Letter to editor

  • This concerns the article “Sublimation of Iodine: Rise and fall of a misconception,” by Michael P. Jansen, Chem 13 News, October 2015, page 4.

There should be no misconception about iodine being similar to carbon dioxide as only undergoing sublimation and not existing as a liquid at atmospheric pressure. As a student* and then as a teacher,** I used the lab called “Sublimation of Iodine”. It was done by placing a few solid crystals of iodine into a test tube and heating in a hot water bath, a beaker filled halfway with water (100 oC). The test tube had a polyester wool plug in it to allow air and vapours out, but keep the solid iodine in. I also used a glass wool plug. The iodine vapours (violet) appear, no liquid is seen, and crystals of iodine appear on the upper test tube — deposition occurring.

One of the lab questions was “The melting point of iodine is 113.5 oC. How do you know that the iodine did not melt?” At this point, the iodine in a test tube could be melted by heating it in a Bunsen flame. Now I recommend to do so in a hot oil bath heated to 115 oC on a hot plate.

The point is that at one time there was no misconception. One wonders how the idea that iodine only sublimes got started.

To do that old lab now, place a 150 mL beaker half full of water on a hotplate and heat it to near boiling. Carefully float a small crystallizing dish with a few crystals of iodine in it on top of the water. Cover the beaker with a watch glass or evaporating dish, and place an ice cube on top of the watch glass or evaporating dish. The iodine will sublime, the ice will melt, and the water will evaporate and condense. The only change of state not taking place is freezing.

To clean up use a minimum amount of 3.0 mol/L potassium iodide solution to dissolve the iodine from the demonstration. Iodine is more soluble in aqueous potassium iodide than in water. Collect this solution in a beaker. Rinse the setup with distilled or deionized water to collect any clinging solution. This KI3(aq) solution may now be used for starch testing and staining onion skin cells in other courses.

It should also be noted that iodine should be stored by itself in a cupboard that is ventilated. If stored with other chemicals it will discolour the paper labels, which contain starch sizing. Iodine also reacts with iron, discolouring any iron lids or containers, shelves, etc.

*As a student my old text was The Methods of Science, 1962, G. Erwin et al. It was a grade 10 Science textbook in Ontario.

**When I was teaching I used Discovering physical science, 1983, William Andrews et al.