Semiheavy water

Semiheavy water is water whose molecule contains one protium atom and one deuterium atom, giving it the formula HDO. It is one of water’s isotopologues.

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Deuterium: Deuterium is the stable hydrogen isotope whose nucleus contains one proton and one neutron. Replacing one protium atom in ordinary water with deuterium produces HDO.
Isotopic exchange: Isotopic exchange is the redistribution of isotopes among molecules through chemical reactions or molecular encounters. Exchange between H₂O and D₂O generates HDO.
Isotope hydrology: Isotope hydrology uses natural isotopic variations in water to study the movement, sources, and history of water. HDO contributes to isotope measurements used to trace hydrological processes.
Light water: Light water is ordinary water composed predominantly of H₂O molecules. It contains protium rather than the deuterium present in HDO.
Protium: Protium is the hydrogen isotope whose nucleus contains a single proton and no neutrons. The protium atom in HDO pairs with deuterium.
H₂O: H₂O is a water molecule containing two protium atoms and one oxygen atom. It is the protium-only isotopologue that exchanges with heavy water.
Isotope-ratio mass spectrometry: Isotope-ratio mass spectrometry measures the relative abundances of isotopes in a sample. It can quantify deuterium enrichment in water samples containing HDO.
Heavy water: Heavy water is water composed primarily of D₂O molecules. Its two deuterium atoms distinguish it from semiheavy water.
Isotopologue: An isotopologue is a molecular species that differs from another by the isotopes present in its atoms. HDO is distinguished from other water molecules by its isotope combination.
D₂O: D₂O is a water molecule containing two deuterium atoms and one oxygen atom. Its exchange with ordinary water forms semiheavy water.