I continue to write a weekly science column for the Tomah Journal and 2 newspaper in Juneau County are running some columns, soon to be joined by the Prairie du Chien Courier Press. The following question came from a student in the Warrens, Wisconsin area.
Why do some chemicals explode when they are mixed?
All chemical explosions are rapid and violent oxidation-reduction (redox) reactions that produce huge amounts of heat and gases that expand rapidly. A redox reaction is simply a reaction in which two atoms exchange electrons. Oxidation involves loss of electrons. Reduction means a gain in electrons. All explosions are exothermic, meaning heat is given off or liberated.
It is the speed of the reaction that makes the difference between ordinary combustion and an explosion. Consider wood that is burning in the fireplace or campfire. The same oxidation-reduction process is taking place in the burning wood as is occurring with a firecracker. However, the heat and gas released with the burning wood happens much slower, not rapidly enough to cause an explosion as in the firecracker. Another example of a very slow redox reaction would be steel that rusts.
Gunpowder was the first man-made explosive, invented by the Chinese in the ninth century. A combination of sulfur and charcoal was the fuel, and saltpeter (potassium nitrate) was the catalyst that provided the oxygen. Gunpowder, or black powder, is a slow reacting explosion, causing a subsonic shock wave. That’s good for propelling a bullet out of a gun barrel. Deflagration is the term used to describe the slower burning explosion.
Nitroglycerine was discovered in 1860s. Nitroglycerine is unstable, so Alfred Nobel invented the safer-to-handle dynamite in 1866. Dynamite and TNT produce fast acting explosions with a resultant supersonic shock wave. Detonation is the term used for a first acting explosion. If dynamite were to be used in a gun barrel, it would blow the gun barrel apart.
That prefix nitro is a clue that an explosion of some kind is possible. We burn 10 percent nitromethane fuel in our radio controlled airplane engines. Nitromethane fuel is used in some dragster race cars.
A fertilizer bomb destroyed the Federal Building in Oklahoma City in April, 1995 with the loss of 168 lives. Timothy McVeigh used over 4,800 pounds of ammonium nitrate fertilizer and nitromethane. There’s that word nitro again.
The worst industrial accident in the United States occurred on April 16, 1947 in the port of Texas City, Texas. The freighter SS Grandcamp, loaded with 7,700 tons of ammonium nitrate exploded at 9:17 AM. The denotation killed 581 people and injured over 5,000. The blast knocked out windows in Houston, a distance of 40 miles.
An explosion of twice the magnitude of the Texas City disaster was the N1 rocket disaster in the Soviet Union on July 3, 1969. The Russians planned to use the N1 rocket to beat the Americans to a moon landing. A loose bolt was ingested into a fuel pump of one of the thirty engines, triggering the massive blast. After this lost, the Soviets never mounted a serious effort to go to the moon. Email questions and comments to: firstname.lastname@example.org