"Ice is less dense than water because of its intermolecular forces."
Water contains hydrogen bonds (a type of
intermolecular force of attraction) between the H (hydrogen) of one atom
and the O (oxygen) of another atom. As the water gets colder and the
kinetic energy of the molecules decreases, the hydrogen bonds keep the
water molecules apart, forming hexagonal structures with water molecules
at each vertex. In between the water molecules is nothing. In liquid
water, the molecules of water can be much closer together; the hydrogen
bonds are more flexible. Therefore, the solid ice, with its molecules
kept at a fairly fixed distance and the crystals holding lots of
"nothing" among the water molecules, is less dense than the liquid
water.
