Heavy Water Ice Cubes Do Not Float


Introduction

Figure 1 shows a photo of heavy water ice cubes sinking in ordinary water (Source: Science Photo Library). I find this an interesting photo. Let’s discuss it a bit.

Figure 1: Heavy Water Ice Cubes Sink in Water (Left) While Ordinary Ice Cubes Float in Water (Right).

Figure 1: Heavy Water Ice Cubes Sink in Water (Left) While Ordinary Ice Cubes Float in Water (Right).

Background

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.

Figure 2: Forms of Hydrogen and Their Formation into Heavy Water and Ordinary Water.
isotopes_lg

Analysis

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.

Figure 3: Calculation of the Density DIfference Between Heavy Water and Ordinary Water.

Figure 3: Calculation of the Density DIfference Between Heavy Water and Ordinary Water.

Properties of Water Properties of Heavy Water Properties of Water Density of Heavy Water

Conclusion

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.

About mathscinotes

I am an engineer who encounters interesting math and science problems almost every day. I am not talking about BIG math here. These are everyday problems where a little bit of math really goes a long way. I thought I would write some of them down and see if others also found them interesting.
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2 Responses to Heavy Water Ice Cubes Do Not Float

  1. Pingback: Agua, molécula fundamental (IV): Filosofía de la Química | Enseñanza, Historia, Filosofía y Divulgación de la Química

  2. JoeSnow says:

    The problem with the photo, though, is that it makes an apples to oranges comparison. Naturally heavy water ice does not float in ordinary water but put that same heavy water ice into a glass of heavy water and it will float just fine. It’s like putting a wrench in water and watching it sink but if you put that same wrench in mercury, it will float because the mercury is denser than the steel the wrench is made of.

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