Chlorine

Chlorine, symbol Cl, greenish-yellow gaseous element. In group 17 (or VIIa) of the periodic table (see Periodic Law), chlorine is one of the halogens. The atomic number of chlorine is 17.

Elementary chlorine was first isolated in 1774 by the Swedish chemist Carl Wilhelm Scheele, who thought that the gas was a compound; it was not until 1810 that the British chemist Sir Humphry Davy proved thatchlorine was an element and gave it its present name.

At ordinary temperatures, chlorine is a greenish-yellow gas that can readily be liquefied under pressure of 5170 torr, or 6.8 atmospheres, at 20° C (68° F). The gas has an irritating odor and in large concentration is dangerous; it was the first substance used as a poison gas in World War I (1914-1919) (see Chemical and Biological Warfare).Free chlorine does not occur in nature, but its compounds are common minerals, and it is the 20th most abundant element in the earth's crust. Chlorine melts at -101° C (-149.8° F), boils at -34.05° C (-29.29° F) at one atmosphere pressure, and has a specific gravity of 1.41 at -35° C (-31° F); the atomic weight of the element is 35.453.

Chlorine is an active element, reacting with water,organic compounds, and many metals. Four oxides have been prepared: Cl2O, ClO2, Cl2O6, and Cl2O7. Chlorine will not burn in air, but it will support the combustion of many substances; an ordinary paraffin candle, for example, will burn in chlorine with a smoky flame. Chlorine and hydrogen can be kept together in the dark, but react explosively in the presence of light. Chlorine solutions in waterare familiar in the home as bleaching agents.

Most chlorine is produced by the electrolysis of ordinary salt solution, with sodium hydroxide as a by-product. Because the demand for chlorine exceeds that for sodium hydroxide, some industrial chlorine is produced by treating salt with nitrogen oxides or by oxidizing hydrogen chloride. Chlorine is shipped as a liquid in steel bottles or tank cars.It is used for bleaching paper pulp and other organic materials, destroying germ life in water, and preparing bromine, tetraethyl lead, and other important products.[1]

Water Desalinization

To meet the ever-increasing demands for fresh water, especially in arid and semiarid areas, much research has gone into finding efficient methods of removing salt from seawater and brackish waters. In theU.S., desalinization research is directed by the Bureau of Reclamation, Department of the Interior. Several processes are being developed to produce fresh water cheaply.

Three of the processes involve evaporation followed by condensation of the resultant steam and are known as multiple-effect evaporation, vapor-compression distillation, and flash evaporation. The last-named method, the mostwidely used, involves heating seawater and pumping it into lower-pressure tanks, where the water abruptly vaporizes (flashes) into steam. The steam then condenses and is drawn off as pure water. In 1967, Key West, Florida, opened a flash-evaporation plant and thus became the first city in the U.S. to draw its fresh water from the sea.

Freezing is an alternate method, based on the differentfreezing points of fresh and salt water. The ice crystals are separated from the brine, washed free of salt, and melted into fresh water. In another process, called reverse osmosis, pressure is used to force fresh water through a thin membrane that does not allow the minerals to pass. Reverse osmosis is still undergoing intensive development. Electrodialysis is being used to desalt brackish waters. Whensalt dissolves in water, it splits into positive and negative ions, which are then removed by electric current through anion and cation membranes, thus depleting the salt in the product water. Although developmental work on electrodialysis is continuing, a number of commercial plants are in operation. In 1962 Buckeye, Arizona, became the first town to have all its water supplied by its own...