# More heavy water stuff **URL:** \ **Category:** Cecil's Columns/Staff Reports\ **Created:** [December 9, 2003, 8:18pm UTC](https://boards.straightdope.com/t/more-heavy-water-stuff/218007 "2003-12-09T20:18:16Z")\ **Posts on this page:** 5\ **Page:** 1
**Author:** ![Ludovic](https://avatars.discourse-cdn.com/v4/letter/l/7ab992/32.png) [@Ludovic](https://boards.straightdope.com/u/Ludovic)\ **Post date:** [December 9, 2003, 8:18pm UTC](https://boards.straightdope.com/t/more-heavy-water-stuff/218007/1 "2003-12-09T20:18:16Z")
From the [Mailbag](http://www.straightdope.com/mailbag/mheavywater.html): > [@](#): > > Despite the fact the **light water and heavy water are chemically identical** , heavy water is mildly toxic. How can this be? Since heavy water is heavier than normal water, the **speed of chemical reactions involving it is altered** somewhat, as is the strength of some types of bonds it forms. This affects certain cellular processes, notably mitosis, or cell division, due to the **difference in binding energy** in the hydrogen bonds needed to make certain proteins. Hmmm, it would appear that, since deuterium has different binding energies and different speeds of reaction, that it is NOT chemically identical to protonium. ---
**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\ **Post date:** [December 9, 2003, 8:57pm UTC](https://boards.straightdope.com/t/more-heavy-water-stuff/218007/2 "2003-12-09T20:57:15Z")
“Chemically” refers to the fact that light and heavy water both participate in the same chemical reactions, and have the same electron structure. It is the electron structure that determines the way a substance reacts with other substances. Although the mass difference is resposible for altering reaction times and binding energies, this doesn’t affect the fundamental chemistry. All atomic isotopes of a given element are considered chemically identical, despite similar differences in reaction times and binding energies–it just happens that these differences are more pronounced in hydrogen and it’s isotopes. ---
**Author:** ![cjclark](https://avatars.discourse-cdn.com/v4/letter/c/7bcc69/32.png) [@cjclark](https://boards.straightdope.com/u/cjclark)\ **Post date:** [December 10, 2003, 3:23am UTC](https://boards.straightdope.com/t/more-heavy-water-stuff/218007/3 "2003-12-10T03:23:13Z")
The electronic structure of hydrogen and deuterium is slightly different. Measuring the mass difference between the two by measuring their spectra is a classic university freshman physics lab experiment. As for D2O and H2O, they have significantly different rotational and vibrational spectra, for reasons that are pretty obvious when you think about it a second. I’m not sure how different their electronic spectra are or if the difference is even measureable. My personal experience with deuterium is its use to replace hydrogen in various compounds neutron scattering spectra. The claim was usually made that the deuterized compounds were very chemically similar to the “normal” variety but not identical. After all, the whole point of using a deuterized agent is that it is different from one with the normal D/H distribution. To open a different can of worms, are chiral isomers “chemically identical?” They will have the exact same physial properties (melting, boiling, triple point, heat capacities, spectra, etc.), but they don’t pass the “same chemical reactions” test since chiral molecules have different biological activity (due to specific reactions with chiral biological chemicals). ---
**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\ **Post date:** [December 10, 2003, 3:51am UTC](https://boards.straightdope.com/t/more-heavy-water-stuff/218007/4 "2003-12-10T03:51:07Z")
For purposes of the Staff Report, I decided that “chemically identical” would mean having the same chemical formula and participating in the same chemical reactions, producing the same products. Unfortunately, there as as many ways to define this as there are people writing about it. Perhaps my wording could have been clearer, nevertheless, I think the meaning is clear enough in the context of the Report. ---
**Author:** ![Ludovic](https://avatars.discourse-cdn.com/v4/letter/l/7ab992/32.png) [@Ludovic](https://boards.straightdope.com/u/Ludovic)\ **Post date:** [December 10, 2003, 2:36pm UTC](https://boards.straightdope.com/t/more-heavy-water-stuff/218007/5 "2003-12-10T14:36:58Z")
I guess in the chiral example above, might one say they were “physically identical” but not “chemically identical”, physical and chemical meaning highly specific things that one would need to further define in a lay text, (and perhaps use a different word in industry press?) Even though neither chiral compounds nor isotopes are either when you really press it. Of course, when you really press it the other way, I _did_ know what you meant: the reason I wrote here is perhaps more for further definition on the term “chemically identical” rather than to nitpick. Thanks for not reaming me out for nitpicking 🙂