The explosive that goes off with the touch of a featherS

Nitrogen triiodide is expensive to make. It's also useless in any practical sense. But it's cool to watch — just the touch of a feather and it explodes into purple smoke. All it takes is ammonia and iodine.

Fortunately for everyone who enjoys not having to scrub purple stains out of their furniture with burnt hands, it's tougher to get hold of than it looks. Iodine is vaguely familiar to those who remember the days when it was used to paint wounds and prevent infection, but that iodine is so diluted in water that it's mostly useless for this experiment. Expensive and hard-to-get iodide crystals are necessary instead; they're highly concentrated with a lot of iodine atoms. Ammonia, otherwise known as ammonium hydroxide, is a lot of hydrogen and oxygen, with a bit of nitrogen attached. To make the solution, iodide crystals are crushed in a beaker and ammonia is poured over them.

Over the next two hours, the iodide crystals grab the nitrogen off the ammonia. Each nitrogen atom gets three iodide atoms, and they sit there and stew. As long as the crystals stay wet, the set up isn't too volatile. Before they begin drying on their own, the maker — usually a science teacher wishing to impress a class, since the concoction is useless for anything else — pours the iodide crystals onto filter paper and places them somewhere to dry completely. Then comes the fun part.

The explosion goes off at, literally, the tap of a feather. Sometimes it even goes off when blown on too hard. The reaction gives off heat and purple and orange smoke, stained by iodine. It's an impressive boom, but a useless one. The set-up can't be transported, or even touched — it's too explosive to be a good explosive. The explosion is dangerous only to people in labs. There have been instances of people over-soaking it, until the reaction goes off in the bottle, that have caused a lot of damage. Other than that, its volatility makes it useful only for entertainment explosions.

Iodine Image: Paul

Via MIT, RSC.