potassium

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A soft, silver-white, highly or explosively reactive metallic element that occurs in nature only in compounds. It is obtained by electrolysis of its common hydroxide and found in, or converted to, a wide variety of salts used especially in fertilizers and soaps. Atomic number 19; atomic weight 39.098; melting point 63.65°C; boiling point 774°C; specific gravity 0.862; valence 1. See Table at element.

Century Dictionary and Cyclopedia

1. n. Chemical symbol, K (for kalium); atomic weight, 39.1. The metallic base of the alkali potash, a substance not occurring uncombined in nature, but in various combinations widely diffused and of the highest importance. See potash. Potassium is silvery-white, and has a decided metallic luster. Its specific gravity is 0.875, and it is the lightest of all the metals with the exception of lithium. At the freezing-point of water it is brittle and has a crystalline fracture; at the ordinary temperature it is soft and may easily be cut with the knife. It was first obtained by Davy, in 1807, by the electrolysis of potash; but its preparation in the large way is effected by the ignition of a mixture of charcoal and potassium carbonate in a mercury bottle or iron tube coated with clay. In perfectly pure and dry air it undergoes no change; but in ordinary air it soon becomes coated with a film of potassium hydrate and carbonate. Its affinity for water is so great that when brought into contact with it immediate decomposition is effected, and sufficient heat evolved to set on fire the liberated hydrogen, which burns with the characteristic violet flame of potassium. Next to cæsium and rubidium it is the most electropositive element. It is a most powerful reducing agent, and hence has been largely employed for separating other metals from their various combinations; but at the present time sodium, being cheaper, is more generally employed for that purpose. Among the most important salts of potassium are the chlorid or muriate, KCI, mined at Stassfurt, Germany, and used as a fertilizer as well as the starting-point for the manufacture of other potash-salts; potassium chlorate, KClO₃, which is used in the arts as an oxidizing agent and in the manufacture of explosives; potassium nitrate, KNO₃, niter or saltpeter, made at present by the double decomposition of sodium nitrate and potassium chlorid, which is used in medicine and pyrotechny, but chiefly in the manufacture of gunpowder; potassium carbonate, K₂CO₃, which, under the commercial names of potash and pearlash, is largely used in the manufacture of soap and glass, and as a basis for making other potash-salts; potassium cyanide, KCN, a violent poison, used in photography and as a reducing agent; and potassium bichromate, K₂Cr₂C₇, red chromate of potash, much used in dyeing and calico-printing.

Wiktionary

1. n. A soft, waxy, silvery reactive metal that is never found unbound in nature; an element (symbol K) with an atomic number of 19 and atomic weight of 39.0983. The symbol is derived from the Latin kalium.

GNU Webster's 1913

1. n. (Chem.) An Alkali element, occurring abundantly but always combined, as in the chloride, sulphate, carbonate, or silicate, in the minerals sylvite, kainite, orthoclase, muscovite, etc. Atomic weight 39.0. Symbol K (Kalium).

WordNet 3.0

1. n. a light soft silver-white metallic element of the alkali metal group; oxidizes rapidly in air and reacts violently with water; is abundant in nature in combined forms occurring in sea water and in carnallite and kainite and sylvite