Water is a colorless, odorless, tasteless liquid that also
occurs commonly in the solid state (as ice) and in the gaseous
state (as steamor water vapor). It is a very stable compound that
undergoes a number of important reactions. It reacts with some
metals to form elemental hydrogen and an inorganic base.With
some metals, such as sodium and potassium, the reaction is
quite violent. With other metals, such as iron, the reaction
occurs only very slowly. Water is also a weak electrolyte, ionizing
to form hydrogen ions (H+) and hydroxide ions (OH– ). The
hydrogen ions occur in solution as hydronium ions (H3O+).
Water is also a strong dipole. A dipole is a molecule in
which electrical charges are sufficiently separated from each
other that one part of the molecule is negatively charged and
another part, positively charged. Water’s dipole character is
responsible for many of its special characteristics. The positive
end of one water molecule is attracted to the negative
end of a second water molecule, resulting in the formation of
a weak (‘‘hydrogen’’) bond between the two molecules.
For example, water has a very high boiling point for a
substance with relatively small molecules. The high boiling
point is a result of the fact that heat added to water
must first be used to break hydrogen bonds between water
molecules before providing enough energy to vaporize the
molecules. Similarly, the phenomenon known as surface
tension is caused by hydrogen bonding. Surface tension
is the tendency of a liquid to act as if it is covered with a
thin film. Some insects are able to walk on water because
its surface tension is so great. The surface tension is
caused by the attractive forces between adjacent water
molecules.
Water is also an excellent solvent. A solvent is a substance
capable of dissolving other substances. Chemists
sometimes refer to water as ‘‘the universal solvent’’ because
it is able to dissolve so many other substances. That statement
is an exaggeration, but does reflect the compound’s
ability to dissolve more substances that probably any other
single compound. Water’s ability to dissolve other substances
is at least partly a result of its strong dipole character.
The positive or negative end of a water molecule
attaches itself to the negative or positive end of the substance
to be dissolved. The force of attraction exerted by
the water molecule is sufficient to tear apart the particles of
which the second substance is composed causing them to
dissolve in the water.
Water can be made by a variety of chemical reactions,
including:
• The oxidation of hydrogen: 2H2 + O2 ! 2H2O;
• The reaction between an acid and a base, as, for
example: NaOH + HCl ! NaCl + H2O;
• The combustion of an organic material, as, for example:
CH4 + 2O2 ! CO2 + 2H2O.
Because water occurs so abundantly, none of these
reactions is required for the commercial production of the
compound. Water makes up about 70 percent of the Earth’s
surface in the oceans, lakes, rivers, ponds, glaciers, ice caps,
and other reservoirs. The problem is that only a very small
fraction of that water—about 3 percent—is fresh water. The
remaining 97 percent is salt water. And even the 3 percent of
fresh water in lakes, rivers, and other resources is impure, in
the sense that it contains other substances dissolved and
suspended in it.
Thus, the primary concern in obtaining adequate supplies
of pure water for household, personal, commercial,
industrial, or other uses is the purification of water, not its
synthesis. Purification of water is achieved by a number of
processes, including chlorination, filtration, distillation, or
purification by some type of ion exchange mechanism.
