At atmospheric pressure, iodine turns straight from a solid into a gas. This is called sublimation. I tried it out for the first time today. I put a few iodine crystals into a conical flask. There was a boiling tube of ice in the neck of the conical flask. This is called a "cold finger". I heated gently. The iodine sublimed into a purple gas. This then turned straight back into a solid when it hit the ice-cold glass.
What I had forgotten was that this only works for iodine because melting and boiling are dependent on pressure as well as temperature. Look at this diagram for water
https://en.wikipedia.org/wiki/Triple_point#/media/File:Phase_diagram_of_water.svg Normal air pressure is around 100 kPa, as shown by the red line on the diagram. If we dropped the pressure to 100 Pa then ice would turn straight into steam at about - 20 degrees Celsius. That sounds weird. But it is what the iodine has done. On the other hand, if we increased the pressure in the flask, then the iodine would melt into a liquid before boiling into a gas.
