ANDREW LAMBERT PHOTOGRAPHY / SCIENCE PHOTO LIBRARY ANDREW LAMBERT PHOTOGRAPHY / SCIENCE PHOTO LIBRARY
Iodine subliming and crystallizing in a heated test tube. Iodine at room temperature is a black solid. Gentle heating results in the formation of a purple gas. This solid-gas transition is known as sublimation. The vapour recrystallizes on the cool surfaces of the tube, and is purified in the process. Iodine, found in seaweed and sea water, is an essential part of the diet.
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