Chemistry Reference
In-Depth Information
FIGURE 83
Barometer.
A most basic and widely used instrument for measur-
ing atmospheric pressure is the centuries-old “mercury barometer.” This device
measures the atmospheric pressure as that pressure that exactly balances, at any
specific temperature, the pressure exerted by the height of a column of mercury
measured in millimeters: the higher the mercury level, the greater the air pres-
sure. At sea level and at a temperature of 25°C, for example, the height of a
mercury column is 760 mm and the pressure exerted is known as one “standard
atmosphere.” Pressure values higher than 760 mm of mercury are characteristic
of places below sea level, or at sea level where, as a result of particular weather
conditions, there is an increase in the atmospheric pressure. Values below
760 mm are typical of places at altitudes higher than sea level, or at sea
level but affected by special weather conditions that reduce the atmospheric
pressure.
Composition of the Atmosphere
The
air
that envelops the earth is made up of a mixture of gases (see Fig. 84).
In the lowest layer, the
troposphere
, the air is made up almost entirely of three
elements, nitrogen, oxygen, and argon, although others, some of them air
pollutants (see text below) are present in small but significant amounts. The
exact composition of the air varies slightly, however, depending on the alti-
tude above sea level, the location, and the time of the year.
Nitrogen.
Nitrogen
, a colorless, odorless, and tasteless gaseous element, is
the main component of the atmosphere, which makes up about 78% of
its volume; since it is also an important constituent of living organisms,
