Upon freezing (i.e, transforming from a liquid to a solid upon coolng), most substances experience an increase in density(or, corre- pondingly, a decrease in volume). One exception is water, which exhibits the anomalous and fa- miliar expansion upon freezing approximately volume percent expansion. This behavior may be explained on the basis of hydrogen bonding Each H2O molecule has two hydrogen atoms that can bond to oxygen atoms: in addition, its single O atom can bond to two hydrogen atomsof other H10 molecules. Thus, for solid ice, each water molecule participates in four hydrogen bonds as shown in the three-dimensional schematic of Figure 2.16a; here hydrogen bonds are denoted by dashed lines, and each water molecule has 4 nearest-neighbor molecules. This is a relatively open structure-i.e., tha molecules are not closely packed together-and, as aresult, the A watering can that ruptured along a side panel bottom panel soam. Water that was left in the can during a cold late-autumn night expanded as it iroze and caused the rupture. (Photography by S Tinner.)