Modern inorganic chemistry

270 GROUPVI

oxygen atom of each water molecule is surrounded by four hydrogen

atoms arranged approximately tetrahedrally, two (in the molecule)

attached by covalent bonds, and two from adjacent molecules by

longer hydrogen bonds. As the temperature is increased hydrogen

bonds begin to break and at 273 K there are insufficient to maintain

H H

Figure 10.3

the crystalline lattice and the solid melts. The liquid formed at

273 K has a quasi-crystalline structure. Between 273 K and 277 K

the hydrogen bonds rearrange and the 'crystal' structure changes;

the molecules pack more closely together so that the density increases.

But above 277 K (where the density reaches a maximum value) the

effect of thermal agitation of the 'molecules' becomes increasingly

important and there is an overall expansion.

SOLVENT PROPERTIES

The high permittivity (dielectric constant) makes water a highly
effective solvent for ionic crystals, since the electrostatic attractive
forces between oppositely charged ions are reduced when the crystal
is placed in water. Moreover, since water is not composed of ran-
domly arranged molecules but has some degree of 'structure', the
introduction of charged ions which attract the polar water molecules,
produces a new 'structure', and a fraction of the water molecules
become associated with the ions—the process known as hydration.
Energy is evolved in this process—hydration energy—and this
assists the solution of both ionic and partly covalent substances:
in the latter case hydrolysis may also occur (see below). There are,
however, many non-ionic substances for which water is a good
solvent; this is because the molecules of such substances almost
always contain hydrogen and oxygen atoms which can form
hydrogen bonds with water molecules. Hence, for example, sub-
stances containing the —OH group, for example alcohols, carboxylic
acids and some carbohydrates, are soluble in water, provided that
the rest of the molecule is not too large.