Like all matter, water is made up of atoms. Atoms attach together, or
bond, to form molecules. Two hydrogen atoms bonded to an oxygen atom form
a water molecule. Water has several properties that make it a very unique
When electrons are not shared equally in a covalent bond, the molecule
is described as polar. Water molecules are ploar. This means that while
water molecules are neutral as a whole, one end of the water molecule
tends to have a positive charge while the other has a negative charge.
The oxygen end has a slight negative charge while the hydrogen end has
a slight positive charge. Each end of a water molecule is attracted to
the opposite charged end of another water molecule. Water's polarity is
responsible for the "stickiness" or cohesion between the molecules.
Cohesion of water causes capillary attraction, which is the ability of
water to move upward in small spaces. Cohesion makes it possible for water
to move up the fibers of a plant. This is how plants get the water they
need to survive. In addition, it moves water upwards in soil. Cohesion
of water also causes surface tension, water's invisible skin which allows
water striders to walk on water
Water is considered the "universal solvent" because its bipolar
molecule enables it to dissolve a wide variety of substances. Solubility
is affected by polarity. Polar substances can dissolve other polar substances.
Non-polar substances dissolve other non-polar substances. Polar substances
and non-polar substances, however, do not mix.
Another property of water is density during phase changes. The density
of most substances increases when a liquid becomes a solid. Solid water
is actually less dense than liquid water. It is for this reason that ice
floats. Can you imagine a world where ice sank? Lakes would freeze from
the bottom up, killing many fish. Frozen water in the polar regions would
sink and change the ocean levels. The fact that ice floats is essential
for the survival of many aquatic ecosystems and ultimately life on Earth.