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.

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