<P> Water enriched in the heavier oxygen isotopes O and O is also commercially available, e.g., for use as a non-radioactive isotopic tracer . It is "heavy water" as it is denser than normal water (H O is approximately as dense as D O, H O is about halfway between H O and D O)--but is rarely called heavy water, since it does not contain the deuterium that gives D O its unusual nuclear and biological properties . It is more expensive than D O due to the more difficult separation of O and O. H O is also used for production of fluorine - 18 for radiopharmaceuticals and radiotracers and for positron emission tomography . </P> <P> Tritiated water contains tritium (H) in place of protium (H) or deuterium (H), and therefore it is radioactive . </P> <Table> Physical properties of isotopologues of water <Tr> <Th> Property </Th> <Th> D O (Heavy water) </Th> <Th> HDO (Semiheavy water) </Th> <Th> H O (Light water) </Th> </Tr> <Tr> <Td> Freezing point </Td> <Td> 3.82 ° C (38.88 ° F) (276.97 K) </Td> <Td> 2.04 ° C (35.67 ° F) (275.19 K) </Td> <Td> 0.0 ° C (32 ° F) (273.15 K) </Td> </Tr> <Tr> <Td> Boiling point </Td> <Td> 101.4 ° C (214.5 ° F) (374.55 K) </Td> <Td> 100.7 ° C (213.3 ° F) (373.85 K) </Td> <Td> 100.0 ° C (212 ° F) (373.15 K) </Td> </Tr> <Tr> <Td> Density at STP (g / mL) </Td> <Td> 1.1056 </Td> <Td> 1.054 </Td> <Td> 0.9982 </Td> </Tr> <Tr> <Td> Temp. of maximum density </Td> <Td> 11.6 ° C </Td> <Td> Unverified </Td> <Td> 3.98 ° C </Td> </Tr> <Tr> <Td> Dynamic viscosity (at 20 ° C, mPa s) </Td> <Td> 1.2467 </Td> <Td> 1.1248 </Td> <Td> 1.0016 </Td> </Tr> <Tr> <Td> Surface tension (at 25 ° C, N / m) </Td> <Td> 0.07187 </Td> <Td> 0.07193 </Td> <Td> 0.07198 </Td> </Tr> <Tr> <Td> Heat of fusion (kJ / mol) </Td> <Td> 6.132 </Td> <Td> 6.227 </Td> <Td> 6.00678 </Td> </Tr> <Tr> <Td> Heat of vaporisation (kJ / mol) </Td> <Td> 41.521 </Td> <Td> Unverified </Td> <Td> 40.657 </Td> </Tr> <Tr> <Td> pH (at 25 ° C) </Td> <Td> 7.44 ("pD") </Td> <Td> 7.266 ("pHD") </Td> <Td> 7.0 </Td> </Tr> <Tr> <Td> pK (at 25 ° C) </Td> <Td> 7.44 ("pK D O") </Td> <Td> Unverified </Td> <Td> 7.0 </Td> </Tr> <Tr> <Td> Refractive index (at 20 ° C, 0.5893 μm) </Td> <Td> 1.32844 </Td> <Td> Unverified </Td> <Td> 1.33335 </Td> </Tr> </Table> <Tr> <Th> Property </Th> <Th> D O (Heavy water) </Th> <Th> HDO (Semiheavy water) </Th> <Th> H O (Light water) </Th> </Tr>

At what temperature would a sample of heavy water freeze