The weight of water

It's the UN's international year of the periodic table, so I've been writing and thinking about the elements. While writing a piece for Nature Chemistry about the hidden depths of the periodic table (the more than 3000 isotopes that could be stacked onto their elemental spots), I wandered across an interesting set of papers on heavy water and isotopic tracing, which along with some questions my students had in the intro chem course led to another piece for Nature Chemistry (The weight of water).

Heavy water (D₂O) is water where the hydrogens have been replaced with deuterium, an isotope of hydrogen that weighs about twice as much as standard hydrogen. Heavy water weighs just over 10% more than regular water, a tablespoon weighs only about a gram more, so it is probably not noticeable should you heft a glass of it.  (Heavy water was a key element of some nuclear weapons programs, check out the harrowing story of the raids on a Norwegian heavy water production facility in WW II.)

In one of the papers I read, future Nobelist George de Hevesy stuffed some twenty (albeit tiny) goldfish into 60 ml of water, in another he reports drinking heavy water and making thousands of distillations of urine to recapture the water, measure its density and track deuterium through the human body.

Drinking your experiement sounds dicey, but in small amounts heavy water is safe to drink, and as I recently learned, used in human metabolic studies in doses of about 10 ml. An interesting question raised in the papers I read was about the taste of heavy water. One report suggests a burning sensation might be felt when drinking it, another (by Harold Urey, who discovered deuterium) suggests it tastes like undeuterated water. But other reports say it tastes sweet.



With a bit of help from my youngest son, I set up a repeat of Urey's blind taste test. And was surprised to find I could indeed taste the difference. It is sweet.

And for the next few weeks, until the last of the extra deuterium clears my systems, I'll be just a little bit heavier than usual.

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Shameless self-promotion, George de Hevesy is one of the Catholic scientists featured in the audio series I did last year with Brother Guy Consolmagno, SJ, the director of the Vatican Observatory.