Earth’s Water
The Water Planet
Why is water important to life?
Scientists are always looking for water in other areas of
our solar system. Water is necessary for life. Scientists look
for water in other areas of the solar system as a first step to
finding life in these areas.
Water is important on Earth for other reasons. Earth’s
climates are affected by ocean currents that move thermal
energy around Earth. You might know thermal energy by
another name: heat. Large bodies of water, such as large
lakes, affect local weather patterns. Many organisms live in
water habitats. People also use water for moving goods on
boats and for recreation.
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Biological Functions
Water is necessary for the life processes of all living
organisms. From the smallest unicellular bacterium to the
giant blue whale, all organisms need water for life. The body
of a jellyfish is about 95 percent water. About 60 percent of
the mass of the human body is water. Even plant seeds that
seem dry have a small amount of water inside them.
Transport One of the main roles of water in an organism
is to transport materials. Water carries nutrients, such as
proteins, to cells and within cells. Water also carries wastes
away from cells.
Photosynthesis Water is necessary for chemical reactions,
such as photosynthesis, to take place within living
organisms. Photosynthesis is a reaction that occurs in light.
During photosynthesis, carbon dioxide and water react and
produce sugar and oxygen. Photosynthesis takes place in
plants, algae, and some bacteria. Organisms that carry out
photosynthesis are the beginning of most food chains.
Body Temperature Regulation Water helps prevent an
organism’s body temperature from becoming too high or too
low. In humans, water from the skin, or sweat, changes to a
gas. Sweat releases thermal energy, transferring it to the air.
This helps keep the body cool.
Warming Earth
One reason life can exist on Earth is that Earth’s atmosphere
traps thermal energy from the Sun. This process, shown below,
is called the greenhouse effect. Some of the Sun’s energy
that reaches Earth’s surface is absorbed and then emitted
back toward space. Gases in the atmosphere, such as water
vapor (H2O), methane (CH4), and carbon dioxide (CO2),
absorb some of this energy and emit it back toward Earth.
CH4
COa
The Sun’s energy
reflected off atmosphere
H2O
The Sun’s energy
absorbed by Earth
The Sun’s energy
reflected off Earth
back toward space
Gases in the atmosphere
absorb energy emitted by
Earth and radiate it in all
directions. Some energy
gets radiated back toward
Earth. This warms Earth
over time.
Energy emitted
by Earth
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There is more water vapor in the atmosphere than any
other greenhouse gas. Without the greenhouse effect, Earth’s
average surface temperature would be below freezing,
about -18ºC. All of the water at Earth’s surface would be ice.
Organisms could not survive at that temperature.
Keeping Earth’s Temperature Stable
Think about what happens at the beach on a hot, sunny
day. If you walk barefoot across the sand, you might burn
the bottoms of your feet. Where do you find relief from the
hot sand? If you head to the water, you will find that it is
much cooler. Why does the water have a lower temperature
than the sand?
Water has a high specific heat. Specific heat is the amount of
thermal energy needed to raise the temperature of 1 kg of a material by
1°C. In fact, the specific heat of water is about six times
higher than the specific heat of sand. This means the water
would have to absorb six times as much thermal energy to
have the same temperature as the sand. The water at a beach
will always be cooler than the sand on the beach.
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Water’s high specific heat is important to life on Earth.
Water vapor in the air helps control the rate at which air
temperature changes. The temperature of water vapor
changes slowly. As a result, the temperature change from
one season to the next is gradual.
Large bodies of water, such as oceans, also heat and cool
slowly. This provides a stable temperature for aquatic
organisms. The climate in coastal areas is affected by the
stable temperatures of the ocean. The local weather patterns
of inland areas near large lakes are affected as well. Examples
of how water is important to life are summarized in the table
below.
Importance of Water to Life on Earth
Importance to Life
Examples
Biological functions
• transport of nutrients and wastes to and
from cells
• photosynthesis
• body temperature regulation
Keeps Earth warm
• greenhouse effect
• air temperature regulation
Stabilizes Earth’s
temperature
• gradual temperature change from one
season to the next
• high specific heat causes large bodies of
water to heat and cool slowly
• stable temperature for aquatic organisms
Atmosphere
Glaciers
68.7%
Lakes, rivers, streams,
swamps, atmosphere
about 1%
Groundwater
30.1%
Freshwater
3%
Oceans
97%
Glaciers
Lake
Rivers
Groundwater
Ocean
Water on Earth
Distribution of Water on Earth
The figure above shows that most of Earth’s water is in
oceans. Only about 3 percent is freshwater, water that is not
salty. Freshwater is located at Earth’s surface, in the ground,
and in ice caps and glaciers. Only about 1 percent of all
water on Earth is in lakes, rivers, streams, swamps, and the
atmosphere.
Structure of the Hydrosphere
The hydrosphere is all the water on and below Earth’s surface and
in the atmosphere. The figure above shows the many parts of
the hydrosphere. Water is in oceans, lakes, rivers, streams,
and underground. Water beneath Earth’s surface is called
groundwater. Clouds are a collection of tiny water droplets
or ice crystals. Ice, or water in the solid state, is in ice caps
near Earth’s poles and in glaciers. Water vapor, or water in
the gaseous state, is in the atmosphere.
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You have just read several reasons why water is important
for life. You also use water every day. People use water for
bathing, cooking, and drinking. About 70 percent of Earth’s
surface is covered by water. How is all this water distributed?
Water Changes State
Water is the only substance that exists in nature in three
states—solid, liquid, and gas—within Earth’s temperature
range. Water can easily change state within the hydrosphere.
Snow and ice are both solid states of water. Snow and ice
melt to liquid water.
When enough thermal energy is added to liquid water, it
changes to a gas and enters the atmosphere. When water
changes from one state to another, thermal energy is either
absorbed or released. Thermal energy always moves from an
object with a higher temperature to an object with a lower
temperature.
Between Solid and Liquid
When thermal energy is added to ice, the water molecules
gain energy. If enough thermal energy is added, the ice will
reach its melting point and change to a liquid.
If thermal energy is released from liquid water, the reverse
happens. As the thermal energy is released, the water molecules
lose energy. When the molecules lose enough energy, the
liquid reaches its freezing point and ice forms.
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Between Liquid and Gas
As thermal energy is added to liquid water, the molecules
gain energy. When enough energy is added to the water, it
reaches its boiling point. At its boiling point, water changes
to water vapor, a gas.
It takes less energy for molecules at the surface of water
to break free from surrounding molecules. Therefore, water
at the surface can change to a gas at temperatures below the
boiling point. It evaporates. Evaporation is the process of a liquid
changing to a gas at the surface of the liquid.
When water vapor molecules lose thermal energy,
condensation occurs. Condensation is the process of a gas
changing into a liquid. For example, you might have noticed
water droplets on the outside of a glass of ice water. The
temperature of the water vapor in the air in contact with the
glass is lowered by the glass of ice water. The water vapor
condenses into liquid water on the outside of the glass.
The Water Cycle
The series of natural processes by which water continually moves
throughout the hydrosphere is called the water cycle. Water
changes from one state to another as it moves through the
water cycle.
Driving the Water Cycle
The Sun and gravity are the two main factors driving the
water cycle. Energy from the Sun causes water at Earth’s
surface to evaporate. Gravity causes the water to fall back to
Earth as precipitation. On Earth’s surface, gravity moves
water from higher to lower areas. Water eventually returns to
the oceans and other storage areas in the hydrosphere. Then
the cycle continues, as shown below.
Evaporation
Water on Earth’s surface evaporates because energy from
the Sun breaks the bonds between water molecules. Liquid
water changes into water vapor and enters the atmosphere.
Evaporation occurs throughout the hydrosphere.
Transpiration The evaporation of water from plants is called
transpiration. Plants absorb most of the water they need
from the ground. When plants have a good supply of
water or air temperatures rise, plants transpire more rapidly.
They release even more water vapor into the atmosphere.
Transpiration usually occurs through the leaves.
Precipitation
Condensation
Runoff
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Evaporation
Condensation and Precipitation
Water vapor from transpiration and evaporation rises in
the atmosphere. As the water vapor rises, it cools and
condenses into a liquid. Water vapor condenses around dust
particles in the atmosphere and forms droplets. The droplets
combine and form clouds.
The water droplets eventually fall to the ground as rain. If
the temperature is low enough, the droplets will freeze. Then
they reach Earth’s surface as other forms of precipitation,
such as snow, sleet, or hail.
Runoff and Storage
When precipitation reaches Earth’s surface, the water
flows downhill as a result of gravity. Water from precipitation
that flows over Earth’s surface is called runoff. Runoff enters
streams and rivers and eventually reaches lakes or oceans.
Some precipitation soaks into the ground and becomes
groundwater.
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Water moves constantly through the water cycle.
However, most water remains in storage areas for long
periods of time. A storage area of the water cycle is called a
reservoir. Reservoirs can be lakes, oceans, groundwater,
glaciers, and ice caps.