I have spent many years teaching chemistry to high school students and always enjoyed carrying out demonstrations. For students, watching chemicals perform their magic live is far more memorable than looking at equations on the blackboard. But teaching is also learning, as I discovered years ago when I wanted to show my students how soft a metal sodium is, and of course I planned to subsequently demonstrate its classic, fiery reaction with water. Sodium reacts with water to produce sodium hydroxide and hydrogen gas which ignites due to the heat generated by the reaction. The flaming piece of sodium skitters about the surface of the water impressing onlookers.
Luckily, I was wise enough to use the two-way fume hood setup. I cut the block of sodium, exposing its lustrous interior–the surface slowly oxidizes with time even when it’s kept under oil. But I had made the mistake of not taking tongs with me. I had actually used my pocketknife to pull it out of its bottle, and after cutting it, I poked my knife into one of the halves and placed the knife and sodium into a beaker of water.
Any time an exothermic reaction is carried out, the vessel should be checked for cracks. I only noticed a dark line in the glass after I dropped in the knife and sodium. At the time, I was also unaware of the precautionary trick of placing sand at the bottom of a beaker. It helps prevent overheated pyrex from cracking. My last error was my worst one: sticking a knife into too large a piece of sodium and immersing the knife with the impaled piece of sodium into water is a bad idea. The weight of the knife causes the chunk to sink into water. Without the added weight, sodium, with a density of 0.97 g/cm3, would not have sunk, and not all of its surface would have come into contact with water; the reaction would have been slower and more controlled. The hydrogen produced when sodium reacts with water would have ignited, but the reaction would not have generated as much pressure as when sodium is submerged.
To complete the disaster, our technician had recently left methanol in an open beaker in the fume hood. There was an immense explosion heard across our small school as the methanol vapour ignited. The observing students gaped in astonishment but were unharmed since the protective pane on their side could not be opened. The fume hood glass on my side did not break, but all the glassware within it did. I had never totally lowered my window, and sodium hydroxide (NaOH) produced by the reaction along with bits of glass projected towards my belly. A couple of pieces actually got stuck in my shirt. When I unbuttoned it, I saw no blood but found NaOH reacting with my belly.
Although I washed it off, by the time I got home that evening I had developed a second rash. Coincidentally, during that week I had experienced an allergic reaction. (It proved to be the new detergent my wife had bought). The next evening I told the dermatologist to ignore the NaOH rash and try to figure what was up with the rest of the redness. He looked at me as if I was from a different planet. But the reason I was in one piece and that my students were home safe was that in spite of all the errors committed, I had had the good sense of conducting a risky experiment in the safer environment of a fume hood.
Enrico Uva is a retired chemistry teacher who still enjoys learning chemistry, other sciences and mathematics.Â
