How is heavy water different from ordinary water?

Heavy water, or deuterium oxide ($D_{2}O$), differs from ordinary water ($H_{2}O$) primarily in its isotopic composition and physical properties. While ordinary water consists of two atoms of common hydrogen and one of oxygen, heavy water replaces the ordinary hydrogen with deuterium—a hydrogen isotope containing one proton and one neutron, which doubles the mass of the hydrogen nucleus.12

This difference in mass results in several distinct characteristics:

• Molecular Weight: Heavy water has a molecular weight of approximately 20, compared to about 18 for ordinary water.1
• Physical Properties: Heavy water is denser than ordinary water and has higher melting and boiling points. For instance, its melting point is 3.81 °C, whereas ordinary water melts at 0 °C.3
• Chemical Reactivity: While deuterium forms compounds equivalent to those of ordinary hydrogen, it reacts more slowly. This kinetic-isotope effect makes deuterium useful as an isotopic tracer in scientific research.23

In practical applications, heavy water serves as a neutron moderator in nuclear power plants because it slows down neutrons without absorbing them significantly. Notably, high concentrations of heavy water are toxic to plants and animals, as it disrupts delicate biological processes.13