbon dioxide to form crystals of sodium bicarbonate. Heating the crystals yields sodium carbonate, or soda ash. The process is named for its inventor, Ernest Solvay, a Belgian chemist.

Caustic soda is commercially produced from brine by electrolysis; that is, decomposing the brine by passing an electric current through it. The primary product of the electrolysis is chlorine, while caustic soda is formed as a by-product.

Baking soda is produced commercially by treating a saturated solution of sodium carbonate with carbon dioxide, which precipitates the less soluble sodium bicarbonate. It is also produced by purifying the bicarbonate crystals obtained through the Solvay process.

The chemical formula for sodium carbonate is Na2CO3; sodium hydroxide, NaOH; sodium bicarbonate, NaHCO3.