To Sublime is Sublime

Last week we discussed precipitation reactions in my General Chemistry class. I tried to impress upon the students how surprising it would seem to someone observing such a chemical reaction for the first time. Mix two clear and colorless solutions together, i.e., two substances that look just like liquid water, and out pops a solid! Most of the solids are white in standard demos (and perhaps the jaded student does not find this all that interesting) but lead iodide (PbI₂) is yellow. I wonder if an early alchemist observing a similar reaction might have wondered if this was a route to making gold. I’m not sure my students were all that impressed (having seen this reactions in lab the week before). They might even have found writing out the chemical equations tedious. At the very least I might have amused them with my opening Power Point slide shown below as they were filing into class. Lame, but it still elicits chuckles.

Let’s stop to think about this for a moment. We often see solids dissolve in water (salt and sugar for example) but in everyday experience we rarely encounter solids appearing where you only had liquids to begin with. It’s almost magical. If you had a chemistry set as a kid, you might have had the magical experience of making your own precipitate or growing crystals. Think about the child-like wonder in seeing something surprising that might not have conformed to your everyday experience! (In the age of the iPad with fancy addictive games, it’s unclear to me if today’s kids will find it fascinating.) I’m pretty sure the alchemists did – especially since they did not know the underlying chemistry. I wonder if part of why the students are jaded is because the explanation seems so mundane and straightforward. (Maybe I need to spice it up!) Knowledge should be exciting, not boring!

What questions did students have on this unit? The first one, predictably, was “Do we have to memorize the solubility rules?” This was the final thing we talked about in class where my emphasis was not on memorizing but in trying to draw general principles from the rules – really a list of observational facts. That’s how you put together a theory! (Or perhaps a working hypothesis that can be tested!) But more on that in a later post since I’ve been thinking about philosophy of science and the makings of a scientific theory. In any case, I hope the students understood the idea of how to formulate such a theory based on what they had learned earlier in the semester. We had discussed lattice energies of ionic compounds, packing in solid crystalline structures, and dynamic equilibrium. I waved my hands and drew pictures on the board as I drew students into the discussion. I was at least interesting enough that they were paying attention instead of watching the clock (because we ran over by 2 minutes and they didn’t notice). I suppose I could have an assessment question to check if they really did “get it”.

A related phenomena that I think the alchemists would find particularly interesting is sublimation. Some solid substances will turn directly into gases (skipping the liquid state) upon heating. Dry ice (solid carbon dioxide) is an example familiar to most people. We also covered this in class but I neglected to make the connection to alchemy and teleportation! (More on the latter in a moment.) Iodine works well in a demo because of the lovely purple hue as it sublimes. I did not do the demo this year in my lecture class (noxious fumes) but I have done so when I teach lab (with the apparatus in a fume hood). If you put a watch glass over beaker so you can see the crystals reforming on the base of the watch glass. (Putting a small piece of ice on the watch glass hastens the deposition.) Other substances exhibiting this behavior include arsenic, camphor and ammonium chloride (sal ammoniac). Arsenic and sal ammoniac would have been familiar ingredients to the medieval alchemist, and camphor was available to Europe as a spice through the Arab traders.

Why might sublimation and deposition be of particular significance to the alchemist or magician? I think the process is akin to teleportation! A solid substance disappears into vapors, only to be reformed somewhere else. In the TV series Once Upon A Time, when the evil queen teleports she is engulfed by a swirling purple smoke that resembles the color of iodine vapor! (I wonder if that’s where they got the idea.) Furthermore, sublimation could also be used as a tool for purification, much like distillation – a process core to alchemist’s work. Since alchemy was often infused with the religious symbol of Christianity, one might see the analogy a soul rising upward (or heavenward). If my students lived in the medieval world and witnessed such alchemy, they might think it sublime!