http://www.ck12.org Chapter 14. The Properties of Gases
fluid will change height to a different level that is proportional to the pressure of the sample. Because this was one
of the first standardized ways to measure pressure,millimeters of mercury (mm Hg)is actually a common unit of
pressure. This unit is also sometimes referred to as atorr, short for Torricelli. Standard atmospheric pressure (1
atm) is defined as 760 mm Hg, which is approximately equal to the average atmospheric pressure at sea level. The
SI unit for pressure is the Pascal, where (1 Pascal = 1 newton/m^2 ). Because this is such a small unit, kilopascals
(kPa) are also commonly used. The relationships between various units of pressure are shown below:
1 atm = 1.01325 bar = 101.325 kPa = 760 mm Hg = 760 torr = 29.92 in Hg = 14.696 psi
Example 14.1
The pressure in a bicycle tire is measured to be 46 psi at room temperature. What is this pressure in atm?
46 psi× 141 .atm 7 psi= 3 .1 atm
Atmospheric Pressure
Atmospheric pressurecan be thought of as the force exerted by a hypothetical column of gas that stretches from
sea level to the outer reaches of the atmosphere. If we divide the total weight of the air in the column (the force) by
the area that it is pressing down on, we get a pressure of about 1 atm.Figure14.3 illustrates this type of calculation.
FIGURE 14.3
Illustration of atmospheric pressureIf we go up in altitude, there is less air in between the ground and the edge of the atmosphere. The smaller amount
of force means that atmospheric pressure is lower at higher altitudes (seeFigure14.4).
As shown here, pressure decreases to less than 25% of ground-level atmospheric pressure when we get to an altitude
of about six miles. Since this is a common cruising altitude for commercial airplanes, pressurized cabins are needed
for the comfort and survival of the passengers. This is because the density of the air decreases along with the
decreasing air pressure, so there is less oxygen in a breath of air. You’ve probably noticed this effect in high-
mountain areas, as it often feels harder to breath.
Example 14.2
If you were to ascend from sea level to the top of a 10,000-foot high mountain (about 2 miles), what would the
atmospheric pressure be? What percent of standard atmospheric pressure is this value?