Upon freezing (transforming from liquid to a solid upon cooling),most substances experience an increase in density (or, correspondingly,a decrease in volume). One exception is water, which exhibits the anomalous and familiar expansion upon freezing approximately 9 volume percent expansion. this behavior maybe explained on the basis of hydrogen bonding. each H2O molecule has two hydrogen atoms that can bond to oxygen atoms in addition, its single oxygen atom can bond two hydrogen atoms of other H2O molecule. Thus for solid ice, each water molecule participates in four hydrogen bonds as shown in the three-dimensional schematic of Figure 2.16 here hydrogen bonds are denoted by dashed lines, and each water molecule has 4 nearest-neighbor molecules. This is a relatively open structure, the molecule are not closely packed together and as a result