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While teaching at Carleton College I liked telling my students that iodine is my favorite element, largely because of its colorful chemistry (1). On retirement, I made an iodine thermometer, comprising a few grams of the solid sealed in a 12-liter round-bottom flask. It sits in my back yard, giving a visual indication of the temperature with increasing sublimation pressure. In the morning, it is nearly colorless, but the Arizona sun works wonders as the day progresses (2). At night, condensation results in countless little violet-black crystals on the inner surface. 
Figure 1. An iodine thermometer showing small iodine crystals that were deposited on the inner surface of the flask by repeated solar vapor-solid transitions.
Figure 2. An iodine thermometer at midday provides a visual indication of the temperature with increasing sublimation pressure—the deeper the color, the higher the temperature. The Santa Rita mountains of Arizona are in the background.
In case anyone wants to make an iodine thermometer, here’s the procedure I followed. Use about a half-teaspoon of solid iodine. The trick is to keep water from getting in. Seal a glass plate to the neck of the flask, using silicone goop. When it is dry, set the inverted flask into a plastic jar lid and pour in plaster of paris. When that is dry, further seal it with more silicone. Just sealing with epoxy or silicone doesn’t hold up because the iodine attacks it. Literature Cited- Cooper, J. N.; Ramette, R. W. J. Chem. Educ. 1969, 46, 872.
- Goldsmith, R. H. J. Chem. Educ. 1995, 72, 1132.
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