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Thought experiment

Given two glasses of water, how would one detect which glass contains heavy water, and which contains potable water without using complicated laboratory equipment?

Something like the way a piece of litmus paper (yes, it is laboratory equipment and easily available to the layman at that!) may be used to test whether a solution is an acid, or alkali.

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    $\begingroup$ heavy water has higher boiling and melting points and is slightly toxic for eukaryotes. It is also heavier (1.1 g/ml) then normal water. $\endgroup$
    – permeakra
    Commented Oct 21, 2012 at 5:43
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    $\begingroup$ This is just a hunch, but I imagine if you filled a small waterbomb from each glass, and placed it in the opposite glass, you would see one sinking (in the normal water) and one floating (in the heavy water) $\endgroup$
    – wim
    Commented Oct 21, 2012 at 11:05
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    $\begingroup$ Not sure how big your glass is, but would a 7 dollar hydrometer be "complicated laboratory equipment"? A small volumetric flask (10 ml) and a balance would be less than $50. $\endgroup$
    – MaxW
    Commented Oct 27, 2015 at 3:58
  • $\begingroup$ Why would you oppose heavy water and potable water, to begin with? $\endgroup$
    – Ivan Neretin
    Commented Jun 3, 2018 at 18:17

7 Answers 7

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There's a very simple test for $\ce{D_{2}O}$ that springs to mind - ice cubes made with heavy water sink in light water. I assume you're talking about differentiating a glass of essentially pure heavy water from, say, tap water, in which case this test should work rather well and requires equipment no more sophisticated than a freezer, ice cube tray and some glasses.

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  • $\begingroup$ Cool (+: But could there be something still simpler - akin to the boiled egg floats in salt-water? As an extreme example, a person out trekking may not have a freezer ^ power-source handy. $\endgroup$
    – Everyone
    Commented Oct 21, 2012 at 4:52
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    $\begingroup$ @Everyone - You could make a custom float that floats on heavy water but sinks in light water. Quantitative floats similar to this are used, for instance, to estimate alcohol concentrations in distillates from hobby stills. $\endgroup$
    – Richard Terrett
    Commented Oct 21, 2012 at 5:43
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    $\begingroup$ There is a much simpler test. Just weigh each one, or even lift up each one and feel which is heavier. $\endgroup$
    – Canageek
    Commented Oct 21, 2012 at 23:15
  • $\begingroup$ This would not work if the regular water has enough dissolved impurities... $\endgroup$
    – Alex
    Commented Nov 11, 2022 at 19:07
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If you don't have access to a freezer, you could allow sun light to pass through your glass vessels and then through a prism. The heavy water will absorb less orange red light and the difference might be observable.

enter image description here

One of these might help (not complicated).

P.S. I did not want to post this as an answer because I'm not sure if it's realistic but I can not see were to add a comment.

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    $\begingroup$ You can't add comments unless you have earned enough reputation. Besides that, I think this qualifies as an answer, although you will need a considerable layer thickness to see the differences. $\endgroup$
    – Antimon
    Commented Oct 21, 2012 at 20:53
  • $\begingroup$ @Antimon Thanks for the clarification Sb. I was uncertain if the naked eye would be able to tell the difference between H2O and D2O. Do you think this might work better if one substituted the sun with an artificial light source that emitted at a wavelength that was absorbed, Like a 660nm LED? $\endgroup$
    – Cargo
    Commented Oct 21, 2012 at 21:19
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    $\begingroup$ The problem lies in the Beer-Lambert law and the low extinction coefficient of water, not in the light source. As you can see in the spectra you posted, there is no significant absorption or characteristic bands in the visible region. $\endgroup$
    – Antimon
    Commented Oct 21, 2012 at 21:50
  • $\begingroup$ Pity one could not make use of that bump at around 950 nm, like with a long darkened tube with a InGaAs photodiode at one end and a GaAs IR LED at the other. $\endgroup$
    – Cargo
    Commented Oct 22, 2012 at 0:11
  • $\begingroup$ Why couldn't you? $\endgroup$
    – Antimon
    Commented Oct 22, 2012 at 8:40
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Mass. Heavy water is noticeably heavier then light water. Simply weigh an equal amount of each water and see which weights more.

Light water weights 1 g/mL (0.99700 g/mL at 25 °C)

Heavy water weights 1.107 g/mL at 25 °C

So if you have 100 mL you will see an 11 g difference, assuming pure samples.

(The Science and Technology museum in Ottawa actually has samples you can lift to feel this).

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Old question, but in the interim Cody's lab has released this video (among others) on tasting heavy water. His conclusion was that $\ce{^2H_2}\ce{^{16}O}$ tasted sweeter than $\ce{^1H_2}\ce{^{16}O}$ but $\ce{^1H_2}\ce{^{18}O}$ didn't. He wasn't able to tell the difference after eating. Of course, chemists absolutely freak out at the suggestion that one attempt to identify chemicals by tasting them, normally an extremely dangerous practice.

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(Completely untested since, sadly, I have no access to heavy water.) Slowly pour the contents of one glass into the other. The index of refraction of heavy water is 1.328, that of light water is 1.333. Not much, but maybe just enough to see the inhomogeneities. If the decanted glass's contents sink, it had the heavy water. Otherwise, light water.

But if you have even a cheap kitchen scale, that would probably be the better test, since the densities differ by about 10%, as opposed to the index of refraction, which differs by far less.

Or, just add a drop of food colouring to one of the glasses. Or a drop of milk. Just to help out with the contrast.

Do the test with just a small sample from each glass if you want to avoid contaminating the expensive heavy water.

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  • $\begingroup$ What would you be looking for? $\endgroup$
    – Alex
    Commented Nov 11, 2022 at 19:29
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The simplest way of checking the nature of your water is related to the difference in density. If you dissolve $80$ g NaCl in $1$ liter ordinary water $H_2O$, you obtain a solution with a density $1.056$ g/mL. You may add a little bit of an ink to make the demonstration more visual. Now you pick up maybe 1 mL of this NaCl solution with a pipette, and let fall one drop in the water sample. One drop of this solution will go to the bottom of pure water $H_2O$. But it will float on $D_2O$ as it has a density $1.107$ g/mL.

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  • $\begingroup$ No, it won't float -- it will get mixed in the other glass either way. $\endgroup$
    – Alex
    Commented Nov 11, 2022 at 19:14
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If you have two glasses—knowing that one contains deuterated water, the other contains potable water; one might determine the contents by freezing both samples in a freezer at $\pu{0 ^\circ C}$. Since the melting point of the potable water is $\pu{0 ^\circ C}$ and the melting point of the deuterated water, $\ce{D2O}$, is $\pu{3.8 ^\circ C}$, the $\ce{D2O}$ will take longer to melt assuming they are side-by-side.

I would add, having done this, practically speaking: this means you can hold the glass of frozen $\ce{D2O}$ in one hand and $\ce{H2O}$ in the other and while the regular water melts, the frozen $\ce{D2O}$ takes noticeably longer to melt. And on a side note: imagine some of the toxicity in Eukaryotes is due to the subtle differences in thermal properties between these two water species.

Be aware: $\ce{D2O}$ must be in an enclosed container, otherwise it will exchange heavy hydrogen in the water for atmospheric light hydrogen in atmospheric moisture. If left uncovered, the experiment may work a couple times at first; however, it will become less effective (as the deuterium exchanges and evaporates).

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  • $\begingroup$ Why was this downvoted? If you're doing so can you atleast give a reason in comments, so that other people can understand why this answer was not helpful $\endgroup$
    – Micelle
    Commented Mar 15, 2020 at 19:41
  • $\begingroup$ probably because it's yet-another abstract principles-answer, largely repeating what's already been answered, and still not addressing the question of a field test for two glasses of water, one of which contains more D2O than the other. i.e. without a 'pure specimen body of D2O in vessel #2' $\endgroup$
    – New Alexandria
    Commented Jan 23, 2022 at 17:52
  • $\begingroup$ Probably because you can just put the two into a refrigerator, at a temperature where one would melt and the other one not -- this way you don't have to go through the trouble of covering it, observing it while it melts, etc. $\endgroup$
    – Alex
    Commented Nov 11, 2022 at 19:17

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