I need to know, is real water made up of heavy water which is formed with deuterium, and light water which is water by protium?
We know that hydrogen has three major isotopes, including protium and deuterium.
We also know that water, $\ce{H2O}$, is made up by hydrogen and oxygen.
All this might mean that real water itself can naturally contain some heavy water - Deuterium oxide. Is this correct?


2 Answers 2


Water has formula H2O. Oxygen has 3 stable isotopes (99.76% 16O, 0.039% 17O, 0.201% 18O), and hydrogen has two (99.985% 1H, 0.015% 2H). Thus, there are 9 natural isotopic configurations for water: 3 possibilities for oxygen, multiplied by 3 possibilities for 2 hydrogens with 2 possible isotopes.

Out of those 9 possible configurations, only 4 have a natural abundance larger than 1 ppm:

                        enter image description here

So, pure water is a mixture of 9 different isotopic configurations. You will note that D2O, which is what is usually called heavy water, is not actually among the 4 more common isotopic configurations given by natural abundance.

Also of note: water that is not “heavy water” is sometimes called “light water”, but never “soft water”. “Soft water” is a term used for a chemical process of natural (non pure) water that has been depleted from certain metal cations. (It is used in contrast to “hard water”.)

  • $\begingroup$ I've submitted an edit to the question which, among other things, changes "soft water" to the more correct "light water". $\endgroup$
    – KeithS
    Aug 27, 2013 at 15:52
  • $\begingroup$ Don't forget that water is highly dynnamic, an O-H bond exists for about 10exp-13 seconds only. This makes the question of molecules in water soemwhat philosophic. $\endgroup$
    – Georg
    Aug 28, 2013 at 11:16
  • $\begingroup$ @Georg yes, water is “highly dynamic”, but your order of magnitude for O–H bond lifetime (0.1 ps) is very wrong… even hydrogen bonds have lifetime of the order of 10 ps $\endgroup$
    – F'x
    Aug 28, 2013 at 11:55

Theoretically, you are correct; water, being chemically composed of hydrogen and oxygen, can contain any isotope of hydrogen (or oxygen) and so there are many isotopic combinations of water. However, if you look at hydrogen's molar mass (which is calculated based on a weighted average of the atomic masses of the two stable isotopes), you'll see that it's much closer to 1g than to 2. This is because Earth's hydrogen content is dominated by the protium isotope, with an atomic mass of 1 dalton. If there were a 50-50 mix between protium and deuterium we'd expect the molar mass to be 1.5g, but as it is, Earth's hydrogen content is only approximately 0.015% deuterium.

So, we could expect, on average, a slightly lower percentage of water to have at least one deuterium atom, because a very few have two. As shown by F'x's chart, the concentration of naturally-occurring deuterium oxide is measured in the parts per billion (22ppb). That's actually a lot of heavy water, if you were to get all of it in one place; there are about 31 trillion kilograms of naturally occurring deuterium oxide on the planet (31 km3). That's enough to fill Lake Mead, created by the Hoover Dam. However, to get one kilogram, or liter, of $\ce{D2O}$ by separating it from light water, you would need to process 45 million kilograms of water. Put another way, there is a little over one gallon of natural deuterium oxide (5L to be more precise) in a typical Olympic swimming pool.


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