Shrinking and Growing

In the last experiment, the volume of the water increased as it turned to ice, and the volume of the oil decreased, or stayed the same, as it froze. Why do these two liquids act so differently?

Most liquids contract (get smaller), at least a little, when they freeze. Molecules in a liquid move in all directions very quickly. They slow down as they get colder, and get really lazy around the freezing temperature. When the molecules are about to turn into a solid, they begin to crowd together and stick to one another. This arrangement of molecules takes up less space than the liquid form, and the volume of the solid that results after freezing is less than the volume of the original liquid.

Water (H₂O) acts differently because of its molecular structure. A water molecule has its oxygen atom in the center, with hydrogens on either side. However, the molecule is not straight, but rather, is bent at an angle of 105^o. So the water molecules cannot pack tightly like most other molecules can. They end up with a structure that looks like a honeycomb, in a six-sided, or hexagonal arrangement. This structure actually takes up more space than the original water, and so solid water has a larger volume than liquid water.

Here's a video you can watch that shows the motion of water molecules in ice (solid), water (liquid), and steam (gas).