| Water has important
effects on all biological systems. What makes water so unique
are two very important properties.
Water is a polar molecule
 A water molecule
is formed when two atoms of hydrogen bond covalently with an atom
of oxygen. In a covalent bond electrons are shared between atoms.
In water the sharing is not equal. The oxygen atom attracts the
electrons more strongly than the hydrogen.This gives water an
asymmetrical distribution of charge. Molecules that have ends
with partial negative and positive charges are known as polar
molecules. It is this polar property that allows water to separate
polar solute molecules and explains why water can dissolve so
many substances.
Water
is highly cohesive .
The positive regions in one water will attract the negatively
charged regions in other waters. The dashes show the hydrogen
bond. In a hydrogen bond a hydrogen atom is shared by two other
atoms. The donor is the atom to which the hydrogen is more tightly
linked. The acceptor (having a partial negative charge) is the
atom which attracts the hydrogen atom. Click here
or on the image to your left to view a movie of two water
molecules.
Liquid
water has a partially ordered structure in which hydrogen bonds
are constantly being formed and breaking up.
See a flash movie of water molecules in action.
Hydrogen
bonds are much weaker than covalent bonds. However, when a large
number of hydrogen bonds act in unison they will make a strong
contributory effect. This is the case in water.
View Flash animation of this image.
On the
other hand ice has a rigid lattice structure.
In liquid water each molecule is
hydrogen bonded to approximately 3.4 other water molecules. In
ice each each molecule is hydrogen bonded to 4 other molecules.
Compare
the two structures below. Notice the empty spaces within the ice
structure.
In ice Ih, each water forms four
hydrogen bonds with O---O distances of 2.76 Angstroms to the nearest
oxygen neighbor. The O-O-O angles are 109 degrees, typical of
a tetrahedrally coordinated lattice structure. The
density of ice Ih is 0.931 gm/cubic cm. This compares with a density
of 1.00 gm/cubic cm. for water.
There are eleven different forms
of crystalline ice that are know. The hexaganol form known as
ice Ih is the only one that is found naturally. The lattice structure
of ice 1h is shown here.
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