Water
The Water Molecule
Properties of Water
The Water
Molecule. Water is a remarkable substance that is important not only as a
source of moisture, but also as a means of transferring energy in the atmosphere.
Water is the only common substance that is liquid at
ordinary temperatures on the earth, but it is also found in all three physical states, solid,
liquid, and vapor, at various places on the earth's surface, and sometimes
in the same place at the same time (think of an iceberg floating in the ocean, with water
vapor in the surrounding air).
Water is a molecule composed of two atoms of hydrogen bonded
with one atom of oxygen: H2O. The 
oxygen atom and the two hydrogen atoms share electrons in a strong
covalent bond.
Water molecules as a whole have no net charge, but the oxygen end has a slight negative
charge (since the electrons tend to stay on the side of the large oxygen nucleus),
and the hydrogen end has a slight positive charge.
For this reason, water is referred to as a polar molecule. It has a positive
end and a negative end (analagous to the north and south poles of a bar magnet).
This is a very important fact about water, because it means that the molecules can bond
together in hydrogen bonds. The negative (oxygen) end of one water molecule
forms a slight bond with the positive (hydrogen) end of another water molecule.
Water molecules floating
Water
molecules bonded to each
around by themselves:
other with hydrogen bonds:
This hydrogen bond is not as strong as the bonds that hold
the water molecule itself together, but the attraction of water molecules for each other
is a very important factor determining the properties of water.
Properties of Water.
Because of the way water molecules are arranged and can bond with each other, water
has several important properties:
- Since water molecules are attracted to each other to each
other by hydrogen bonding , they tend to stick together: this is called cohesion.
- Because of this cohesion, water has high surface tension.
If you have ever seen an insect walking on the surface of a pond, and wondered what was
going on, it is because of surface tension. Hydrogen bonds are holding the molecules
of water together, forming a surface "skin" that an insect can walk on (if it
spreads its weight out over the surface). Surface tension is responsible for water
sticking together in small drops. In a slowly dripping faucet, watch the drop
get bigger and bigger, until it finally falls into the sink. This happens because
the attraction of the water molecules to each other is strong, holding them together until
the drop gets to be a certain size, when the pull of gravity overcomes the cohesion of the
water molecules.
- Water has good adhesion: water molecules are attracted
to other polar molecules, and to solid surfaces. Water molecules adhere to soil particles,
to fibers in a towel, and to many other substances.
- Hydrogen bonding also water makes capillary action
possible: the ability of water to pull itself upward through small openings against the
pull of gravity. The water molecules adhere to the surface of the material in the opening,
and cohere to each other. You might think of it as if a mountain climber were
clinging to the side of the mountain with one hand, and holding on to his climbing
companion with the other: he is adhering to the mountain, and cohering to his companion.
Spill some water, then dip the end of a paper towel into it. What happens to
the water? It climbs upwards through the paper towel. This is capillary
action. Capillary action helps water move upward in soil.
- Water is a good solvent: water molecules can also
attract ions, or charged particles. This makes it able to dissolve other substances
easily.
- Water has a high specific heat. Specific heat is the
amount of heat required to raise the temperature of one gram of a substance 1 degree
Celsius. The specific heat of water is 1 cal/g, much higher than the specific heat of most
other substances (ammonia is an exception). For example, the specific heat of most earth
materials, such as rock and soil, is about 5 times as great as that of water. This means
that water can absorb more solar energy without its temperature increasing. Go to the
beach on a hot summer day (who said all scientific study has to be tough?), and notice the
temperature of the sand and the water. In the middle of the afternoon, the sand will
feel hot, and the water cool. At midnight, the sand will feel cool, and the water
temperature will be about the same. The sand changes its temperature through the course of
the day, but the water temperature doesn't change much. So a large body of water has a
moderating influence on climate.
- Water is one of the few substances that expands when it
freezes. Most substances become denser and contract when they are cooled, and expand
when they are heated. Water is less dense when it is frozen, so its solid form is lighter
than its liquid form; in other words, ice floats. This is great if you are a fish: you can
survive in the liquid water under a cover of ice on the surface. Otherwise, ice
would sink to the bottom of lakes and oceans, and it would be very hard to melt it.
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State and Latent Heat
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mailto:jthorn@mail.sdsu.edu
03/10/01
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