Britannica Student Encyclopedia - 2010

They are known as solitary wasps.

They may build nests in the ground, lay

eggs inside plants, or even sneak their

eggs into the nests of other insects.

Many people fear the sting of wasps.

However, wasps usually sting only

people who swat at them or touch their

nest. In fact, wasps can be helpful. They

feed many troublemaking insects to

their young. They also help to spread

pollen when they visit flowers to drink

nectar. Spreading pollen allows flowers

to make seeds.

#More to explore

Ant • Bee • Insect

A potter wasp constructs a nest out of mud.

BRITANNICA STUDENT ENCYCLOPEDIA Wasp 21

 

Watch

#see Clock.

Water

Water is the most important liquid on

Earth. It covers almost 75 percent of

Earth’s surface in the form of oceans,

rivers, and lakes. All plants and animals

need water to live.

People have many uses for water besides

drinking. They use it for washing and

cooking. They use it to irrigate crops

and lawns, to clean streets, and to operate

air-conditioning units and heating

systems. They also use the power of

flowing water to produce electricity.

Makeup and Physical States

Water is made of tiny units called molecules,

which are combinations of even

smaller units called atoms. A molecule

of water is made of two hydrogen atoms

and one oxygen atom. The scientific

formula for water is H2O.

Water can be found in three physical

states: liquid, solid (ice), or gas (steam or

vapor). The molecules in all three states

are constantly moving. The speed of this

movement determines water’s physical

state. In ice, the water molecules vibrate

but basically stay in place. In liquid

water, the molecules move more quickly

but stay near each other. In vapor, the

molecules move so quickly that they fly

away in all directions.

Heat causes water to change from one

physical state to another. When ice is

heated, the water molecules move faster

and farther apart. This causes the ice to

melt into liquid. When liquid water is

heated, the molecules speed up even

more. Molecules at the surface of the

liquid begin to break loose and fly into

the air. In this way the liquid evaporates,

or becomes vapor.

Everyone should drink water every day.

Water makes up about 60 percent of an

adult’s body by weight. Children’s bodies

have an even higher percentage of water.

River water gushes down rapids. Liquid

water can flow freely. Water also occurs as

a solid, called ice, and as a gas, called

steam or water vapor.

22 Watch BRITANNICA STUDENT ENCYCLOPEDIA

 

This process also works in reverse. Cooling

temperatures slow down the molecules

so that vapor turns back into

liquid. Very cold temperatures turn liquid

into ice.

Boiling and Freezing

Boiling is the formation of bubbles of

vapor inside liquid. As water boils, the

bubbles of vapor rise to the surface and

escape into the air. At most places on

Earth, freshwater boils when it reaches

212° F (100° C). Once it reaches this

temperature, the water cannot get any

hotter. Any heat added to the water just

causes more water to evaporate.

Freezing is the transformation of liquid

water into ice. Freshwater freezes at

32° F (0° C).Water is lighter as a solid

than as a liquid. This is why ice floats

and why it forms on top of a lake

instead of at the bottom.Water also

expands when it freezes. This is why

water pipes can burst on very cold

nights. The water inside the pipes

pushes outward as it turns into ice.

The Water Cycle

Water on Earth is always moving. It

moves from Earth’s surface into the

atmosphere and then returns to the surface.

This movement is called the water

cycle.

The sun, air, and gravity work together

to create the water cycle. Heat from the

sun causes water to evaporate from the

Most of Earth’s water is in the oceans. But water is constantly being recycled. Through a

process called evaporation, water moves from Earth’s surface into the air as water vapor.

The vapor can form clouds, which make rain and snow. Rain and snow bring water back

to the ground again and into lakes, streams, and oceans. Some of the water seeps underground,

where it is called groundwater.

BRITANNICA STUDENT ENCYCLOPEDIA Water 23

 

surface of lakes, streams, oceans, and

plants. The water vapor moves into the

atmosphere. In the cool air high above

the ground, the water vapor changes

into droplets of water. Large groups of

these droplets are called clouds. Gravity

pulls the droplets back to Earth as rain.

The rain falls into oceans and lakes,

enters rivers, and seeps into the ground.

#More to explore

Cloud • Evaporation and Condensation

• Lake • Molecule • Ocean • Rain

• River

Waterfall

A waterfall is a place in a river where

water spills suddenly downward.Waterfalls

are known for their beauty and awesome

power.

The world’s tallest waterfall is Angel

Falls, in Venezuela. Its water plunges

3,212 feet (979 meters). Angel Falls and

other large waterfalls are sometimes

called cataracts. Cataracts often drop

straight down. The falling water may

not even touch the rock behind it.

Smaller or less steep waterfalls may be

called cascades. Cascades often fall over

a number of short rock walls.

The action of flowing river water is one