ABSTRACT
Water is the most powerful solvent of geochemical materials and plays the main role in their
continuous redistribution in the environment, below, at, and above the land surface. This
capability of water is the result of a unique structure of the water molecule, which has an
asymmetric distribution of the hydrogen nuclei with respect to the oxygen nucleus and the two
pairs of unshared electrons as illustrated in Figure 4.1. The water molecule has a shape of a
distorted tetrahedron, with the oxygen atom located at its center. The covalent bonds between
the hydrogen atoms and the oxygen atom form an angle of 104.58, while this angle in the molecule of ice is 1098, resulting in a regular tetrahedron, which is one of the reasons for the firm (solid) structure of ice. The centers of gravity and electric charges in the water molecule are
asymmetric, which makes water molecule a dipole. Two corners of the tetrahedron with
the hydrogen atoms are strongly positive because of the lack of electrons (hydrogen nuclei
share electrons with the oxygen nucleus), while two corners with the unshared electron pairs
that belong to the oxygen atom are strongly negative. Water molecules are connected between
themselves with strong hydrogen bonds: hydrogen atom (proton) of one water molecule is
bonded to the negatively charged side of another water molecule. This hydrogen bonding is one
additional reason for the unique behavior of water. For example, breaking hydrogen bonds to
boil and evaporate water takes considerable energy-water has higher specific heat than any
common substance (1 cal=g per 18C, or 4.186 J=g 8C). The polarity of molecules, in general, is quantitatively expressed with the dipole moment,
which is the product of the electric charge and the distance between the electric centers.
Dipole moment for water is 6.17 1030 C m (coloumb-meters), which is higher than for any other substance and explains why water can dissolve more solids and liquids and in greater
concentrations than any other liquid. Figure 4.2 illustrates schematically the process of
dissolution of ionized substances, such as sodium chloride, by water. Salt ions are easily
and quickly separated by shells of dipole water molecules, which explains their high
solubilities in water. Organic substances with polarized molecules, such as methanol, are
also highly soluble in water: hydrogen bonds between water and methanol molecules can
readily replace the very similar hydrogen bonds between different methanol molecules and
different water molecules. Methanol is therefore said to be miscible in water (its solubility in
water is infinite for practical purposes). On the other hand, many nonpolar organic mol-
ecules, such as benzene and carbon tetrachloride for example, have very low water solubility.
