Hydrogen - Heavy Water

Heavy water is the oxide of deuterium ($^2_1H$ or $D$), represented by the formula $D_2O$. It was discovered by Harold Urey in 1932.

Preparation: It is primarily prepared by the prolonged electrolysis of ordinary water ($H_2O$) containing a small amount of alkali like $NaOH$.

Physical Properties: $D_2O$ is a colorless, odorless, and tasteless liquid. Its physical constants (Boiling Point: $374.42 \text{ K}$, Freezing Point: $281.0 \text{ K}$, Density: $1.106 \text{ g/cm}^3$) are higher than those of ordinary water due to the higher molecular mass of deuterium.

Kinetic Isotope Effect: Chemical reactions of $D_2O$ are slower than those of $H_2O$ because the $D-O$ bond is stronger than the $H-O$ bond, requiring higher activation energy.

Biological Properties: Heavy water is injurious to humans, animals, and plants. it retards the growth of tobacco seeds and inhibits the development of tadpoles.

Nuclear Moderator: $D_2O$ is extensively used in nuclear reactors to slow down fast-moving neutrons to thermal speeds, facilitating a controlled nuclear chain reaction.

Exchange Reactions: When compounds containing reactive hydrogen are treated with $D_2O$, hydrogen atoms are replaced by deuterium, e.g., $NaOH + D_2O \rightarrow NaOD + HDO$.