D2O

D2O, also known as heavy water, is water in which the two hydrogen atoms are the isotope deuterium, giving the chemical formula D2O. Deuterium is a stable hydrogen isotope with one proton and one neutron. Heavy water's physical properties differ slightly from ordinary water: it has a higher melting point (about 3.82 °C) and boiling point (about 101.4 °C), and a higher density (approximately 1.105 g/mL at 25 °C). It remains fully miscible with H2O.

Production of heavy water occurs on an industrial scale through isotope exchange and electrochemical processes. Common methods include exchange between water and deuterium-containing compounds such as hydrogen sulfide (the Girdler-Sulfide process) and electrolysis to concentrate the deuterium content. Distillation and other separation techniques are often used in combination to achieve higher enrichment.

The principal use of D2O is as a neutron moderator and coolant in certain nuclear reactors, notably CANDU reactors, where it enables the use of natural uranium fuel. Beyond reactors, heavy water serves in scientific research and industry as a solvent and tracer. It is used in deuterium labeling and in metabolic studies, and in nuclear magnetic resonance (NMR) applications as a deuterium source or solvent in experiments that require reduced background proton signals.

Safety and environmental aspects: D2O is not radioactive, but large-scale replacement of body water with heavy water can disrupt biological processes due to the kinetic isotope effect. It is chemically toxic at high levels, but ordinary handling poses no special hazards beyond standard chemical safety.