childrens_illustrated_encyclopedia_template_US_layer_v1
556
We are surrounded by water. More than 70 percent of earth’s
surface is covered by vast oceans and seas. In addition, 10 percent of the
land—an area the size of south america—is covered by water in the form
of ice. However, little new water is ever made on earth. The rain that
falls from the sky has fallen billions of times before and will fall
billions of times again. It runs down the land to the sea,
evaporates (changes into vapor) into
the clouds, and falls again as rain
in an endless cycle. Water has a
huge effect on our planet and
its inhabitants. all plants
and animals need water to
survive; life itself began in
earth’s prehistoric seas. seas
and rivers shape the land over
thousands of years, cutting cliffs
and canyons; icy glaciers dig out
huge valleys. Water is also
essential to people in homes
Water for life and factories, and on farms.
all plants and animals,
including humans, are
made largely of water
and depend on water
for life. For instance,
more than two-thirds
of the human body is
water. To replace water
lost by urinating,
sweating, and breathing,
we must drink water
every day to stay healthy.
no one can survive for
more than four days
without water.sTaTes oF WaTer
Pure water is a compound of two
common elements—hydrogen
and oxygen. In each water
molecule there are two
hydrogen atoms and one
oxygen atom; scientists
represent this by writing
H 2 o. Water is usually in a liquid
state, but it can also be a solid or
a gas. If left standing, water slowly
evaporates and turns into water
vapor, an invisible gas. When
water is cooled down enough,
it freezes solid and turns to ice.Water 556-
salty water boils at a higher
temperature and freezes at
a lower temperature than
freshwater, which is why salt
is put on roads in the winter
to keep ice from forming.WaTer vaPor
Water boils at 212°F (100°C). at this
temperature it evaporates so rapidly that
water vapor forms bubbles in the liquid.
Water vapor is invisible; visible clouds of
steam are not water vapor, but tiny droplets of
water formed when the hot vapor hits cold air.ICe
Water freezes when the
temperature drops below 32°F
(0°C). Water expands, or takes
up more space, as it freezes.
Water pipes sometimes burst in
very cold winters as the water
inside freezes and expands.The force of surface
tension holds water
molecules together so
that they form small,
roughly spherical drops.Water
surFaCe TensIon
The surface of water seems to
be like an elastic skin. You can see
this if you watch tiny insects, such as
water striders, walking on water—
their feet make hollows in the
surface of the water, but the
insects do not sink. This “skin”
effect is called surface tension.
It is caused by the attraction
of water molecules to each
other. surface tension
has another important
effect: it causes water
to form drops.Molecules at the surface
have other molecules pulling on them only from
below. This means there is a force pulling on this
top layer of molecules, keeping them under
tension like a stretched rubber band.
In the body
of the liquid, each
water molecule is
surrounded by others,
so the forces on them
balance out.US_556_Water_1.indd 556 09/02/16 5:29 pm