Three isotopes of hydrogen have been found by scientists in nature, and each of them can combine with oxygen. Hence, one may speak of three kinds of water: protium, deuterium, and tritium waters: H₂O, D₂O and T₂O, respectively.

There may also be �mixed� waters containing, say, an atom of protium and an atom of deuterium, or an atom of deuterium and one of tritium in their molecules. This increases the list of waters: HDO, HTO, and DTO.

But the oxygen contained in the water is also a mixture of three isotopes: oxygen-16, oxygen-17 and oxygen-18, the first being by far the most common. Taking into account these varieties of oxygen, another 12 possible waters can be added to the list.

When you draw a cup of water from a lake or river, you probably never suspect that you have in your cup eighteen different kinds of water. And so water, no matter where it comes from, is a mixture of different molecules, the lightest being H₂O¹⁶ and heaviest, T₂O¹⁸. Chemists can now prepare each of these eighteen kinds of water in the pure form.

Hydrogen isotopes differ perceptibly in their properties. And what about the different kinds of water? They differ in some ways, too. For example, they have different densities, and different freezing and boiling points.

And still the relative content of the different kinds of water in nature is always and everywhere different. For example, tap water contains 150 grams of heavy deuterium water, D₂O, per ton. But its content in the water of the Pacific Ocean is noticeably higher: about 165 grams.

A ton of ice from the Caucasian glaciers contains 7 grams of heavy water more than a cubic metre of river water. In a word, the isotopic composition of water is different everywhere. The reason for this is that there is a mighty process of isotope exchange occurring continuously in nature.

The different hydrogen and oxygen isotopes keep replacing one another under various conditions. Is there any other natural compound with as large a number of varieties? There is not.

Of course, we have to do mainly with protium water. But the other kinds of water cannot be disregarded. Some of them are widely used in practice, especially heavy water D₂O. It is used in nuclear reactors for moderating neutrons which cause uranium fission. Besides, scientists use various kinds of water for investigations in the field of isotope chemistry.

Eighteen kinds, and no more?

Actually, the varieties of water may be much more numerous. Besides the natural isotopes, there are also man-made radioactive isotopes of oxygen: oxygen-14, oxygen-15, oxygen-19, and oxygen-20. And just recently the number of hydrogen isotopes has also increased: we can now speak of H⁴ and H⁵.

Now if we take into account the man-made isotopes of hydrogen and oxygen, the list of possible waters increases to over 100. You can easily count up the exact number yourself�

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