Does Heavy Water Ice Sink or Float?

While regular ice floats in water, heavy water ice cubes sink in regular water. Ice made from heavy water would, however, be expected to float in a glass of heavy water.

Heavy water is water made using the hydrogen isotope deuterium rather than the usual isotope (protium). Deuterium has a proton and a neutron, while protium only has the proton in its atomic nucleus. This makes deuterium twice as massive as protium.

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What Is Heavy Water?

By Anne Marie Helmenstine, Ph.D.

Several Factors Affect the Behavior of Heavy Water Ice

Deuterium forms stronger hydrogen bonds than protium, so the bonds between hydrogen and oxygen in heavy water molecules would be expected to impact the water heavy water molecules pack when the substance changes from a liquid to a solid.

1. Even though deuterium is more massive than protium, the size of each atom is the same, since it is the electron shell that determines it's atomic size, not the size of an atom's nucleus.
2. Each water molecule consists of an oxygen bonded to two hydrogen atoms, so there is not a huge mass difference between a heavy water molecule and a regular water molecule because most of the mass comes from the oxygen atom. When measured, heavy water is about 11% denser than regular water.

While scientists could make a prediction whether heavy water ice would float or sink, it required experimentation to see what would happen. It turns out heavy water ice does sink in regular water. The likely explanation is that each heavy water molecule is slightly more massive than a regular water molecule and heavy water molecules may pack more closely than regular water molecules when they form ice.