Heavy water, also known as deuterium oxide (D₂O), is a form of water where the usual hydrogen atoms (protium, ^1H) are replaced by deuterium (^2H or D), a heavier isotope of hydrogen that contains one proton and one neutron, making it roughly twice as heavy as ordinary hydrogen

. Because of the presence of deuterium, heavy water has distinct physical and chemical properties compared to normal water (H₂O). It is about 10–11% denser, has a higher melting point (about 3.82°C compared to 0°C for ordinary water), and a slightly higher boiling point. The hydrogen-oxygen bonds in heavy water are stronger due to the heavier isotope, affecting its biochemical behavior

. Heavy water is not radioactive in its pure form since deuterium is a stable isotope. It naturally occurs in very small amounts in normal water—about one molecule of heavy water per 20 million molecules of ordinary water

. A key application of heavy water is as a neutron moderator in certain types of nuclear reactors, such as CANDU reactors. It slows down neutrons effectively without absorbing many of them, allowing reactors to use natural (unenriched) uranium as fuel. This property makes heavy water valuable in nuclear energy production

. Biologically, heavy water can be toxic to multicellular organisms if it replaces a large fraction of normal water (over 50%) because it disrupts cellular processes like enzyme function and cell division. However, small amounts are harmless, and some bacteria can survive in heavy water environments

. In summary, heavy water is water composed of deuterium atoms instead of ordinary hydrogen, giving it unique nuclear and physical properties, with important uses in nuclear reactors and scientific research. It is chemically similar but physically heavier and biologically distinct from regular water