WATER
Think about the different
ways you use water. You drink water when you are thirsty. You take a bath and
wash your clothes with water. You water the grass or other plants. You swim in
water. Water pouring over huge dams may even make the electricity that lights
up your home.
About three-quarters of
Earth’s surface is water. Living things are mostly made up of water. Without
water, there would be no life on Earth.
What Is Water?
Water is a chemical. Chemists say water is
a compound, a combination of different materials. Water is a combination of
oxygen and hydrogen atoms. An atom is a tiny bit of matter much too small to
see. Water is made of one atom of oxygen and two atoms of hydrogen. The three
atoms make the tiniest possible drop of water, called a molecule.
Water Molecule
A water molecule is made up of one oxygen
atom and two hydrogen atoms.
Water can be a liquid,
a solid, or a gas. Liquid water flows. Solid water is ice. Water in the form of
a gas is called water vapor.
Where Is Water Found On Earth?
Liquid water fills the
ocean, lakes, ponds, rivers, and swamps. Water droplets form rain clouds.
Liquid water makes beads of dew on the grass. It seeps down into the ground. It
fills underground lakes and streams.
Oceans 
Ice falls as hail or crystals
of snow. Ice forms on ponds and frosty windowpanes. Huge sheets of ice make
glaciers and icecaps at the North and South poles.
Water vapor is always
present in the air. Water vapor makes clouds in the sky. Water vapor makes fog
that hangs close to the ground. It is the steam that comes out of a teakettle.
Where Do We Get Drinking Water?
Not all water is safe
to drink. Water in lakes and rivers can be polluted. It can have harmful
chemicals or germs that cause disease. Long ago, many people living in cities
got sick or died from drinking dirty water.
Scientists have learned how
to clean, or purify, drinking water. They learned that boiling water could kill
germs. They learned that adding certain chemicals could kill germs. Engineers
learned how to keep dirty water in sewers, away from drinking water.
Governments passed laws to keep factories from polluting water.
People still get sick
from drinking polluted water. Many poor people still do not have pure drinking
water.
Could We Run Out Of Water?
Nature recycles water all
the time. Water in oceans, lakes, and rivers evaporates, or turns into a gas
and rises into the air. The water vapor eventually turns back into a liquid and
falls as rain. The water cycle keeps the total amount of water on Earth the
same. But most of this water is salt water in the ocean. People need fresh
water for drinking and for growing food on farms. Ocean water is too salty to
drink. It is too salty to use for watering plants.
Some places have more
fresh water than others. People who live near big lakes or rivers have more
fresh water than people who live in the desert. Places where a lot of rain
falls have more fresh water.
Places that usually have
enough fresh water sometimes have a drought. Very little rain falls during a
drought. People run short of water. People everywhere should be careful not to
waste water.
WATER CYCLE
Maybe you recycle cans, glass, and paper.
Did you know that nature recycles, too? One of the things nature recycles is
water. Water goes from the ocean, lakes, and rivers into the air. Water falls
from the air as rain or snow. Rain or snow eventually find their way back to
the ocean. Nature’s recycling program for water is called the water cycle.
In Storage
The water cycle has four stages:
storage, evaporation, precipitation, and runoff. Most of the water on Earth is
in the first stage, storage. Water on Earth gets stored in oceans, lakes, rivers,
ice, and even underground. The oceans store the majority of this water.
Evaporation:
Into The Air
Water goes from storage into the
atmosphere (the air that surrounds Earth) by a process called evaporation. When
water evaporates, it changes from a liquid into an invisible gas. The gas is
called water vapor. Water vapor goes up into the atmosphere.
Most of the water that gets into
the air evaporates from the surface of the oceans. Water also evaporates from
rivers and lakes. Water can also go from ice caps and icebergs into the air.
Ice changing into water vapor is called sublimation.
Precipitation:
Back To Earth
Water returns to Earth as
precipitation—rain, snow, or other moisture. Precipitation requires ice or
liquid water. Water vapor can change into tiny ice crystals or drops of water
when the air gets cold enough. Clouds are collections of tiny ice crystals or
water droplets.
When the ice crystals or drops of water
in a cloud get heavy enough, they fall to Earth as precipitation. Rain, snow,
sleet, and hail are all forms of precipitation. Most precipitation falls into
the oceans and goes right back into storage.
Runoff:
Moving Around On Land
Water that falls on land always
flows from high places to lower ones. This flow is called runoff. Precipitation
that falls on land runs downhill. The water cuts channels as it flows.
Some water seeps into the ground. It
fills cracks between rocks. Underground water also flows from places that are
full to places that have less water.
Water from land flows into streams.
Streams flow down mountains. Streams join together to make rivers and
eventually the water flows into storage in the ocean. Then the water cycle
starts all over again.
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