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Deuterium oxide (D₂O), aka “heavy waterâ€�, is the form of water that contains two atoms of the ²H, or D, isotope. The term heavy water is also used for water in which ²H atoms replace only some of the ¹H atoms. In this case, rapid exchange between the two isotopes forms twice as many “semiheavyâ€� HDO molecules as D₂O.

Harold Urey, the 1934 Nobel Prize laureate in chemistry, . In 1931, he and his colleagues at Columbia University (New York City) carefully distilled 5 L of liquid nitrogen to produce 1 mL of molecular deuterium. Shortly afterward, they produced D₂O from ordinary water by using prolonged electrolysis.

For decades, D₂O has been extremely useful in many chemical applications. The difference between a reaction rate in D₂O solvent versus that in H₂O often provides clues as to the reaction’s mechanism. This is especially important if water is one of the reactants.

In some nuclear reactors, D₂O is used to slow down neutrons so that they react with fissionable ²³⁵U rather than nonfissioning ²³⁸U, thus eliminating the need for uranium enrichment. D₂O is superior to H₂O for this use because of its â‰�6 times greater thermal neutron capture cross section.