Physical Characteristics of

Gases

Pressure

Objectives

1. Define pressure and relate it to force

2. Describe how pressure is measured.

3. Convert units of pressure.

4. State the standard conditions of temperature and pressure.

Pressure

To adequately describe a gas, you must know…• Volume• Temperature• Number of particles• Pressure

area

force Pressure

Atmospheric PressureThe 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 PressureUnit Symbol Std. Atmospheric

pressure

Millimeters of Hg mm Hg 760

Centimeters of Hg cm Hg 76

Inches of Hg in. Hg 29.92

Torr Torr 760

Pascals (N/m2) Pa 101,325

Atmospheres atm 1

Pounds per square inch psi 14.69

bars bar 1.01325

•The Pascal is the SI unit

Pressure Conversions• Convert 35 psi into units of atmospheres.atm

psi1

35 atm

psi

atmpsi 4.2

69.14

135

Standard Temperature and Pressure• Conditions used by scientists for comparison.

1 atm

0oC