Chemistry Reference
In-Depth Information
Over recent years, scientists have become aware
of a reduction in the amount of ozone in our
atmosphere and of the formation of 'ozone holes'
in the stratosphere (Figure 11.5). The reduction
of ozone in our atmosphere has led to an increased
risk of skin cancer as more harmful ultraviolet
radiation has reached the surface of the Earth. This
is a different type of problem from the greenhouse
effect and associated global warming, which is
caused mainly by an increase in the amount of
carbon dioxide in the atmosphere. For a further
discussion of the greenhouse effect and global
warming see p. 212.
The structure of the atmosphere
The atmosphere is 100 km thick (Figures 11.1 and
11.2) and it is divided into four layers:
●
troposphere
●
stratosphere
●
mesosphere
●
thermosphere.
The gases in the atmosphere are held in an envelope
around the Earth by its gravity. About 75% of the
mass of the atmosphere is found in the layer nearest
the Earth called the
troposphere
. Beyond this layer,
the atmosphere reaches into space but it becomes
extremely thin beyond the mesosphere.
The composition of the atmosphere
If a sample of dry, unpolluted air was taken from
any location in the troposphere and analysed, the
composition by volume of the sample would be
similar to that shown in Table 11.1.
Table 11.1
Composition of the atmosphere.
Component
%
Nitrogen
78.08
Oxygen
20.95
Argon
0.93
Carbon dioxide
0.04
Neon
0.002
Helium
0.000 5
Krypton
0.000 1
Xenon plus minute amounts of other gases
0.000 01
Measuring the percentage of oxygen in the
atmosphere
When 100 cm
3
of air is passed backwards and
forwards over heated copper turnings it is found that
the amount of gas decreases (Figure 11.6). This is
because the reactive part of the air, the oxygen gas,
is reacting with the copper to form black copper(
ii
)
oxide (Figure 11.7). In such an experiment, the
volume of gas in the syringe decreases from 100 cm
3
to about 79 cm
3
, showing that the air contained
21 cm
3
of oxygen gas. The percentage of oxygen gas
in the air is:
Figure 11.5
This diagram comes from NASA's ozone monitoring
programme TOMS (Total Ozone Mapping Spectrometer). The ozone hole
over the Antarctic (shown in purple and pink on the diagram) is largest
in the Antarctic spring. Note: Dobson Units are a measure of the total
amount of ozone in a vertical column from the ground to the top of the
atmosphere.
Questions
1
Produce a time line for the formation of the atmosphere,
starting with the formation of the Earth 4500 million
years ago.
2
Use your research skills to fi nd out:
a
more about the ozone hole problem
b
what precautions are necessary to prevent an increase in
skin cancers.
21
100
× 100 = 21%
