Introduction:
Properties of Gases
What do you know about gases?
• Particle Diagrams
• Ar(g), H2(g), CO2(g)
• Gases take shape and volume of
their container
• Gases flow
• Gases have low densities
Properties of Gases
1. Gases have mass
2. Gases take shape & volume of their
container
3. Gases are compressible
4. Gases move through each other
easily
 “Diffusion”
perfume, skunks!
5. Gases exert pressure
Gases take shape of container
Why don’t these balloons
keep expanding until
they pop?
What is pressure?
• Pressure = force per unit area
• P = Force
Area
Force = mass x acceleration
Would you rather have your foot stepped on by
someone wearing sneakers or stilleto heels?
What is air pressure?
• pressure exerted on us by weight
of gases above our heads (& all
around our body)
• at sea level:
air pressure = 1 atmosphere
Torricelli
• discovered air pressure
• invented mercury barometer in 1643
• 1st person to propose correct
explanation for wind
Mercury Barometer 1
To report air
pressure: take
short-cut & report
height of Hg column
at 1 atm column is:
29.92 in Hg
76 cm Hg
760 mm Hg
How does the downward pressure
of the Hg in the column compare
to the pressure of the
atmosphere?
water barometer has to be 13.6
times taller than Hg barometer
(DHg = 13.6 g/ml) because …
DH2O = 1.0 g/ml
How does air pressure at top of Mt. Whitney
(14,494 ft) compare to air pressure at John
Jay?
It’s less
What about Death Valley
(86 m below sea level)?
It’s greater
When you drink through a straw,
you reduce the pressure in the
straw
• Why does the liquid in the cup go up the
straw?
• Could you drink a soda this way on the
moon? Why or why not?
Units of pressure = 1 atm
• 14.7 lb/in2
• 29.9 in Hg
• 1.013 bars
U.S. pressure gauges
U.S. weather service
Physics & Astronomy
CHEMISTRY:
1 atm = 760 torr = 760 mm Hg = 101.3 kPa
SI Units
What causes the pressure of a gas
in a closed container?
Impacts of gas
molecules with walls
of container
Anything that ↑ # of
impacts per second
or force of each
impact ↑ pressure
Microscopic View
Light molecules move
faster & hit walls more
often
Heavy molecules hit
walls with greater force
These 2 effects
essentially balance out
**Gas pressure doesn’t depend on the
identity of the gas**
Pressure Depends on
1) concentration
• # of gas molecules per unit volume
(moles!!)
and
2) temperature
How fast do molecules in the air
move?
• Depends on mass
• Light molecules are faster than
heavy molecules at same
temperature
• Temperature = measure of average
KE of particles in system
Molecular Speeds at 298 K
•
•
•
•
•
H2
He
O2
Ar
Xe
1.93 X 105 cm/sec
1.36 X 105 cm/sec
4.82 X 104 cm/sec *
4.31 X 104 cm/sec
2.38 X 104 cm/sec
1080 miles per hour
Pressure – Microscopic View
• Gas molecules hit walls of container
• Pressure depends on
– Number of impacts per unit time
– Force of each impact
Pressure – Macroscopic View
• Pressure depends on:
• how many gas molecules per volume and
• temperature
• same amount of gas exerts different
pressure at different temperatures
• Tires
• Bike
• Car
• etc
Describing Gas Phase System
Need 4 variables to describe gas
phase system from macroscopic or
lab view
1. Pressure (P)
2. Volume (V)
3. Temperature (T)
4. Amount of gas (# moles)
Exit Ticket
1. List 5 common properties of all gases?
2. Draw a particle diagram substance in gas
phase
3. Use picture from #2 to explain at least 2
properties of gases
Bonus:
At 0oC, a He atom is moving at 120 meters/sec.
How fast is this in miles per hour? Show all
work!