When Chemists Attack

Iodine is a rather neat element. It’s a nice – if a little boring looking – crystalline solid at room temperature. Chunks of iodine are similar in appearance to things you might find in the bins of rocks at places like Black Market Minerals at Barefoot Landing.

Solid iodine

Iodine is interesting because it is easy to make solid iodine go into the gas state. Plus, unlike many gases, iodine vapor has a distinct purple color and is easy to see. Solid iodine slowly sublimes (goes from the solid state to the vapor) at room temperature. It’s easy to accelerate this process by supplying a little heat.

If the iodine vapor comes into contact with a cool surface, it will deposit (resolidify) on the surface, forming pretty crystals. (A similar thing happens when water vapor comes into contact with a cool surface, although in that case you usually get liquid water.)

To show this, I tried to replicate a picture of a demonstration from one of my older chemistry books. I took some solid iodine and put it into a beaker, then set the beaker on a hotplate. On top of the beaker, I put a watch glass (curved piece of glass that looks something like a lens) and some ice – to provide a nice, cool surface.

Setup

To speed up the production of iodine vapor, I turned on the heat (just a little). You can just barely make out the purple iodine vapor in the beaker.

A little vapor is visible

If the hotplate’s temerature gets to about 114 ^oC (about 237 ^oF), the iodine will begin to melt, forming a dark purple liquid. The amount of iodine in the vapor state goes up, too!

More vapor is visible. If you value your nose, keep it away from this vapor.

What’s impressive about this demonstration is the sheer number of phase changes that are going on at once.

Pick a phase, any phase!

At the bottom of the beaker, you have some solid and liquid iodine. Since the hotplate is providing heat energy, you have the solid iodine melting and subliming. The liquid iodine is also evaporating. Near the top of the beaker (and to some extent on the sides of the beaker – which are cooler than the bottom), you have deposition of iodine vapor, forming solid iodine crystals. (It also looks like some iodine may have condensed on the hotter parts of the beaker nearer the bottom, then frozen after the beaker was removed from the hotplate.) That about covers it!

… not counting the ice, that is. The ice at the top of the beaker is melting, removing energy from the iodine vapor as it deposits on the bottom of the cold watch glass.

Deposited iodine crystals, forming from purple iodine vapor

The crystals formed on the watch glass are flat and shiny – almost metallic in appearance. They’ve grown to look a bit like perverted stalactites.

Deposited iodine crystals, closer view

A few words of caution if you attempt this experiment yourself. Iodine may look harmless (it won’t blow up on you – provided you keep it away from combustibles), but iodine solid can cause chemical burns on skin contact, and iodine vapors are very bad for the lungs. This sort of experiment needs a fume hood, and solid iodine shouldn’t be handled directly.

This entry was posted on Monday, September 11th, 2006 at 2:04 pm and is filed under Science, Science education. You can follow any responses to this entry through the RSS 2.0 feed. Both comments and pings are currently closed.