Physical Properties (with Comparison To Light Water)
| Property | D2O (Heavy water) | HDO (Semiheavy water) | H2O (Light water) |
|---|---|---|---|
| Freezing point (°C) | 3.82 | 0.0 | |
| Boiling point (°C) | 101.4 | 100.7 | 100.0 |
| Density at STP (g/mL) | 1.1056 | 1.054 | 0.9982 |
| Temp. of maximum density (°C) | 11.6 | 4.0 | |
| Dynamic viscosity (at 20 °C, mPa·s) | 1.2467 | 1.1248 | 1.0016 |
| Surface tension (at 25 °C, μJ) | 7.187 | 7.193 | 7.198 |
| Heat of fusion (cal/mol) | 1,515 | 1,436 | |
| Heat of vaporisation (cal/mol) | 10,864 | 10,757 | 10,515 |
| pH (at 25 °C) | 7.43 (sometimes "pD") | 7.266 (sometimes "pHD") | 6.9996 |
| Refractive index (at 20 °C, 0.5893 μm) | 1.32844 | 1.33335 |
Physical properties obvious by inspection: Heavy water is 10.6% denser than ordinary water, a difference which is not immediately obvious. One of the few ways to demonstrate heavy water's physically different properties without equipment is to freeze a sample and drop it into normal water (it will sink). If the water is ice-cold the higher melting temperature of heavy ice can also be observed – it melts at 3.8 °C, and thus holds up very well in ice-cold normal water.
An early experiment reported not the "slightest difference" in taste between ordinary and heavy water; on the other hand, rats given a choice between distilled normal water and heavy water were able to avoid the heavy water based on smell, and it may be possible that it has a different taste.
No physical properties are listed for "pure" semi-heavy water, because it is unstable as a bulk liquid. In the liquid state, a few water molecules are always in an ionised state, which means the hydrogen atoms can exchange among different oxygen atoms. Semi-heavy water can be created by a chemical method but would rapidly transform into a dynamic mixture of 25% light water, 25% heavy water, and 50% semi-heavy water (however if it were made in the gas phase and directly frozen to a solid, this semiheavy ice would be stable).
Read more about this topic: Heavy Water
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