How does heavy water differ from ordinary water?

Heavy water (D₂O) differs from ordinary water (H₂O) primarily in its isotopic composition and molecular weight. While ordinary water is composed of two atoms of common hydrogen and one atom of oxygen, heavy water is composed of two atoms of deuterium—a hydrogen isotope containing one proton and one neutron—and one atom of oxygen.12

Because deuterium has double the mass of ordinary hydrogen, heavy water has a molecular weight of approximately 20, compared to about 18 for ordinary water. This difference in mass influences physical properties, such as volatility; heavy water is less volatile than ordinary water, a property utilized in its separation through fractional distillation.1

Chemically, while deuterium forms compounds equivalent to those of ordinary hydrogen, it reacts more slowly, a distinction that makes it valuable as an isotopic tracer in scientific research. Additionally, heavy water serves as a neutron moderator in nuclear power plants, where it helps sustain fission chain reactions by slowing down high-energy neutrons.213