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Chlorine is a greenish-yellow gas, the normal density of which is 3.214 gm. per litre. The relative density at S.T.P. is therefore 36.03 (O = 16). The relative density calculated from the atomic weight is 35.46, and the somewhat higher observed density may indicate a slight polymerisation: 2Cl2 <=> Cl4.
The density decreases slightly with rise of temperature, and becomes normal at about 240°, remaining normal up to 1200°. The density at 1150° was found by Reinganum (1905) by comparing the volumes of gas displaced from a small quartz Victor Meyer apparatus, in one case filled with oxygen and in the other with chlorine. They were equal, hence no dissociation had occurred. Crafts (1880) obtained the same result by displacing oxygen by chlorine, or chlorine by oxygen, in a porcelain apparatus at 1350°. According to Victor Meyer and Langer (1885), at 1400° the density of chlorine fell to 29-29, which would correspond with a 21 per cent, dissociation into atoms: Cl2 <=> 2Cl. Pier (1908), from specific heat measurements, assumed a dissociation of Cl2 above 1450°, but the value given by Victor Meyer and Langer appears to be too high. At very low pressures the dissociation is appreciable at 700°-900° (Henglein, 1922).
Chlorine, when cooled in solid carbon dioxide and ether, condenses to an amber-yellow liquid, boiling at -34.5°- On cooling in liquid air, this forms a pale yellow solid, melting at -100.9°. The critical temperature of chlorine is 140-9°; its critical pressure is 76.1 atm. The gas is liquefied at 0° by a pressure of 3.66 atm.; at 20°, 6.57 atm. pressure is required.
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