Physical
Characteristics of
Gases
Pressure
Objectives
1.
2.
3.
4.
Define pressure and relate it to force
Describe how pressure is measured.
Convert units of pressure.
State the standard conditions of temperature
and pressure.
Pressure
To adequately describe a gas, you must know…
• Volume
• Temperature
• Number of particles
• Pressure
force
Pressure 
area
Atmospheric Pressure
The gases that make up the
atmosphere exert
pressure on their
surroundings. It is a
result of:
1. Collisions between gas
particles.
2. The weight of the
atmospheric gases
above the earth’s
surface.
Measuring Pressure
• Atmospheric pressure is
measured using a barometer.
• Atmospheric pressure
depends on elevation and
weather.
• Normal atmospheric pressure
at sea level: 760 mm Hg
Measuring Pressure
• A manometer can be
used to measure the
pressure of an enclosed
gas.
• The pressure is
measured relative to
atmospheric pressure.
Units of Pressure
Unit
Symbol
Std. Atmospheric
pressure
Millimeters of Hg
Centimeters of Hg
Inches of Hg
Torr
mm Hg
cm Hg
in. Hg
Torr
760
76
29.92
760
Pascals (N/m2)
Atmospheres
Pounds per square inch
Pa
atm
psi
101,325
1
14.69
bars
bar
1.01325
•The Pascal is the SI unit
Pressure Conversions
• Convert 35 psi into units of atmospheres.
1atm
35 psi 
 2.4atm
14.69 psi
Standard Temperature and Pressure
• Conditions used by scientists for comparison.
1 atm
o
0C