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States matter
States matter Utter Urben
M
P. G. Urben is an industrial chemist currently working with fine chemicals. He is also the editor of Bretherick’s Handbook of Reactive Chemical Hazards.
1074-9098/00/$20.00 PII S1074-9098(01)00264-7
y first chemistry set contained materials and instructions for making old-fashioned black powder. The attempt, on gram scale, was almost entirely safe, extremely interesting, and highly informative. Buying a modern chemistry set for my godson, I find that safety has emasculated it— any possible experiment is now neither remotely hazardous, interesting, or seriously informative. Every schoolboy started with gunpowder, and was disappointed᎑his would burn but not explode. The first thing learnt thereby was that solids must be incredibly well-mixed, which also means extremely finely ground, to effect violent reaction. You put a lot of work in to get power out! Later, he might have realized that a liquid phase is commonly involved in supposed solid reactions. The chemistry of black powder is complex, but certainly nothing happens until the sulfur component is molten. Sulfur appears to contribute little to the thermodynamics of the overall explosion (if you ever get one). Saltpetre is an oxidant and charcoal the fuel it oxidizes; most of the sulfur emerges reduced. The above intelligence is neglected entirely by commentators on chemical storage—I don’t think they had chemistry sets when they were young. For some reason they pick upon sodium nitrite. Is it next to an organic solid or near another sodium salt such as the thiosulphate? But these are solids and solids do not spontaneously mix. A great deal of hard work goes into increasing the entropy of reactive solids enough for them to become violent. Sodium nitrite is, of course, a component of what is claimed to be the only demonstrable spontaneous reaction between solids; in these days of safety and blissful ignorance you may never have seen it. Shovel some hydroxylamine hydrochloride into a tube, then add some sodium nitrite on top; nothing will noticeably happen although you leave it half an hour. Shake the two together for a few sec-
onds; still nothing. Set the tube down on the bench and after a minute or so it will undergo a vigorous fume-off, hazardous in a sealed container, not in a lidless tube. Several comments are in order, mixing appears required, even though hydroxylamine hydrochloride is itself energetic and unstable. Secondly, both materials cake on storage, which suggests a surface film of water in equilibrium with normal air while water is certainly a product of the reaction. Is this truly a reaction between solids? (I suspect it would no longer work in rigorously dry conditions.) Spilt solids stay put and do not spontaneously mix, or react, on casual contact. To obtain a propagating reaction, it is usual that one or both must be seriously unstable in its own right. Ah, you say, but what about fire conditions? Sodium nitrite is a dangerous oxidant in fires, but that is irrespective of what is immediately adjacent to it. Released oxygen will move to find fuel. In any case, uncommented upon, but far nearer than the next “chemical” there is usually fuel. Is the nitrite in polythene bottles or drums? In polythene bags within fibre drums? Within steel drums? All of these are definitely fuels, but not chemicals it seems, so that must be all right! It almost certainly stands on wood palettes or shelving. Wood was the first fuel known to man, but wood is not a chemical. The ultimate tests of chemical storage are probably earthquakes. They have been found to lead to fires, but one component of the initiating reaction has invariably been a liquid. The other has sometimes been wood. Look first to your liquids, and the solids can shift for themselves—and they won’t! To get a spark to ignite any flammable vapors that may be present habitually demands significant mechanical energy to grind reagents together. And it is a very considerate fire that melts, but does not overheat, fusible reactive solids for long enough to allow them to diffuse into a seriously explosive composition.
© Division of Chemical Health and Safety of the American Chemical Society Published by Elsevier Science Inc.
35
M
P. G. Urben is an industrial chemist currently working with fine chemicals. He is also the editor of Bretherick’s Handbook of Reactive Chemical Hazards.
1074-9098/00/$20.00 PII S1074-9098(01)00264-7
y first chemistry set contained materials and instructions for making old-fashioned black powder. The attempt, on gram scale, was almost entirely safe, extremely interesting, and highly informative. Buying a modern chemistry set for my godson, I find that safety has emasculated it— any possible experiment is now neither remotely hazardous, interesting, or seriously informative. Every schoolboy started with gunpowder, and was disappointed᎑his would burn but not explode. The first thing learnt thereby was that solids must be incredibly well-mixed, which also means extremely finely ground, to effect violent reaction. You put a lot of work in to get power out! Later, he might have realized that a liquid phase is commonly involved in supposed solid reactions. The chemistry of black powder is complex, but certainly nothing happens until the sulfur component is molten. Sulfur appears to contribute little to the thermodynamics of the overall explosion (if you ever get one). Saltpetre is an oxidant and charcoal the fuel it oxidizes; most of the sulfur emerges reduced. The above intelligence is neglected entirely by commentators on chemical storage—I don’t think they had chemistry sets when they were young. For some reason they pick upon sodium nitrite. Is it next to an organic solid or near another sodium salt such as the thiosulphate? But these are solids and solids do not spontaneously mix. A great deal of hard work goes into increasing the entropy of reactive solids enough for them to become violent. Sodium nitrite is, of course, a component of what is claimed to be the only demonstrable spontaneous reaction between solids; in these days of safety and blissful ignorance you may never have seen it. Shovel some hydroxylamine hydrochloride into a tube, then add some sodium nitrite on top; nothing will noticeably happen although you leave it half an hour. Shake the two together for a few sec-
onds; still nothing. Set the tube down on the bench and after a minute or so it will undergo a vigorous fume-off, hazardous in a sealed container, not in a lidless tube. Several comments are in order, mixing appears required, even though hydroxylamine hydrochloride is itself energetic and unstable. Secondly, both materials cake on storage, which suggests a surface film of water in equilibrium with normal air while water is certainly a product of the reaction. Is this truly a reaction between solids? (I suspect it would no longer work in rigorously dry conditions.) Spilt solids stay put and do not spontaneously mix, or react, on casual contact. To obtain a propagating reaction, it is usual that one or both must be seriously unstable in its own right. Ah, you say, but what about fire conditions? Sodium nitrite is a dangerous oxidant in fires, but that is irrespective of what is immediately adjacent to it. Released oxygen will move to find fuel. In any case, uncommented upon, but far nearer than the next “chemical” there is usually fuel. Is the nitrite in polythene bottles or drums? In polythene bags within fibre drums? Within steel drums? All of these are definitely fuels, but not chemicals it seems, so that must be all right! It almost certainly stands on wood palettes or shelving. Wood was the first fuel known to man, but wood is not a chemical. The ultimate tests of chemical storage are probably earthquakes. They have been found to lead to fires, but one component of the initiating reaction has invariably been a liquid. The other has sometimes been wood. Look first to your liquids, and the solids can shift for themselves—and they won’t! To get a spark to ignite any flammable vapors that may be present habitually demands significant mechanical energy to grind reagents together. And it is a very considerate fire that melts, but does not overheat, fusible reactive solids for long enough to allow them to diffuse into a seriously explosive composition.
© Division of Chemical Health and Safety of the American Chemical Society Published by Elsevier Science Inc.
35