Why does ice float?
(Lansing State Journal, October 12, 1994)

This is not the case for water, however.  Ice is less dense than water, therefore it floats.

Ice is less dense than water because of a special type of chemical bonding called hydrogen bonding.  A hydrogen bond is a weak bond between a hydrogen atom and an unbonded pair of electrons from another atom.

We can think of a water molecule as being in the shape of an "X" twisted so that the top half is perpendicular to the botton half.  In the center of the "X" is an oxygen atom, at the bottom tips of the "X" are two hydrogen atoms, and at the top tips of the "X" are two unbonded electron pairs from oxygen.

As you can see, water molecules are prime candidates for hydrogen bonding between each other.

At warm temperatures, water molecules have a lot of energy and are able to move past and mix with each other despite the attractions between the hydrogen atoms and unbonded electron pairs.

As water is cooled down, however, the molecules have less energy and hydrogen bonding takes over.  The molecules form a ordered crystal through hydrogen bonding that spaces the molecules farther apart than when they were in a liquid.  This makes ice less dense than water allowing it to float.

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