heavy water

American  

noun

1. water in which hydrogen atoms have been replaced by deuterium, used chiefly as a coolant in nuclear reactors.

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heavy water British  

noun

1. water that has been electrolytically decomposed to enrich it in the deuterium isotope in the form HDO or D 2 O

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heavy water Scientific  

1. Water in which deuterium, a heavy isotope of hydrogen, takes the place of hydrogen. Heavy water has physical and chemical properties that are like those of ordinary water, but heavy water is 10 percent heavier and has higher freezing and boiling points. Also called deuterium oxide. Chemical formula: D ₂ O.

2. ◆ Semiheavy water is similar to heavy water, but only one of the two hydrogen atoms in each molecule is replaced with deuterium. Chemical formula: DHO.

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heavy water Cultural  

1. Water with a higher average molecular weight than ordinary water. In a molecule of heavy water, the hydrogen atoms are isotopes in which the nuclei each contain a proton and a neutron, and hence are twice as heavy as a normal hydrogen atom.

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Closer Look

The nucleus of most hydrogen atoms consists of a single proton, but in one isotope of hydrogen, called deuterium or heavy hydrogen, the nucleus also contains a neutron and thus weighs nearly twice as much as standard hydrogen. The substance called heavy water is chemically identical to ordinary water (H₂O), except that the hydrogen atoms in the molecule are the deuterium isotopes (D₂O). Heavy water makes up a small percentage (0.02%) of water naturally occurring on Earth. It is an excellent moderator for nuclear reactions, slowing down the fast neutrons produced in a nuclear fission reaction, increasing the likelihood that the neutrons will successfully collide with heavy nuclei to cause further fission. Although heavy water is chemically nearly identical with ordinary water, it is about ten percent heavier and interferes with cell mitosis if consumed in place of normal water. Ice cubes made of heavy water are denser than ordinary liquid water and will sink to the bottom of a glass of cola.

Etymology

Origin of heavy water

First recorded in 1930–35

Example Sentences

The public infrastructure report listed $150 million for “wet” infrastructure repairs, which included replacing aging and leaky water main pipelines.

From Los Angeles Times • Feb. 19, 2026

Fixing Mexico City’s notoriously leaky water system would help in times of drought.

From Seattle Times • Oct. 18, 2023

Glen Ullin, North Dakota, was first in line for money to replace its leaky water pipes before Washington cut funding by one-third this spring.

From Reuters • Sep. 29, 2022

Billie Eilish performed a perfect version of her track Happier than Ever, which began with her singing softly in the living room of an upside down house, while ankle deep in leaky water.

From BBC • Apr. 4, 2022

"Did you go to Mrs. Hethcote's on West 7—th street yesterday to fix a leaky water pipe?" asked Ransom.

From The Trimmed Lamp, and other Stories of the Four Million by Henry, O.