Why does the density of water decrease with decreasing temperature?
The molar volume of water actually goes through a minimum at about 4oC and increases as temperature changes above or below that temperature. The only sense in which it decreases is from 0-4oC (or from supercooled state to 4oC). This is not unique, but certainly is unusual. It has less to do with the arrangements in the liquid than those in the crystal. In the liquid, the molecules can arrange in whatever way best suits the overall kinetic energy of the system, and it happens that this leads to clustering. That water molecules cluster in the liquid is not in dispute, though exactly how they do so definitely is. The brief summary of the various views is that clusters either get larger as temperature decreases, or that there are different types of clusters, and denser cluster types become dominant as temperature decreases. At about 4oC, most of the water molecules are involved in big, dense clusters, and essentially hit a volume minimum. At lower temperatures, the clusters begin to order
