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MARGINALIA

Bonding to Hydrogen

The simplest molecule, made for connection

Roald Hoffmann

Flammable Air

Next, I encountered the gas, Henry Cavendish’s inflammable air, in a high school electrolysis experiment. We ran a current through water with a little salt dissolved in it, collected the unequal volumes of gases formed, each trapped in an inverted tube. Both gases gave small pyrotechnic pleasures—one, hydrogen, with that satisfying pop when a newly extinguished splint came near it; the other, oxygen, revived exuberantly the flame of the same splint.

Primo Levi, in an early chapter in his marvelous The Periodic Table, describes an initiation into chemistry that features the same experiment, with more fearsome results:

I carefully lifted the cathode jar and holding it with its open end down, lit a match and brought it close. There was an explosion, small but sharp and angry, the jar burst into splinters (luckily, I was holding it level with my chest and not higher) and there remained in my hand, as a sarcastic symbol, the glass ring of the bottom.… It was indeed hydrogen, therefore: the same element that burns in the sun and stars, and from whose condensations the universes are formed in eternal silence.

2012-09MargHoffmannFA.jpgClick to Enlarge ImageIn my high school lab I had no idea that I was reliving, with different methods, part of the experiment Antoine Laurent Lavoisier thought important enough over two days in February 1785 to invite a select group of luminaries of French science to witness. In a tour de force of the big science of his day, using some remarkable instruments he had constructed at his own expense, he decomposed water into its constituent hydrogen and oxygen, and followed that by a recombination of the elemental gases thus generated into water. Henry Cavendish had proved that water is formed in the combustion of hydrogen some years before; Lavoisier not only decomposed water, but determined that the cycle of its decomposition and reformation proceeded with conservation of mass. Not everyone was convinced—they should have been—yet with this experiment, a new chemical age dawned. Water and air, those seemingly homogeneous elements of the Greeks, were shown to be a compound and a mixture, respectively.




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