Heavy water (deuterium oxide, 2H2O, D2O) is a form of water that contains a larger than normal amount of the hydrogen isotope deuterium (2H or D, also known as heavy hydrogen), rather than the common hydrogen-1 isotope (1H or H, also called protium) that makes up most of the hydrogen in normal water.[4] The presence of deuterium gives the water different nuclear properties, and the increase of mass gives it slightly different physical and chemical properties when compared to normal water.
| Property | D2O (Heavy water) | HDO (Semiheavy water) | H2O (Light water) |
|---|---|---|---|
| Freezing point | 3.82 °C (38.88 °F) (276.97 K) | 2.04 °C (35.67 °F) (275.19 K) | 0.0 °C (32 °F) (273.15 K) |
| Boiling point | 101.4 °C (214.5 °F) (374.55 K) | 100.7 °C (213.3 °F) (373.85 K) | 100.0 °C (212 °F) (373.15 K) |
| Density at STP (g/mL) | 1.1056 | 1.054 | 0.9982 |
| Temp. of maximum density | 11.6 °C | Unverified | 3.98 °C[11] |
| Dynamic viscosity (at 20 °C, mPa·s) | 1.2467 | 1.1248 | 1.0016 |
| Surface tension (at 25 °C, N/m) | 0.07187 | 0.07193 | 0.07198 |
| Heat of fusion (kJ/mol) | 6.132 | 6.227 | 6.00678 |
| Heat of vaporisation (kJ/mol) | 41.521 | Unverified | 40.657 |
| pH (at 25 °C)[12] | 7.44 ("pD") | 7.266 ("pHD") | 7.0 |
| pKb (at 25 °C)[12] | 7.44 ("pKb D2O") | Unverified | 7.0 |
| Refractive index (at 20 °C, 0.5893 μm)[13] | 1.32844 | Unverified | 1.33335 |
