395
We cannot see, smell, or taste oxygen, yet without it, none of
us could survive longer than a few minutes. It is fortunate, then, that
oxygen is the most common substance on earth. oxygen is a gas. mixed
with other gases, it makes up about one-fifth of the air we breathe.
most of the oxygen in the world, though, does not float free as a
gas. Instead, the oxygen is bound up in combination with other
substances—in a solid or liquid form. this is because oxygen is
chemically reactive: it readily combines with other substances,
often giving off energy in the process. Burning is an example
of oxygen at work. When a piece of timber burns, oxygen is
combining with the wood and giving off heat. oxygen is also
found in water, combined with atoms of another
gas, hydrogen. oxygen can be extracted
from water by passing an electric
current through it. the electricity
breaks the water into its
parts—the gases oxygen
and hydrogen—and
oxygen bubbles off.RespIRatIon
our bodies need
oxygen to release the
energy consumed
when we use our
muscles. the
oxygen we
breathe in is
used to “burn”
the food we eat,
producing energy.
this process is
called respiration.
Blood carries the
oxygen from the
lungs, which
extract it from the
air, to the muscles
where it is needed.oxygen In WateR
seawater contains
dissolved oxygen. Fish
use this oxygen to
breathe. Water flows
over their gills, which
extract the oxygen.
Unlike other fish,
some sharks can
breathe only when
moving in the
water. to avoid
suffocating, they
must swim constantly,
even when asleep.BURnIng
nothing can burn
without oxygen. In outer
space there is no air or
oxygen, so it would
be impossible to
light a fire. the
rocket motors
used to launch
spacecraft need
oxygen to burn
the rocket fuel
and propel the
craft upward.
spacecraft carry
their own supply
of pure oxygen,
which mixes with the
fuel in the rocket motor.
When anything burns in
pure oxygen, it produces a
very hot flame. In welding
machines a fuel gas is
burned with pure oxygen,
producing a flame hot
enough to melt metals.Find out more
chemistry
Human body
lungs and breathing
plants
scienceMountain climbers, astronauts,
and undersea divers carry a
supply of oxygen to breathe.
A special valve releases the
oxygen at the correct
pressure for
breathing.Oxygen
People and
animals
breathe
in oxygen.oxygen cycle
Breathing air or burning fuel removes oxygen
from the atmosphere and gives off carbon
dioxide. plants do the reverse. During the
day, they produce energy for growth by the
process of photosynthesis. the green parts
of the plant take in sunlight,
water, and carbon dioxide to
make new cells, and give
off oxygen. thus,
oxygen continually
passes into and
out of the air.
this is called
the oxygen cycle.Green plants absorb
carbon dioxide
breathed out by
living creatures.US_395_Oxygen.indd 395 22/01/16 4:45 pm