Science can be explosive: Don’t try this at home
You know the old expression “a picture is worth a thousand words?” Well, when it comes to chemical demonstrations, a video is worth a thousand pictures, and a live performance is worth a thousand videos. OK, maybe not a thousand, but several. At one time, demonstrations were an integral part of chemistry lectures, but these days they tend to be rare. Good demos take time to prepare, take time to set up and often require a cleanup. Videos on the other hand are easy to show and many excellent ones are available. But the same way that a movie can never quite capture the thrill of a live stage performance, a video of a combustion reaction just doesn’t have the same impact as a professor performing it live in front of a class.
One of the features of a “live” demo that students find especially appealing is the chance that something might go awry. And with combustion-type reactions that can certainly happen. I know. I once came pretty close to a nasty accident with the classic “burning money” demonstration in which a bill is immersed in a mixture of water and alcohol and is set ablaze. The flames can be clearly seen even in a large lecture hall, and to the students’ amazement, when they are extinguished, the bill is seen to be totally undamaged, although a little wet. How does this happen? It’s a case of the alcohol burning and the water preventing the bill from catching fire. Once when I was performing this, I accidentally tipped over the beaker holding the alcohol-water mixture, which then caught fire. Much merriment ensued when the students saw my flaming hands. Luckily nothing else on the desk caught fire and my hands were protected by the water the same way the bill was. But now when I perform this demo, I always make sure the beaker is securely clamped to a stand.
My little accident was nothing compared with what was probably the worst chemical demonstration accident ever. That occurred in 1957 at Indiana University in Bloomington when a professor was demonstrating the effect of liquid oxygen on the combustion of aluminum in front of a group of high school students to give them a memorable chemical experience. It turned out to be memorable all right, but not in the fashion he envisaged.
Oxygen itself does not burn, but it does support combustion. Aluminum on the other hand burns well, especially when powdered to provide a large surface area for potential contact with oxygen. Dowsing the powdered aluminum with liquid oxygen provides ideal combustion conditions. Most of the liquid oxygen quickly evaporates, but some gets trapped in the crevices of the tiny aluminum particles. The classic demonstration of the “liquid oxygen effect” involves using a candle to set fire to a sample of powdered aluminum in a metal crucible after dowsing it with liquid oxygen. The usual result is a bright flare that shoots straight up in a harmless but spectacular fashion.
Up to 1957, this experiment was a standard one, frequently performed at universities around the world without any problem. But on that fateful day in Indiana, instead of just burning brightly, the mixture detonated, hurling fragments of the iron crucible and the stone tabletop on which it had been sitting throughout the auditorium, and causing injuries that ranged from the loss of an eye to severe lacerations and even crushed bones.
It turns out that under the right, or in this case wrong, conditions, the reaction inside the mound of aluminum can be so rapid and produce so much heat that the air trapped inside the crucible expands with tremendous speed and produces a shock wave. And that is an explosion! Although with great care it can be performed safely, this is one demonstration I wouldn’t touch with a 10-foot, nay, a 100-foot pole. But it is perfectly safe on a video.
There’s another demonstration that I once performed but do not care to repeat. The “thermite reaction” is just too dangerous. But it is a doozy. Once more, powdered aluminum is involved, this time mixed with finely ground iron oxide, familiar to us as “rust.” When this mixture is ignited, the oxygen is transferred from the iron oxide to the aluminum, yielding aluminum oxide and metallic iron. Ignition is not easy, but can be achieved with a strip of magnesium. Magnesium lights readily and the heat generated ignites the thermite mixture, which then proceeds to produce a tremendous amount of heat with the temperature of the products reaching some 2,200C! Glowing bits of iron spew out in a shower of sparks, impressing everyone around, especially those who have an encounter of the first kind.
Why would this reaction be worthy of a demonstration? There is of course the “gee whiz” effect. But there’s more. The thermite reaction has some practical applications. Soon after it was patented by German chemist Hans Goldschmidt in 1895, the reaction was put to use in welding, especially railroad tracks. And humans, being humans, soon found a military application in incendiary bombs, used by both the Germans and the Allies during the Second World War. Thermite hand grenades were also developed for the destruction of captured military equipment as well as for the emergency destruction of sensitive equipment in the face of an impending risk of being captured by the enemy. Exploding a thermite grenade in the barrel of a cannon will ensure that the weapon is permanently disabled. And exploding one inside a helicopter ensures its destruction. U.S. Navy SEALS probably used thermite grenades to destroy the secretly developed stealth helicopter that crashed during the assault on Osama bin Laden’s compound in Pakistan.
Conspiracy theorists also maintain that the thermite reaction was used by the “master architects” of 9/11 to bring down the twin towers from the inside because, according to these sages, burning jet fuel is not hot enough to melt structural steel. Their evidence? A video that shows an automobile engine being melted by a thermite reaction. Such absurd conspiracy theories deserve to crash and burn.