Tornadoes, Rainbows, and Chemistry

Excerpted from A Feast of Science: Intriguing Morsels from the Science of Everyday Life

Excerpted from A Feast of Science: Intriguing Morsels from the Science of Everyday Life by Dr. Joe Schwarcz. © 2018 by Dr. Joe Schwarcz. Published by ECW Press Ltd. Reprinted with permission.

Moses, with a little help from above, proceeded to part the waters as a pillar of fire barred the pharaoh’s soldiers from pursuing the Hebrews. A truly memorable scene from Cecil B. DeMille’s 1956 classic, The Ten Commandments. In the film, the fire was the clever handiwork of animators, but it could have been made more realistic with a little help from chemistry. I’m thinking of a classic demonstration known as the methanol tornado. But before we get into this, let me repeat one of my mantras:

There are no safe or dangerous chemicals, only safe or dangerous ways to use them.

And methanol is a classic example. This simple alcohol is not only extremely flammable, it is highly toxic through ingestion or even skin exposure. That, though, does not mean that it cannot be used instructionally with the proper precautions.

Feast of Science book cover with a molecule on a plate

The demonstration requires a metal mesh garbage can, some sort of platform that can be made to spin, a small glass dish with a few milliliters of methanol, a chemical that can color a flame, and a source of ignition. The waste basket is placed on the spinning platform with the methanol container at the bottom. Spinning the platform after igniting the methanol causes a pillar of fire to rise in an impressive fashion and the addition of a bit of strontium nitrate to the methanol produces a striking red color. Seems to me that a close up of this fire tornado can make for a great movie special effect, but when you play with fire, as the saying goes, you can get burned. That’s just what happened to eight children and an adult in a science museum in Reno, Nevada.

Exactly what occurred isn’t clear, but the basic problem was that the stock bottle of methanol, from which a little had been poured out for the experiment, was left on the demonstration table. Somehow, when the methanol in the experimental dish was being ignited, the invisible vapors from the bottle also caught fire. This then ignited the methanol in the bottle, causing it to tip over and spill its burning contents onto the floor where the spectators were sitting. This demonstration is performed routinely in science museums and educational institutions around the world with no problem, but the glitch here was the stock bottle of methanol that was left nearby. That is an emphatic no-no!

While the accident with the methanol tornado experiment was an extremely rare occurrence, another demonstration with methanol, called the Rainbow Flame Experiment, has caused numerous accidents, injuring students and teachers when appropriate safety procedures are not followed, usually through a teacher’s ignorance. The experiment is a very popular one because it teaches students how to test for the presence of certain metals by virtue of the color they impart to a flame. The most common way of carrying out the experiment involves setting up a series of small glass dishes with each one containing a tiny amount of methanol in which a metal salt has been dissolved. When the methanol-metal samples are ignited, a rainbow of colors is produced. A salt that has sodium produces a yellow color, copper produces blue, boron green, potassium lilac, lithium carmine, and strontium red. 

The principle here is that the heat from the flame excites electrons in the metal ion to higher energy states, and when they drop down to lower energy levels they emit the energy they had absorbed in the form of light. Since the difference between energy levels is a property of the atoms in question, the specific wavelengths of light emitted, in other words the colors, can be used to identify the presence of a metal. The brilliant colors of fireworks, the bright red strontium flame of an emergency roadside flare, and the yellow glow of a sodium vapor highway lamp are all due to such electronic transitions.

The accidents while performing the rainbow experiment have occurred when a container of methanol that should never have been anywhere in the vicinity ignited. Commonly, the experimenter attempted to revitalize the flame by pouring more methanol into the dish, setting the stream on fire and creating a flashback into the bottle. This is similar to the disaster that can take place when someone squirts lighter fluid onto a lit barbecue and ends up barbecuing themselves. 

The frightening number of rainbow accidents has led chemical safety experts to recommend that using methanol as the source of fuel be abandoned. I would rather see educators be properly educated in carrying out such demos, and to have them emphasize to the students that the methanol bottle must be returned to its appropriate place of storage before proceeding with the experiment. It is true, however, that the chemical principle involved can be demonstrated without methanol. Just place a small amount of the salt in question on a platinum, gold, or silver wire that is then passed through a Bunsen burner. These metals do not  
produce a color of their own so any observed change is due to the sample being investigated. 

There is another good reason to perform the experiment with a Bunsen burner. Students can be told the fascinating story of how Robert Bunsen actually developed the iconic burner in order to study the colors produced by metals in its flame. Isaac Newton had previously shown how a prism can be used to separate white light into the colors of the rainbow, and Bunsen applied this principle to separate the colors of a flame into their individual components. Together with the physicist Gustav Kirchhoff, he developed the spectroscope, an instrument still used today to identify unknown substances by the colors they produce when heated. The composition of stars is actually determined by examining the light they emit through a spectroscope. A heavenly invention one might say. 

Read Lyle Sadavoy's book review of Feast of Science