The Who, What, When, Where and Why of Chemistry
Chemistry is not a world unto itself. It is woven firmly into the fabric of the rest of the world, and various fields, from literature to archeology, thread their way through the chemist's text.
So what's this kid doing in the high school auditorium after school? He's drilled holes and put pipes into a cooler, there's some kind of heating device or trigger. Wires. And it looks like a boat load of some sort of chemical in that bowl that he's dumping in there. And then...and it shoots out some kind of gas. Kids scream. The gas begins to cover the stage.
"What's happening?" wants to know the teacher who hears the commotion from the hallway. "I'm testing a fog machine I built for the class play."
Yes, at first glance the situation looks potentially perilous. But a quick question, followed by a bit of common sense and the teacher is reassured that all is well.
Now that everyone is sure that there is no bomb, what should happen to the kid?
A. Pull the child into the principal's office and demand that he sign a statement admitting his guilt.
B. Call the police, who will arrest him and charge him with building an explosive device.
C. Call the police, who will arrest him and charge him with building a "hoax bomb"
D. Nominate him for a theater award for special effects, for having designed and built an inexpensive fog machine to use for the school's upcoming production of Grease.
The kid is my kid and the school's response was D. But imagine if my kid wasn't white and male. If his name were Ahmed Mohamed or Kiera Wilmot? There might have been handcuffs, felony charges, letters home to parents about "the incident". If someone had called the police, would they have arrested him because he couldn't explain why he'd built one, when they could have rented a fog machine? (The police thought it suspicious when Ahmed Mohamed couldn't tell them anything more than his device was a clock.) Why would you build a fog machine, or a clock? He must have built it for a purpose, nefarious almost certainly.
Perhaps the purpose was to understand how these machines work? There is an amazing amount of joy in showing that you understand something well enough to build a working apparatus. To tweak and fix.
As a parent, I want the school to exercise an abundance of caution. But once you're sure it's just a clock — or a fog machine — perhaps it's time to slow down, and engage some common sense. Is there anything else that suggests this kid would build anything danger? Besides his name, or the color of her skin, or his religion.
Scientists and engineers are not hatched full grown from eggs in labs. As kids, they tinker and think and build and design, with Legos and parts from Radio Shack and Home Depot. They are in theater and on robotics and Science Olympiad teams. We need to get as excited about what they do as we are about how the football team is doing.
A friend posted the link to this demonstration, wondering if it was safe. (Do listen to the children in the background - their cries of "kraken" at 1:02 are worth it. Science is great fun!)
The caption that came with it noted that it was a mixture of ammonium dichromate ((NH4)2Cr2O7 )and HgSCN (mercurous thiocyanate).1 Mercury and chromium, probably not something you want to eat I told my friend. The whole thing made me curious, just what were those tentacles come out of the burning pile? And what chemical reactions were driving it?
It's a coupled set of decomposition reactions. The volcano comes from the decomposition of ammonium dichromate
(NH4)2Cr2O7(s) → Cr2O3(s)+ N2(g)+ 4H2O(g)
The reaction produces a lot of heat, which makes the particles being thrown off by the rapid expansion of the two gases (nitrogen and water vapor) glow.
The heat then triggers the decomposition of the mercury compound:
2 Hg(SCN)2(s) → 2HgS + 4CS2 + carbon nitrides
The erupting tentacles are an example of intumescence2, a property of mercury thiocyanates noted long ago by the venerable Friedrich Wöhler3. It's a well known demonstration, often called Pharaoh's Serpents. Many material intumesce when heated, and thus produce their own insulation. Some passive fire protection systems rely on this property of polymers, by which they essentially rapidly produce their own insulating layer upon heating, or by swelling up to block air ducts to prevent smoke and other gases from spreading too quickly through a ventilation system.
It works with mercuric thiocynate as well (Hg(SCN)2) — by some accounts even better — and better yet if you toss a bit of potassium nitrate and a bit of fuel in the form of sugars. In other bits of historical trivia, the mercuric thiocyanate was originally made by the aptly named Otto Hermes.
The sale of mercuric Pharaoh's Eggs ceased after some kids ate them with deleterious (fatal) effects.
1. From the Latin verb "to swell" — related to thumb and tuber (as in root vegetables like potatoes)
2. The chemist who showed in 1828 that compounds made by nature do not have some "vital essence" that distinguishes them from the same structure crafted by a chemist from inorganic (never living) materials. Something the Food Babe and hawkers of 'bioidentical' hormones do not get.