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I believe I saw this claim somewhere on the internet a long time ago. Specifically, it was claimed that the difference could be observed by filling one long, straight tube with light water and one with heavy water, and looking through both tubes lengthwise (so that light has to travel through the tubes' lengths before reaching the eye), whereupon the light water would appear blue as it does in the oceans, and the heavy water would not. The explanation given was that heavy water has a different vibrational spectrum because of the greater mass of the $^2$H atom, which seemed perfectly plausible.

However, I am no longer able to find a source for this claim, which is strange because if it were true, surely it would not be so difficult to find a source?

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    $\begingroup$ I didn't even know that normal water is blue. But it seems to be so. $\endgroup$
    – Mindwin Remember Monica
    Apr 4, 2017 at 13:52
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    $\begingroup$ @Mindwin Very interesting article. To me, though, this begs the question "How many meters of a given substance must light penetrate through before we can determine its 'true colour'?" $\endgroup$
    – Steve-O
    Apr 4, 2017 at 15:28
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    $\begingroup$ @Steve-O - the absorption length of water depends a lot on wavelength. At the minimum (418 nm) it takes 227 m to reduce the intensity of the light to 1/e. See this answer on the physics site. Funny that questions about the light absorption of water appear almost simultaneously on both sites. Coincidence? $\endgroup$
    – Floris
    Apr 4, 2017 at 22:35
  • $\begingroup$ A five-hundred-gallon polyethylene tank of pure [RO] water is a distinct strikingly blue color under room lighting $\endgroup$
    – jimchmst
    May 1, 2022 at 3:50

5 Answers 5

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Based on your description, I may have found the article you originally saw, or at least one very similar.

Researchers from Dartmouth College published a paper$\mathrm{^1}$ in which they report, among other things, the results of viewing sunlit white paper through two 3 meter lengths of plexiglass; one filled with $\ce{H2O}$ and one with $\ce{D2O}$. Sure enough, because of the lower frequency of the maximum absorption of $\ce{D2O}$ in the red to near IR wavelengths, the blue color that is characteristic of $\ce{H2O}$ is far less pronounced in $\ce{D2O}$. This website is based on the published paper and additionally shows a photograph of the blue colored $\ce{H2O}$ on the left with the far less colored $\ce{D2O}$ on the right:

Tube of H2O (blue,left) and an empty tube (clear,right)

  1. "Why is Water Blue", Charles L. Braun and Sergei N. Smirnov, J. Chem. Edu., 1993, 70(8), 612
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    $\begingroup$ Just a note: this website (the one linked in Tyberius' answer) contains a version of the same picture shown here, but the description there suggests that the right tube is actually empty. $\endgroup$
    – GOTO 0
    Apr 4, 2017 at 15:51
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    $\begingroup$ @GOTO0 , that is definitely the same picture, thought the fact no source is given is disturbing. The site I linked to is a reproduction of their article published in the Journal of Chemical Education. They fully cite the article, though it only links to an abstract and I don't have free access to the whole thing. The bottom line though is that assuming the Dartmouth College group is not flat out plagiarizing something, their article and descriptions of their work (including the photographed water set up) must be the correct one. They probably wouldn't be happy about the use in the other link. $\endgroup$
    – airhuff
    Apr 4, 2017 at 20:53
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    $\begingroup$ @GOTO0, I have notified the original authors of the link you cited. I will post any response they have regarding the photograph. Thanks again for bringing this to my attention. $\endgroup$
    – airhuff
    Apr 4, 2017 at 21:17
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    $\begingroup$ @airhuff I completely overlooked that the image I would be looking for was in my link. However the description in both the reproduction and the original paper(I have access through school) suggests that the right tube is empty. "The large tube volume and a limited budget precluded checking to see if light transmitted through a D2O filled tube was indeed white, as expected.". Unless the image was obtained after the original article (there are no pictures in the original article) it seems as though it is prohibitively expensive to attempt. $\endgroup$
    – Tyberius ♦
    Apr 4, 2017 at 23:30
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    $\begingroup$ @Tyberius , thx so much for your efforts looking into this. I did think that picture looked pretty low quality for a publication. A good high res picture would have made all this pretty obvious. The text sure seems clear about it though. I do hope to hear back from the primary author from Dartmouth, but I'm not holding my breath. $\endgroup$
    – airhuff
    Apr 5, 2017 at 0:32
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This does seem to be the case. I don't have images of the different types of water, but I did find this overlaid IR-visible spectrum of water and heavy water:

Water and heavy water spectrum

As you stated, the presence of deuterium shifts the absorbance spectrum of heavy water further into the IR region, rendering it colorless.

The website I found this on (http://www.webexhibits.org/causesofcolor/5B.html) goes into great detail on why water, and various other compounds, exhibit particular colors.

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  • $\begingroup$ The spectra show very clearly why $\ce{D2O}$ is preferred over $\ce{H2O}$ in certain NIR experiments, it is simply more transparent in the respective spectral region, or in other words, has a different color. $\endgroup$
    – Snijderfrey
    May 1, 2022 at 15:57
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If you saw it during the 1990s online era, it might well have been reference links on my old website, from this article on physics misconceptions in grade school.

I'd been doing some textbook-consulting back then, and this page was my effort to stir up trouble/perform viral education/convert innocent children into critical thinkers. I was camping out on appropriate Newsgroups as well as many 1990s physics forums, and encountering controversy from those who refused to accept that water could have a blue color (since after all, no grade-school science text ever mentioned this.) And yes, the non-blue of D2O was a continuing topic of the time.

Besides JCE journal and the Dartmouth paper, another reference from the article is Water Absorption Spectrum, from LSB Univ., which takes an in-depth look at H2O vibrational absorption, and specifically points out the flat spectrum of D2O, and its coloration entirely due to Rayleigh scattering.

enter image description here

Unforunately nobody maintains heavy water in kiloliter, white-interior, open-top tanks. :) If you're going to set one up yourself, I recommend a white-painted stepped-depth setup, to display the color effect as with this example with H2O, or another.

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  • $\begingroup$ Aren't the walls of the pool blue in your "example with H2O", invalidating the proof? $\endgroup$
    – Ruslan
    Apr 23, 2022 at 17:50
  • $\begingroup$ @Ruslan in what way? If the water was removed, would the top of the lower step still be blue? Let me find white marble or white tile swimming pools i.pinimg.com/564x/f3/00/cc/f300ccbf3870a4c07833693fb95cfe75.jpg or media.architecturaldigest.com/photos/5cdf2d7f711f00f6203fc2f1/… $\endgroup$
    – wbeaty
    Apr 29, 2022 at 4:23
  • $\begingroup$ Yeah, these are much more convincing examples. I suppose you may be right about the top step, but still it's better to clear any possible doubts and use something without blue background (as you do now). $\endgroup$
    – Ruslan
    Apr 29, 2022 at 6:10
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The problem with the answer given in both of the other two responses (so far) is that they claim that water has the unusual property of having transmitted red light filtered/absorbed so that the transmitted light is blue. I have zero problem with that. The authors of the J.Chem Ed. piece go on to talk about the looking at reflected light from a Colorado lake or the Caribbean. There, you have to be careful, reflected light is clearly not the same as transmitted light and the inclusion of those scenes is a distraction. The two tubes are clearly indicated to be left = water, right = air (due to the cost of D2O being above their budget). I also have zero problem with the assertion that the visible absorption spectra of heavy water is, essentially, flat (although I'd prefer a higher resolution spectra as well as a baseline which is level). (This statement ignores non-linear optical effects, which is quite reasonable.) However there's a clear difference between saying there's little absorbance and that the heavy water is colorless. Scattering will occur. Even in ultra-pure D2O, scattering will occur. see Rayleigh Scattering. It is inherent in the electrical nature of matter. So, blue wavelengths will be more scattered than the reds, regardless of other specific interactions. Heavy Water is colorless and the sky is not blue. If you're ok with that statement, then ok, we're done but perhaps you should take a look outside on a clear day, if you are lucky enough to live somewhere where we haven't fouled the air with pollution. Now, it takes miles of sky, I'm not sure how much liquid water it would take to have a noticeable scattering effect. Anyway, a reductive approach to the question of color should, sooner or later synthesize all of the various causes of color back into the (gesalt) whole. If X is a function of A,B,& C, then showing that one material's X is dominated by A while another material's X isn't doesn't say squat about what X is for the second material. You need to include all contributions. For what it's worth, being completely colorless would be a balancing act amongst all the factors (A, B, C, ..) and would be quite unusual. We usually just consider the color to be relevant to some expected optical path length, so color is typically context dependent. Reflected, transmitted, short path length, long, white light, colored light, etc., etc.

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  • $\begingroup$ agreed the "color of water" is just like the "color of the sky" mainly due to Rayleigh Scattering, no reason to think that D2O would have markedly different properties on the scattering. $\endgroup$
    – Dan S
    Apr 5, 2017 at 12:56
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    $\begingroup$ I actually agree that this is a good answer, though I think you could format it better and try to limit your answer to one cohesive point to make it better. $\endgroup$
    – Tyberius ♦
    Apr 6, 2017 at 5:11
  • $\begingroup$ "'color of water' is just like the 'color of the sky' mainly due to Rayleigh Scattering" @DanS not true. However, that was the conventional explanation from pre- ~1930s, never mind that the deep underwater environment isn't colored sunset-red! The situation is non-ideal, since thick water is a blue-colored filter and often a white sand bottom is present. Also we usually don't encounter utterly silt-free water. E.g. famous blue Crater Lake contains large-particle "glacier milk," and would appear milky white if not for the water's inherent blue color. $\endgroup$
    – wbeaty
    Apr 6, 2017 at 9:11
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    $\begingroup$ @alphonse Separate the examples? Caribbean beach, white sand & 0-10M depths: we see brilliant turquoise/blues against the submerged white background. Rayleigh colors are overwhelmed by water's blue-filter effect. Next, deep ocean, which appears black if no scattering. But Rayleigh scattering returns light from +10M depths, which light is deep blue from water's absorption spectrum. Next, deep D2O oceans appear sky-blue from Rayleigh scattering, and divers encounter a sunset-red-lit environment. D2O Caribbean beach scenes might appear aerogel-gray: shallow D2O over a white sand reflector. $\endgroup$
    – wbeaty
    Apr 6, 2017 at 9:30
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Pure water in large quantities is a definite striking blue color. The first time I saw a large PE tank of reverse osmosis water I was startled by the blue color. It is blue because of the second overtone of the O-H stretch at about 3400 wavenumbers absorbs near 700nm removing some red light. The O-D stretch is about 2400 wave numbers so its overtones will still be in the near infrared and does not affect the visible spectrum.

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