gether by ionic bonds (an ionic bond is formed in a chemical reaction when
electrons are exchanged rather than shared). When sodium and chlorine re—
act, sodium gives up negative electrons and becomes a positive ion; chier—ine
gains these electrons and becomes a negative ion. The attraction between the
positively—charged
sodium ion and the negatively—charged chlorine ion holds
the two atoms together in a crystal of sodium chloride. Dissolved in water,
the positive and negative ions are attracted to the positive and negative re—
gions of the water molecule. The attraction of the sodium and chlorine ions to
each other is overcome to a degree by their attraction to water, effecting
their separation.
Water’s importance as a solvent is enhanced by the fact that chemically it
is a generally inert solvent: Its properties are not changed by most of the substances it dissolves, nor does it change most of the substances which dissolve
in it. Such a powerful solvent as an acid, on the other hand, will react with a
compound which water would simply dissolve. After dissolving sodium chlo—
ride, for example, water could regain its former purity (evaporation would
separate the water from the salt); one acid, however, could not be freed of its
impurities without undergoing another chemical reaction. This is very important when considering biological systems in particular. Because, in general,
water is chemically inert, it can transport various essential substances, dis—
solved but otherwise unchanged, into living cells. Most chemical reactions
within plants and animals take place only when substances are in solution.
Renewable.
The fact that water remains essentially unchanged also explains why it is a renewable resource. A river may carry a heavy load of pol—
lutants in suspended or dissolved form down to the sea, yet when water evaporates from the surface of the ocean under ordinary circumstances it is
essentially pure and reusable.
Cohesive and Adhesive.
Because of its hydrogen bonding, water also has
cohesion (the ability to stick to itself) and adhesion (the ability to stick to
some solid substances)—properties that are important in nature [9, 10]. Ex—
cept for mercury, water has the highest surface tension of all liquids because
of the strong attraction of the polar water molecules on the surface for each
other—that is, their strong hydrogen bonds. The formation of a water droplet
demonstrates water’s cohesion; the droplet would be perfectly spherical ex—
cept for the force of gravity, and a sort of membranelike lm is formed
around it by surface tension.
The surface of lakes and ponds provides a habitat for a variety of algae,
protozoans,
and insects that are able in one way or another to take advantage
of water s surface tension. The ability of insects such as water-strid61‘S, Of
pond-skaters, to
move on top of water without breaking the surface is one ex—
the water and hanging upside
er example.
Water adheres to or wets glass because the hydrogen in the water is at—
38
An Uncommon Friend
‘
electrons are exchanged rather than shared). When sodium and chlorine re—
act, sodium gives up negative electrons and becomes a positive ion; chier—ine
gains these electrons and becomes a negative ion. The attraction between the
positively—charged
sodium ion and the negatively—charged chlorine ion holds
the two atoms together in a crystal of sodium chloride. Dissolved in water,
the positive and negative ions are attracted to the positive and negative re—
gions of the water molecule. The attraction of the sodium and chlorine ions to
each other is overcome to a degree by their attraction to water, effecting
their separation.
Water’s importance as a solvent is enhanced by the fact that chemically it
is a generally inert solvent: Its properties are not changed by most of the substances it dissolves, nor does it change most of the substances which dissolve
in it. Such a powerful solvent as an acid, on the other hand, will react with a
compound which water would simply dissolve. After dissolving sodium chlo—
ride, for example, water could regain its former purity (evaporation would
separate the water from the salt); one acid, however, could not be freed of its
impurities without undergoing another chemical reaction. This is very important when considering biological systems in particular. Because, in general,
water is chemically inert, it can transport various essential substances, dis—
solved but otherwise unchanged, into living cells. Most chemical reactions
within plants and animals take place only when substances are in solution.
Renewable.
The fact that water remains essentially unchanged also explains why it is a renewable resource. A river may carry a heavy load of pol—
lutants in suspended or dissolved form down to the sea, yet when water evaporates from the surface of the ocean under ordinary circumstances it is
essentially pure and reusable.
Cohesive and Adhesive.
Because of its hydrogen bonding, water also has
cohesion (the ability to stick to itself) and adhesion (the ability to stick to
some solid substances)—properties that are important in nature [9, 10]. Ex—
cept for mercury, water has the highest surface tension of all liquids because
of the strong attraction of the polar water molecules on the surface for each
other—that is, their strong hydrogen bonds. The formation of a water droplet
demonstrates water’s cohesion; the droplet would be perfectly spherical ex—
cept for the force of gravity, and a sort of membranelike lm is formed
around it by surface tension.
The surface of lakes and ponds provides a habitat for a variety of algae,
protozoans,
and insects that are able in one way or another to take advantage
of water s surface tension. The ability of insects such as water-strid61‘S, Of
pond-skaters, to
move on top of water without breaking the surface is one ex—
the water and hanging upside
er example.
Water adheres to or wets glass because the hydrogen in the water is at—
38
An Uncommon Friend
‘