Monday, November 2, 2015

Fire Injures Six During Chemistry Demonstration

It was second period last Friday and it would be a four day weekend in Fairfax County, Virginia. A high school chemistry teacher a W.T. Woodson High School was performing what is commonly known as the ‘Rainbow Flame Demonstration.”

In its most exciting (and dangerous) form the “Rainbow Flame” demonstration is performed on an open bench typically using methanol as the flammable solvent. The solvent is poured out on the bench and lit and metal salts are tossed (or spooned) into the flame to change its color. Barium Chloride BaCl2*2H2O makes the flame light green, Calcium Chloride CaCl2 turns the flame orange red, Copper Chloride CuCl2 burns a beautiful blue/green, Lithium Chloride LiCl produces a hot pink flame, Potassium Chloride KCl a light lilac colored flame, Sodium Chloride NaCl produces a yellow flame, and Strontium Chloride SrCl2 produces a red flame. In a darkened room it is really quite spectacular. It is also dangerous.
from a U tube demonstration
When this Rainbow Flame demonstration is carried out on an open bench, the conditions for a flash fire are easily met, and there have been several widely reported classroom accidents. Highly flammable solvents, such as methanol, can produce heavier-than-air vapors that move across surfaces and down toward the floor where they spread undetected among viewers of the demonstration. A flame, spark, or even very hot surfaces can ignite the vapors resulting in a sudden flash fire or worse if there is an open container of solvent. Fire can also flash up when additional methanol is poured on the hot surface for another round. According to Jonathan Sanderson of Science Demo, the cause of a series of accidents reported in the US has been the “demonstrators topping up the flame straight from the methanol bottle, leading to the ignition of a large volume of fuel vapour.”

Fire needs to be treated with respect and handled using proper safety precautions. Numerous accidents have occurred with the Rainbow Flame demonstration. Yet, other instructors have performed the demonstration hundreds of times without incident and I’ve done a version of the demonstration myself. The demonstration can be safely performed under a ventilation hood and strictly controlled by pouring a limited amount of solvent into petri dishes or saucers that contain the various salts, then sealing the solvent bottle and NOT refilling the dishes. A careful and controlled experiment (while wearing safety goggles and an apron) loses some of the wow factor of the open flame. This demonstration is dangerous if it is not performed under a ventilation hood and strictly controlled single round of solvent. Never open a bottle of fuel near a naked flame or source of heat, and that the correct procedure here is to make sure the surface or petri dishes are cold before adding a small amount of fuel, then sealing the fuel bottle and removing it to a safe distance, before lighting the mixture. Safety glasses and aprons are a must especially when dealing with a classroom of students.

The U.S. Chemical Safety Board issued a warning message for this experiment/demonstration back in 2013 along with a video interview with a victim of a Rainbow Flame demonstration fire that can be seen on U tube

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