Heavy Water
Definition and meaning of Heavy Water in chemistry.
Heavy water is water in which both hydrogen atoms are deuterium (²H), the heavier stable isotope of hydrogen, rather than ordinary protium (¹H), giving the compound the formula D2O.
In more detail
Because a deuterium nucleus carries a neutron in addition to a proton, it is roughly twice as massive as protium, making D2O about 10-11% denser than ordinary water and raising its boiling point to 101.4°C and melting point to 3.82°C. Deuterium occurs naturally at only about 1 atom per 6,400 hydrogen atoms, so most natural water contains it as the mixed molecule HDO rather than pure D2O, which must be concentrated industrially, for example by the Girdler sulfide process or electrolysis. Because deuterium's greater mass slows thermal neutrons effectively while absorbing relatively few of them, heavy water is prized as a neutron moderator in certain nuclear reactor designs.
Key facts
| Formula | D2O |
|---|---|
| Field | Physical Chemistry |
| Molar mass | 20.03 g/mol |
| Boiling point | 101.4°C (vs. 100°C for H2O) |
In CANDU (Canada Deuterium Uranium) reactors, heavy water acts as both the neutron moderator and the coolant, which lets the reactor run efficiently on natural, unenriched uranium fuel.
Frequently asked questions
Is heavy water radioactive?
No. Deuterium is a stable, naturally occurring isotope of hydrogen, not a radioactive one. Heavy water's unusual effects come from its extra mass, not from radioactivity.
Is drinking heavy water dangerous?
Small amounts are harmless and pass through the body normally, but replacing a large fraction of an organism's water with D2O can be toxic because the heavier deuterium slows the rates of essential enzyme-catalyzed reactions.