Page 40 - Curriculum Visions Dynamic Book

Iodine
Iodine is named after the violet colour of its vapour.
It is a blue–black solid at room temperature, and is very poorly soluble in water, although its compounds are all soluble. Tincture of iodine, often seen on the bathroom shelf as a dark brown liquid, is iodine dissolved in alcohol. In this form it makes a useful disinfectant.
Iodine is poisonous in high concentrations, but health- giving in trace amounts. It is an essential element to both plants and animals. In animals it is found in the thyroid gland in the throat, where it is used to make vital chemicals that control the rate of body activity. Without sufficient iodine, body activity is slowed down and stunted growth results. Lack of iodine also causes the throat to swell, called a goitre. For these reasons, iodine is added to almost all table salt on sale to the public (sometimes labelled as iodised salt).
Iodine is quite common in the world, especially in sea water. However, it does not make up any kind of rock, so no large mineable deposits exist. It is mainly recovered from ancient salt deposits.
Seaweeds
A number of sea plants contain large natural concentrations of iodine. Sea sponges were traditionally used to reduce the swelling of goitres.
Later it was discovered that the more common seaweed, kelp, also contains large amounts of iodine. Seaweed can be eaten to ensure sufficient iodine in a diet.
Iodine and rain
Iodine combines with many metals, but one
of the most important is silver. Silver iodide crystals have been used in rain-making experiments because they attract moisture and act as condensation nuclei – particles that can “grow” the water droplets that fall as rain.
 Here you can
see the particles of
solid iodine in a tube containing water. Clearly, iodine does not dissolve well in water; however, almost all iodine compounds are soluble.
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