Matter & Energy

Matter is composed of atoms or groups of atoms called molecules. The arrangement of particles in a material depends on the physical state of the substance. In a solid, particles form a compact structure that resists flow. Particles in a liquid have more energy than those in a solid. They can flow past one another, but they remain close. Particles in a gas have the most energy. They move rapidly and are separated from one another by relatively large distances.


Chlorine, symbol Cl, greenish-yellow gaseous element. In group 17 (or VIIa) of the periodic table, 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 that chlorine 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).

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. 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 water are 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.