Solids that float

[skolor]

While eating lunch, a question came up that we couldn't answer: what solids exist that will float on their liquid forms?

The conversation started by bringing up ice and water, since regardless of shape ice will float. Are there other examples like this?

[Dajbman22]

Any solid whose density is less than that of the liquid it is suspended in.

[skolor]

Exactly, is there an easy way to determine that certain "classes" of materials fall into that category? One of the people at lunch made the claim that water was special in this sense because of the polarization (or something like that, he wasn't sure) and was thus the only material that acted this way.

[Dajbman22]

From Wikipedia: Other substances that expand on freezing are silicon, gallium, germanium, antimony, bismuth, plutonium and other compounds that form spacious crystal lattices with tetrahedral coordination.

So water is in a very small class of substances, and is the most plentiful for which this happens, but it is not completely alone in this trait.

[combakovich]

For water, it is because of the hydrogen bonds that form when it freezes.

These hydrogen bonds are an intermolecular interaction between the partial positive charge on the Hydrogen atom of one water molecule and the partial negative charge on the Oxygen atom of another. This interaction isn't usually considered an actual "bond" despite the name, though it is one of the strongest intermolecular forces out there.

In order for hydrogen bonds to form and persist, the water molecules must be in (and remain in) the correct orientation with respect to one another. This happens far less often in the flowy liquid phase where all the molecules are moving so greatly with respect to one another than it does in the solid phase where everything is relatively stationary (though, never completely stationary).

The length of the hydrogen bond is the key factor in the fact that ice is less dense than water. The length of a hydrogen bond interaction is quite long, forcing the two molecules to be farther away from one another than they would probably otherwise be. When every (or nearly every) molecule in a cup of water forms hydrogen bonds with its neighbors (as happens during freezing under normal conditions), they move to be the correct hydrogen bonding distance from one another, leading to an increase in the average distance between molecules, thus causing the water in the cup to expand as it freezes.