Water

 

Water is the most important substance in our daily lives.
Without water, life as we know it would not have been
possible. This essay will examine the water molecule in
order to ascertain how it brought about Earth's thriving
ecosystem and how important it is to us today.
 
Each water molecule consists of one oxygen atom and two
hydrogen atoms. The oxygen atom (or the apex of the water
molecule) bears a slight electronegative charge while
hydrogen possesses a more positive one 1 Because opposite
charges attract, the water molecules are drawn together.
When an oxygen atom is linked to a neighboring molecule's
hydrogen atom, a bond called a hydrogen bond is formed 2.
In an ice crystal the hydrogen bonds govern the shape of
the crystal so that the grid of molecules surrounds
relatively large spaces . In a liquid form, water has no
such spaces; thus ice is less dense and will float on
liquid water. If not for this, great bodies of water would
freeze from the bottom up without the insulation of a top
layer of ice and all life in the water would die.
 
The water molecule is a very small one but because of its
unique properties it behaves like a larger one. The bonds
between water molecules are so strong that water resists
changes in its state (Solid, liquid, gas); thus water has a
higher melting point and a higher boiling point than
another molecule of similar size. If water followed the
example of other molecules its size, it would have a
boiling point of -75øC and a freezing point of -125øC4.
This would mean that, on Earth, water would be a gas all of
the time and life would not be possible.
 
When heat is applied to solid water, some hydrogen bonds
get so much kinetic energy that they break and the ice
melts. Liquid water does not necessarily have all four
hydrogen bonds present at all times but it must retain some
of them 5. For any object to penetrate water, it must be
able to break the hydrogen bonds on the surface of the
water. These bonds resist breaking thus forming a "skin"
that allows small insects to walk on the surface of the
water. Without the cohesiveness of water, those insects
would not have survived.
 
All plant life on Earth benefits from the ability of water
to make a hydrogen bond with another substance of similar
electronegative charge. Cellulose, the substance that makes
up cell walls and paper products, is a hydrophilic
substance ("water-loving") 6. It interacts with water but,
unlike other hydrophilic substances, it will not dissolve
in it. Cellulose can form strong hydrogen bonds with water
molecules 7. This explains why a paper towel will "wick"
water upwards when it comes in contact with it. Each water
molecule will make a hydrogen bond with cellulose and pull
another water molecule up from down below and so on.
Without this feature (capillary action 8), plants would
find it more difficult to transport water up their stems to
the leaves in order to make food through photosynthesis.
 
Water has a very high heat capacity. Most of the heat
introduced to water is used not to set water molecules in
motion (giving them kinetic energy and causing their
temperature to rise), but to move hydrogen atoms around
between neighboring oxygen atoms 9. If all of this heat was
used solely to warm the water, living cells would boil in
their own heat. Every action in a living cell releases some
heat. If the heat was not dissipated by the water, all
living things would cook themselves.
 
In order for water to evaporate from the surface of liquid
water, a certain amount of energy must be expended to break
its hydrogen bonds. Because these hydrogen bonds are so
strong, water requires a lot of heat to boil (100øC). When
water vaporizes, it takes along all of the heat energy
required to break its bonds thus having a powerful cooling
effect on the original body of water 10. It takes very
little water loss to cool water substantially. If humans
had no way of perspiring, their body temperatures would
rise about 70øC in one single day 11.
 
Water is very important because it is as close as we can
get to a "universal solvent." When a crystal of salt is
dropped into a glass of water, the water begins to surround
the sodium and chloride atoms in the salt in what is called
a hydration spere 12. The atoms interact with the water
molecule and leave the surface of the salt crystal, until
the entire crystal is dissolved. This has two effects: the
salt is dissolved and the water molecules are disturbed.
When the water molecules are altered in this fashion they
can no longer take part in the formation of ice crystals
13. This is why oceans, which contain a lot of salt, resist
freezing.
 
Water is a very important substance. Its unique properties
aid all living things to survive in the scheme of nature on
our planet Earth.
 
1. Kirk, David L. Biology Today: Third Edition p. 225 

2. Ibid p. 256 

3. Ibid p. 256
 
4. Ibid p. 257-258 

5. Ibid p. 256 

6. Ibid p. 259
 
7. Ibid p. 259 

8. Ibid p. 259 

9. Ibid p. 257 

10. Ibid p. 257 

11. Ibid p. 257 

12. Ibid p. 259
 
13. Ibid p. 259 
 
Bibliography:
 
Kirk, David L. Biology Today: Third Edition. Random House
Publishers, 1980
 
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