Like regular water, heavy water is composed of two hydrogen atoms bonded two one oxygen atom (H2O). Each hydrogen atom in ordinary water has a nucleus that contains one proton and zero neutrons. This isotope of hydrogen is called protium. Heavy water is also composed of two hydrogen atoms bonded to one oxygen atom, but the hydrogen atoms in heavy water are an isotope of hydrogen with a nucleus that contains one proton and one neutron called deuterium (2H).
Figure 2 illustrates the difference between these forms of water (Source). Ignore the illustration of tritium (3H) for the following discussion.
Let’s discuss why heavy water ice cubes sink in ordinary water. For something to sink in ordinary water, its density must be greater than that of ordinary water. To estimate the density of heavy water, we can make some assumptions:
- Atoms of heavy water and ordinary water are the same size
- Heavy water ice and ordinary ice form exactly the same crystal structure.
Given these assumptions, we can estimate the density difference between heavy water and ordinary water by the percentage difference between the molecular mass of heavy water relative to ordinary water. That calculation is shown in Figure 3. I also list the measured density of heavy water and show it is 11 % more than ordinary water at 25 °C.
Creating heavy water ice cubes and seeing them sink in ordinary water is interesting. It is also expensive since relatively pure heavy water costs about $3 per gram.
I also found this video that illustrates the difference in flotation between heavy water and ordinary water.