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17.  How Many Waters
Are There on Earth!

   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.

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