Expert: George Maxwell - 7/19/2007

Question
When iodine is heated, a solid crystal-like substance is formed around it. This is because the vapur formed goes around, but could you explain to me why this solid is formed?

Answer
Thanks very much for your question.

The easiest way to understand this process is to make a cup of coffee! If you boil a kettleful of water, you'll notice a vapour escaping from the spout: as soon as the temperature inside the kettle reaches 100 degrees centigrade, the water begins to turn in to steam.

The escaping vapour flows from the kettle in to the surroundings, which are a lot colder than 100 degrees. As it touches the cold surroundings (particularly window panes), the water vapour turns back in to a liquid, forming droplets of liquid water.

Exactly the same thing is happening with the iodine, except that iodine goes straight from a solid to a gas (sublimes) when it's heated above a certain point (just over 100 degrees centigrade). You heat up the iodine, and as soon as the container gets to the sublimation temperature, the iodine begins to turn in to a vapor and escape. However, when it escapes, it hits the surrounding surfaces which are much cooler than the sublimation temperature, so the iodine turns back in to a solid (just like the hot water vapour turning back in to a liquid when it hits a cold window).

Basically then, the solid isn't "formed" - it's just moved from the container in which you heat it on to the surroundings. The process depends on the fact that substances are always solid below a certain temperature and always gases above another (assuming the atmospheric pressure is constant).

Hope this helps, but let me know if this isn't clear.
Enjoy the coffee!
Thanks again.