Why does ice float in water when other solids do not float?

Answer 1

The simple answer is that the crystalline structure of ice is less dense than the amorphous structure in water.

The reason has to do with the Hydrogen and Oxygen atoms.

Hydrogen tends to give away it's electron, and thus is considered electro-positive.

Oxygen tends to gain electrons, and is thus considered electro-negative.

Oxygen begins with 6 electrons in its outer shell. By sharing with two hydrogen atoms, it fills the shell with 8 electrons arranged in 4 pairs. Two of these pairs are shared with the hydrogen atoms, and two are unshared.

While you might think of water as having a straight linear planar shape (the two hydrogen atoms repelling each other), it actually ends up with a tetrahedron shape with the 4 corners in 3-D space being the electron pairs, two of which are associated with hydrogen atoms. Thus, it gets the bent shape that is often depicted in diagrams.

Now, the two hydrogen atoms (that are now electropositive) on the water molecule tend to be attracted to the electronegative unshared electron pairs on the oxygen atoms forming what are called Hydrogen Bonds.

In fact, the hydrogen atoms can actually jump from one oxygen to the other, forming different forms of water.

H3O+ <--> H2O <--> OH-

Which is essentially the basis for acid-base chemistry which hydrogen and water are central parts of.

As water cools, this dynamic ability to make and break multiple hydrogen bonds, or interact with multiple oxygen atoms, as well as having the hydrogen jumping from one molecule to another is hindered as it approaches a fixed crystal structure. And, at about 4°C, water reaches its maximum density. As it cools further, it starts expanding as it forms a fixed crystalline structure with essentially 2 hydrogen atoms making hydrogen bonds with each oxygen molecule, and otherwise forming a structure with the maximum distance between individual hydrogen atoms.

Thus, with a little hand waving.... as the ice forms, fewer hydrogen-oxygen interactions form, and the volume increases or the density decreases.

Once formed, ice behaves like other solids and contracts when it cools and expands when it is heated.

See the linked Wikipedia articles on Ice, Water, and sp3 orbital hybridization.

Answer 2

Ice floats in water because it is less dense than liquid water. When water freezes, its molecules form a crystal structure that spaces them farther apart than in liquid form, thereby reducing the overall density of the ice. This unique property allows ice to float on water.