Deuterium has primarily two uses, as a tracer in research and in thermonuclear fusion reactions. A tracer is any atom or group of atoms whose participation in a physical, chemical, or biological reaction can be easily observed. Radioactive isotopes are perhaps the most familiar kind of tracer. They can be tracked in various types of changes because of the radiation they emit.
Deuterium is an effective tracer because of its mass. When it replaces protium in a compound, its presence can easily be detected because it weights twice as much as a protium atom. Also, as mentioned above, the bonds formed by deuterium with other atoms are slightly different from those formed by protium with other atoms. Thus, it is often possible to figure out what detailed changes take place at various stages of a chemical reaction using deuterium as a tracer.
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