the two H atoms in the water molecule and the other two are unshared pairs. The pairs
repel each other by arranging themselves around the spherical surface of the water
molecule in a way that enables them to be as far apart as possible. Try to visualize a
sphere with 4 things arranged around its surface as far apart as possible. They would be
at angles of somewhat more than 100 degrees. In fact, the angle formed by two lines,
each joining the center of an H atom to the center of the O atom in the water molecule
is 105 ̊.
So, the first significant characteristic of the water molecule is that it has a bent
structure with the two H atoms not on opposite sides of the O atom, but located at an
angle somewhat greater than a right angle. A second characteristic of the water molecule
is it is a dipole in which the O atom side of the molecule is relatively more negatively
charged than the side with the two H atoms as shown on the left of Figure 7.1.
Now let us rotate the water molecule 90 ̊ around its vertical axis as shown in the
second structure in Figure 7.1. We see one H atom, but it obscures the other H atom. We
can see the two unshared pairs of electrons, but why bother? They are important because
they can bond to H atoms on other water molecules with a special kind of bond called
a hydrogen bond, as shown on the right in Figure 7.1. Through this characteristic of
hydrogen bonding, water molecules in liquid water and in ice are connected to each other,
which has profound effects upon water’s properties. Furthermore, water molecules can
form hydrogen bonds to molecules dissolved in water, greatly enhancing water’s solvent
properties for some kinds of substances, including many of biological importance.
Because of their polar nature, water molecules surround cations and anions from
ionic compounds dissolved in water. The negative ends of the water molecules are
oriented toward the positive cations and the positive ends of the water molecules toward
the negative anions. This makes water a good solvent for ionic compounds.
7.2. Important Properties of Water
Water has several important properties that are of crucial importance for its role
as a solvent, life medium, environmental behavior, and industrial uses, based upon the
following characteristics of the water molecule:
- Unsymmetrical molecule
- Polar nature of the water molecule
- Ability to form hydrogen bonds
Water is an excellent solvent for a number of materials, including salts, acids, bases,
and substances that have H, O, and N atoms capable of forming hydrogen bonds. As a
result, water is the solvent in biological fluids, such as blood or urine; it acts to weather
minerals and transport dissolved minerals in the geosphere; it transports nutrients to
plant roots in soil; and it has many industrial uses. Water has a very high surface tension
162 Green Chemistry, 2nd ed