My high school chemistry classes were not impressive. We memorized tons of formulas and drew loads of pictures of experimental setups. We became very adept at drawing condensers, Erlenmeyer flasks, and Bunsen burners with a template, but rarely did we get a chance to actually perform an experiment. But there were a few. I do recall heating a mix of manganese dioxide and potassium chlorate to produce oxygen, which we collected and tested with a glowing splint. The ability of oxygen to support combustion was driven home as the splint burst into flame. Potassium chlorate, KClO3, we learned, could liberate its oxygen content when heated in the presence of manganese dioxide, which served as a catalyst. It was an “oxidizing agent.” I was so impressed that I snitched a bit of the chlorate to experiment with at home, carrying it in my pocket. Not a smart thing to do, as you’ll discover.
Although I don’t think I realized it at the time, our experiment was very similar to the one Joseph Priestley performed in 1774, which led to the discovery of oxygen. Priestley heated mercuric oxide, HgO, and collected the gas produced. “What surprised me more than I can well express, was that a candle burned in this air with a remarkably vigorous flame . . . .” Priestley, though, didn’t recognize the gas as an element, calling it “dephlogisticated air,” in light of the prevailing belief that flammable materials contained phlogiston, a substance without color, odor, taste, or weight, given off during combustion. “Phlogisticated” substances were those that contained phlogiston and, when burned, produced “dephlogisticated” air, which is what Priestley thought he had isolated.
Upon inhaling this air, he wrote: “The feeling of it to my lungs was not sensibly different from that of common air; but I fancied that my breast felt particularly light and easy for some time afterwards. Who can tell but that, in time, this pure air may become a fashionable item of luxury.” Priestley was right; salons where people go to breathe oxygen have cropped up, usually making nonsensical claims about the benefit of this practice. Amazingly, he had some pertinent observations about the possible hazards of breathing “dephlogisticated air.” “We may infer from these experiments that though pure dephlogisticated air might be very useful as a medicine, it might not be so proper for us in the usual healthy state of the body, for, as a candle burns out much faster in dephlogisticated than common air, so we might, as may be said, live out too fast, and the animal powers be too soon exhausted in this pure kind of air.”
Today we realize that oxygen of course is necessary for life, but inhaling it also results in the production of free radicals, those rogue molecular species that can wreak havoc in the body. Antioxidants in the form of certain vitamins and plant components can curb the destructive effects of oxygen, which account for the attention they receive in both the popular and scientific literature.
Although Priestley usually gets credit for the discovery of oxygen, he certainly was not the first person to produce the gas. Michael Sendivogius, a Polish alchemist, found back in 1604 that heating saltpeter (KNO3) produced what he called “the elixir of life.” Some historians even believe that Cornelis Drebbel, who, using wood and greased leather, designed the world’s first submarine in 1621, explored the possibility of heating potassium nitrate to supply his crew of twelve oarsmen with breathable air. In Sweden, two years prior to Priestley’s experiment, Carl Wilhelm Scheele produced oxygen from mercuric oxide, recorded his observations, but did not publish them until several years later. Priestley, on the other hand, carefully documented his work and published his results promptly. Antoine Lavoisier correctly interpreted Priestley’s experiment as having produced a new element, but did not attribute much credit to the Englishman. Lavoisier maintained this attitude despite historical evidence that his own experiments with oxygen had been prompted by a meeting he had with Priestley. When taken to task on this issue, Lavoisier commented that “those that start the hare do not always catch it.”
While there is controversy about who discovered oxygen, there is no doubt that it came about by heating some oxidizing agent. This fueled the idea that oxidizing agents could enhance combustion. Potassium chlorate, for example, made matches and sparklers possible. Other uses were discovered as well. Sodium chlorate proved to be a good source of oxygen for combining with chlorine to yield chlorine dioxide, a better bleaching agent for paper than plain chlorine. Amazingly, sodium chlorate even turned out to be an effective weed killer. But this weed killer had an occasional bizarre side effect. It caused farmers’ pants to explode!
Ragwort was a hugely problematic weed in fields in New Zealand where dairy cows grazed in the early years of the twentieth century. Some of the alkaloids the plant contains can cause liver failure and kill cattle. So it was with relish that farmers began to spread sodium chlorate on their fields after learning that the chemical was effective in destroying ragwort. What they didn’t realize was that sodium chlorate was a strong oxidizing agent that, when combined with combustible materials like cotton or wool, could cause violent explosions. Even after laundering, pants that had been exposed to chlorate could still hold enough of its residue to produce dramatic effects. In one widely reported case, a farmer’s pants were drying in front of a fire, and exploded with a loud report. He had enough presence of mind to hurl the remnants out of the house, where “they smoldered on the lawn with a series of minor detonations.” I guess this could have happened to the pants I had on when I snitched the chlorate. Luckily, we didn’t dry our clothes by hanging them in front of a fireplace.