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The reaction of sodium with water is a spectacular and essential classroom demonstration. Many teachers want to show also the more violent reaction of potassium. We propose not to do so because explosions can happen even before the metal is in contact with water. In March 2006 a 62-year-old experienced chemistry teacher wanted to demonstrate the properties of potassium to his class at the secondary school of Goldach, Switzerland. The metal was stored under petroleum in a glass bottle. When he tried to take out a small piece with a knife this piece exploded and burning petroleum splashed on the teacher's chest. The bottle and the potassium in it remained intact. The teacher suffered from severe burns but none of the students were injured. As a consequence, a total ban of the use of potassium at the secondary schools of the Canton of St. Gallen was proposed by the chemistry professors of the School for Teacher Education. The chemistry teachers were urgently asked to dispose of all potassium stocks at their schools and not to use this chemical again. Numerous warnings about the dangers of potassium metal can be found in the literature, not only with regard of its reaction with water. The possible formation of oxide crusts is described in various places. Bretherick's Handbook of Reactive Chemical Hazards (1) and a recent article by DeLaHunt and Lindeman (2) lists potassium dioxide KO2 (or superoxide), potassium peroxide K2O2, and potassium trioxide KO3 as hazardous compounds. They are formed on exposure of the metal to air, during storage under petroleum, and even during storage under vacuum in polyethylene bags (3). They can react with the combustible storage solvent and with potassium metal itself. The contact of the strong oxidants with the strong reducer potassium, occurring at the moment of picking up a piece with tweezers or cutting it with a knife, can lead to heavy explosions. We strongly advise not to remove any crusts manually but to dispose of crusted samples. Potassium with crusts should not be used for classroom experiments. The reaction of alkali metals with water can be demonstrated with the less dangerous lithium and sodium just as well. Sodium does not develop peroxides spontaneously. Video demonstrations of the reaction of potassium (and other elements) with water are very instructive. They can be found in the CD-ROM "Periodic Table Live!" produced by JCE (4). Literature Cited- (a) Urben, P. G. Bretherick's Handbook of Reactive Chemical Hazards, 7th ed.; Elsevier: Amsterdam, 2007; Vol. 1, p 1826. (b) Urben, P. G. Bretherick's Handbook of Reactive Chemical Hazards, 7th ed.; Elsevier: Amsterdam, 2007; Vol. 2, p 240.
- DeLaHunt, John S.; Lindemann, Theodore G. J. Chem. Health Safety 2007, 14 (2), 21.
- Evers, Jürgen; Klapötke, Thomas M.; Oehlinger, G. Chem. Eng. News 2002, 80 (37), 4.
- Journal of Chemical Education (accessed Feb 2008).
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