Hydrogen bonds cause unusual water, ice, and hydrate properties

4 Hydrogen bonds cause unusual water, ice, and hydrate properties 

Along with the attraction of the hydrogen bond, the fact that these strong bonds separate the water molecules rigidly causes the solid density to be less than that of the liquid. In ice only 34% of the volume is occupied by water molecules, in contrast to 37% volume occupation in liquid water this explains the unusual property of a decrease in density on freezing, accounting for the fact that ice floats. 

Because the hydrogen bond is significantly weaker than the covalent bond, when hydrates form or dissociate, only hydrogen bonds are considered between neighboring molecules. Van der Waals forces are present, but are insignificant 

Stillinger (1980) suggested that disconnected icebergs should not be present, but that a more likely model is that of a random, three-dimensional network of hydrogen bonds, rather than long-lived clusters of molecules. Such networks of hydrogen-bonded water molecules and clusters are present when hydrates form or dissociate. These clusters are discussed relative to nucleation of hydrates in the following chapter. 

The thermodynamic and structural processes that occur when water molecules are in the vicinity of hydrophobic entities (water fearing, insoluble in water) are referred to collectively as hydrophobic hydration (Tanford, 1973 Privalov and Gill, 1988 Blokzijl andEngberts, 1993 Chau and Mancera, 1999). Hydrophobic hydration is important in gas hydrate formation, which usually starts with hydrophobic gas molecules (e.g., methane) being introduced into liquid water. 

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