Heavy Water
Heavy Water (D₂O)
Heavy water, chemically called deuterium oxide (D₂O), is a form of water that contains a larger than normal amount of the hydrogen isotope deuterium (²H or D), rather than the common hydrogen-1 isotope (¹H or H) that makes up most of the hydrogen in normal water.
Properties of Heavy Water
The physical and chemical properties of heavy water differ slightly from those of ordinary water (H₂O). Here are some of the key properties:
- Molecular Weight: The molecular weight of D₂O is about 20 g/mol, which is higher than that of H₂O, which is approximately 18 g/mol.
- Boiling Point: Heavy water has a boiling point of 101.4 °C, which is slightly higher than the 100 °C boiling point of normal water.
- Freezing Point: The freezing point of heavy water is 3.8 °C, compared to 0 °C for H₂O.
- Density: At room temperature, heavy water has a density of about 1.105 g/cm³, which is more than the density of ordinary water (about 1.000 g/cm³).
Production of Heavy Water
Heavy water is produced using several methods, including:
- Fractional Distillation: Exploiting the differences in boiling points between H₂O and D₂O.
- Electrolysis: Heavy water is slightly less likely to dissociate into ions than ordinary water, so prolonged electrolysis of water can concentrate D₂O.
- Chemical Exchange Processes: Such as the Girdler sulfide process, which relies on the exchange between hydrogen sulfide (H₂S) and water.
Uses of Heavy Water
Heavy water is used primarily as a neutron moderator and coolant for nuclear reactors. Deuterium atoms in heavy water have a lower absorption cross-section for neutrons than the protium (¹H) in light water. This makes D₂O a good moderator in nuclear reactors, especially those using natural uranium without the need for enrichment.
Differences Between Heavy Water and Ordinary Water
Here is a table summarizing the differences:
| Property | Heavy Water (D₂O) | Ordinary Water (H₂O) |
|---|---|---|
| Molecular Weight | 20 g/mol | 18 g/mol |
| Boiling Point | 101.4 °C | 100 °C |
| Freezing Point | 3.8 °C | 0 °C |
| Density | ~1.105 g/cm³ | ~1.000 g/cm³ |
| Neutron Absorption | Low | High |
| Cost | Expensive | Inexpensive |
Health Effects
While heavy water is not radioactive and not significantly toxic to humans, replacing a large proportion of the body's water with D₂O can be harmful because it can disrupt biological processes. The deuterium in heavy water can replace the normal hydrogen in the body's cells, which can interfere with cellular processes and protein functions.
Examples and Applications
Nuclear Reactors
One of the most notable examples of heavy water use is in the CANDU (CANada Deuterium Uranium) reactor. This type of reactor uses natural uranium as fuel and heavy water as both the moderator and coolant.
Neutron Scattering
Heavy water is also used in neutron scattering experiments in physics and chemistry due to its low neutron absorption cross-section.
Isotope Effects in Chemistry
Heavy water is used in studies of chemical reactions to investigate isotope effects. For example, reactions may proceed at different rates when ordinary hydrogen is replaced with deuterium.
NMR Spectroscopy
In nuclear magnetic resonance (NMR) spectroscopy, heavy water is often used as a solvent to prevent the strong signal of protium from interfering with the observation of other nuclei.
Conclusion
Heavy water is a unique form of water with applications that are critical to the field of nuclear physics and chemistry. Its distinct properties compared to ordinary water make it an invaluable tool in various scientific and industrial processes. However, due to its potential health effects and the complexity of its production, it is handled with care and used primarily in specialized applications.