Iodine (from the Greek, iodines, meaning violet) has an atomic weight of 126.9044, atomic number 53, melting point 113.5° C, and boiling point 184.35° C. As a gas, its density is 11.27 g/1 and as a solid its specific gravity is 4.93 (20° C). This halogen was discovered by Courtois in 1811. It occurs sparingly in the form of iodides in sea water from which it is assimilated by seaweeds, in Chilean saltpeter and nitrate-bearing soil, in brines from ancient sea deposits, and in brackish waters derived from oil and salt wells. Pure grades of iodine can be obtained from the reaction of potassium iodide with copper sulfate. Iodine is a grayish-black, lustrous solid that volatilizes at ordinary temperatures to a blue-violet gas. It forms compounds with many elements. However, it is less active than many of the other halogens which displace it from iodides. Iodine dissolves readily in chloroform, carbon tetrachloride, and carbon disulfide. It is only slightly soluble in water. Iodine is highly irritating to the skin, eyes, and mucous membranes. Its effect on the human body is similar to that of bromine and chlorine. However, it is more irritating to the lungs.
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