Density of Heavy Water
Pure heavy water (D2O) has a density about 11% greater than water, but is otherwise physically and chemically similar.
This difference is caused by the fact, the deuterium nucleus is twice as heavy as hydrogen nucleus. Since about 89% of the molecular weight of water comes from the single oxygen atom rather than the two hydrogen atoms, the weight of a heavy water molecule, is not substantially different from that of a normal water molecule. The molar mass of water is M(H2O) = 18.02 and the molar mass of heavy water is M(D2O) = 20.03 (each deuterium nucleus contains one neutron in contrast to hydrogen nucleus), therefore heavy water (D2O) has a density about 11% greater (20.03/18.03 = 1.112).
Pure heavy water (D2O) has its highest density 1106 kg/m3 at temperature 11.6oC (52.9oF). Also heavy water differs from most liquids in that it becomes less dense as it freezes. It has a maximum of density at 11.6oC (1106 kg/m3), whereas the density of its solid form ice is 1017 kg/m3. It must be noted, the change in density is not linear with temperature, because the volumetric thermal expansion coefficient for water is not constant over the temperature range.