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Chlorine monoxide, discovered by Balard in 1834, is prepared by passing a slow stream of dry chlorine over yellow precipitated oxide of mercury, previously heated to 300°-400°, contained in a cooled tube.
 | Fig: Preparation of chlorine monoxide |
A brown oxychloride of mercury remains, and brownish-yellow chlorine monoxide gas passes on:
2Cl2 + 2HgO = HgO,HgCl2 + Cl2O.
It is condensed in a freezing mixture to an orange-coloured liquid, b. pt. 2.0°. The gas may be collected by downward displacement; it attacks mercury, but only slowly, and is soluble in water.
The gas explodes readily although not very violently on heating, giving a mixture of two volumes of chlorine and one volume of oxygen: 2Cl2O = 2Cl2 + O2. In this way its composition may be determined, the chlorine after explosion being absorbed by caustic soda solution. Liquid chlorine monoxide may explode if the tube containing it is scratched with a file. If perfectly free from organic matter, however, it may be distilled without decomposition. Hydrochloric acid is decomposed by the gas, with production of chlorine: Cl2O + 2HCl = 2Cl2 + H2O. The gas dissolves easily in water, forming a golden-yellow solution containing hypochlorous acid: Cl2O + H2O <=> 2HOCl.
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