Water
Table of
Contents
Blue Planet National Science Standards Correlations ......................................I.
Chapter One: A Water World
The Chemical and Physical Properties of Water..............................................................1
The Role of Water in the Formation of Earth ..................................................................2
The Role of Water in the Origin of Life ...........................................................................3
The Hydrologic Cycle......................................................................................................4
Questions on: A Water World ...........................................................................................6
Lab Activity: Breaking the Tension ...................................................................................7
Chapter Two: Water, Water Everywhere
Saltwater Versus Fresh Water ...........................................................................................8
Oceans.............................................................................................................................8
Inland Lakes and Ponds .................................................................................................11
The Life of a River .........................................................................................................12
Groundwater and Aquifers ............................................................................................14
Glaciers, Sea Ice, and Ice Ages .......................................................................................15
Questions on: Water, Water Everywhere .......................................................................17
Lab Activity: Hard or Soft Water .....................................................................................18
Chapter Three: Defining Climate by Precipitation and Temperature
The Torrid Zones ...........................................................................................................20
Rainforests and Coastal Cloud Forests..................................................................20
Seasonal Wet-Dry Grasslands.................................................................................21
Tropical Dry Climates ............................................................................................21
The Temperate Zones....................................................................................................22
Steppes and Prairies...............................................................................................22
Mediterranean Climate .........................................................................................22
Deciduous Forests ..................................................................................................23
The Polar Zones—High Latitude Climates ....................................................................23
The Coldest Desert..................................................................................................23
The Boreal Forest (Taiga) ......................................................................................23
Tundra Climate ......................................................................................................23
Microclimates ................................................................................................................24
Questions on: Defining Climate by Precipitation and Temperature ..........................25
Lab Activity: Microclimates............................................................................................26
Chapter Four: Ocean Dynamics
Ocean Floor Topography...............................................................................................27
Ocean Currents .............................................................................................................28
Upwelling......................................................................................................................30
El Niño...........................................................................................................................31
Waves ............................................................................................................................32
Questions on: Ocean Dynamics ...................................................................................34
Lab Activity: Downwelling.............................................................................................35
Chapter Five: Tides
The Sun, the Moon, and the Tides .................................................................................36
Differences in Tides .......................................................................................................37
How Tides Affect the Shoreline .....................................................................................37
Tidal Estuaries and Intertidal Zones ..............................................................................37
Questions on: Tides .......................................................................................................39
Lab Activity: Nutrients at High and Low Tide ................................................................40
Glossary............................................................................................................................41
Answer Key .....................................................................................................................44
Safety and Ethical Issues ...........................................................................................47
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1
A Water
World
As far as we know, Earth is the only planet in our solar system that has water on it in
its liquid form.The Earth is covered with liquid water.
The Chemical and Physical Properties of Water
Water is a very simple compound, made from two of the most common elements in
the universe; two atoms of hydrogen bonded to one atom of oxygen. On Earth, water is so
common that it is used as a standard for all sorts of measurements, from specific gravity to
the definition of the Kelvin, the Systeme Internationale (SI) unit of thermodynamic
temperature (Fig. 1).
Water is unusual in one way, though.
FIG. 1
It is liquid between 0º C and 100º C, or
between 32 º and 212 º F—a relatively
large boundary. Both of its component
Water Molecule
atoms are gaseous at the temperatures
where water is liquid, and other common
hydrogen bonds are also gaseous at “liquid
water” temperatures. Oxygen is the key to
water’s state. Liquid oxygen is more
Oxygen
electronegative; it pulls on electrons much
more strongly than hydrogen, leaving a net
positive charge on the hydrogen side of
the molecule, and a net negative charge on
Hydrogen
Hydrogen
the oxygen side. The electrical attraction
“pulls” separate molecules closer together
and raises the boiling point. This attraction
is known as hydrogen bonding.
This electronegative quality also creates cohesion in water molecules, which leads
to some interesting effects. One is surface tension. Take an eyedropper and drop a
single drop of water on a mirror. The water tends to remain in the droplet, rather than
running all over the mirror. Water molecules tend to cling together because the oxygen
atom pulls strongly on the hydrogen atoms within its own molecule and on other water
molecules (Fig. 2).
MP4856 Blue Planet: Water © Milliken Publishing Company
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FIG. 1:
The chemical
elements of water
FIG. 2
FIG. 3
Adhesion
Water molecule
FIG. 2: This water
strider can walk
across a quiet pond
because the
surface tension of
the water creates an
elastic surface.
The animal is denser
than the water, so
buoyancy is not
holding it up.
FIG. 3:
Capillary Action
The other related effect of cohesion is capillary action. When you
put a straw into a glass of water, the level of the water will be higher in
the straw than in the glass. Water will run up a tube against the force of
gravity for short distances. The water initially tries to bond molecularly
with the surface of the tube, a process similar to cohesion called
adhesion. Then, more water molecules join the others through cohesion,
and eventually, the water moves up the narrow tube (Fig. 3).
Water has the second highest specific heat capacity of any known
chemical compound, after ammonia. This means that it takes a high
degree of heat to raise the temperature of water one degree Celsius.
This gives water an unusual role in keeping Earth cool; because there
is more water exposed than land, the water can help to keep the
planet cool by absorbing excess heat that comes to the planet as a
result of solar radiation. Just as it takes a great deal of heat energy to
raise the temperature of water, it also takes a great deal of cold to
lower water’s temperature. For this reason, Earth does not have the
Adhesion
temperature swings most of our neighbors in space are subject to.
An important and unusual property of water is that its common solid
form, ice, floats on its liquid form.This solid phase is not as dense as
liquid water because of the geometry of the hydrogen bonds which are
Cohesion
formed only at lower temperatures. For almost all other substances, the
solid form has a greater density than the liquid form.Water freezes in the
form of crystals, however, which take up more space than the original
water molecules. So, solid water (ice) is less dense than liquid water, and floats on top.
Fresh water at standard atmospheric pressure is most dense above the freezing point,
at 3.98 °C, and sinks by convection as it cools to that temperature. If it becomes colder,
then begins to freeze, it will rise instead. Therefore, deep, still water remains warmer than
shallower freezing water, so that ice in a body of water will form first at the surface and
progress downward, while the majority of the water underneath will remain at a constant
4 °C. This effectively insulates a lake floor from the cold, and keeps lake fish, amphibians,
and reptiles alive over the winter.
Another important feature of water is that it easily dissolves many minerals. For this
reason, water is often called the “universal solvent.” Salt easily dissolves in water, which
is why most of the water on the planet is saline. Other important minerals that dissolve in
water include calcium and its related calcites. It is from these minerals that animals with
hard body parts derive their ability to make their shells and skeletons. And as we shall
soon see, oxygen is also easily dissolved in water.
The Role of Water in the Formation of the Earth
Water arrived on Earth as the Earth formed through accretion, four and a half billion
years ago. None of this water was in its liquid state at the time, however. Most would
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have arrived as frozen chunks from space debris, including comets. Because the Earth was
so hot, the ice quickly became water vapor, which rose into the primitive atmosphere.
Some of the water vapor became a part of the planet, and was eventually “outgassed”
through volcanoes. Today, about 85% of the gas spewed out by volcanoes is water vapor.
Much of the water vapor was undoubtedly lost to space; however, some condensed
and “rained” back down on the hot planet, only to be evaporated again almost instantly.
Over many millions of years, however, this process served to cool down the planet, as the
water absorbed much of the Earth’s heat.
Eventually, warm pools of water began to form on the surface of the planet. This
served to cool Earth and its atmosphere even more. As it did, rain steadily increased. This
was a deluge of global proportions, and continued for a long period of time.
Recent discoveries of certain ancient zircon crystals, which form in the presence of
liquid water, suggest that this process did not take as long as scientists used to believe.
Indeed, the new research shows that, except for its unbreathable atmosphere, Earth was
probably a fairly cool and habitable place by 4.4 billion years ago—around the time it is
estimated that the first living things arose.
The Role of Water in the Origin of Life
The first living organisms arose in the water, and even today, the majority of living
organisms are comprised of water—between 60 and 99%, depending on the organism. Life
arose as a complicated chain of chemical reactions occurred in the warm water, which
ultimately gave rise to amino acids. These amino acids are the building blocks for DNA
and RNA, which are the basic molecules of life.
No one knows exactly how life made the jump from complex molecules that were not
living to complex molecules that could replicate themselves, but the first time this
occurred, the new living organism probably had no competition for survival. There were
no predators, and no competition for the first food sources, which would have been
smaller organic molecules moving toward their own emergence as life. The only real
struggle for survival the new being had was with the environment itself.
And, thanks to water, the environment was habitable in a way that early life on land
could never have been. Water served as a barrier from solar and other space radiation,
which bombarded the young Earth as its atmosphere assembled itself. The ozone layer,
which protects life on land from radiation, was not yet formed.
Also, the atmosphere itself was largely water vapor at this point in Earth’s history.
Water vapor in the atmosphere kept the temperature on the planet moderate—not too
cold at night, not too hot during the day. The type of life that arose on Earth, predictably,
arose in conditions conducive to liquid water. Whether life on other planets require this
sort of temperate environment remains to be seen.
The earliest organisms were anaerobic—that is, they did not require oxygen as part of
their metabolic process. Soon, these tiny anaerobes—called archaebacteria or archaea—
had extreme competition. A new type of bacterium evolved which began to use the light
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