Behavior of Gases
Behavior of Gases
 Pressure
 Boyle’s Law
 Temperature and
Pressure
 Charles’ Law
What is Pressure?
 Pressure is the amount
of force exerted per
unit of area, or P = F/A.
 In other words,
pressure is a measure of
the force felt by the
walls of a container.
 A balloon and a bicycle
tire are considered to
be containers.
 They remain inflated
because of collisions the
air particles have with
the walls of their
container.
Pressure
 This collection of
forces, caused by the
collisions of the
particles, pushes the
walls of the container
outward.
 If more air is pumped
into the balloon, the
number of air particles
is increased.
 This causes more
collisions with the walls
of the container, which
causes it to expand.
Units of Pressure
The SI unit of pressure is called a
pascal (Pa).
Because pressure is the amount of
force divided by area, one pascal of
pressure is the amount of force divided
by area or one Newton per square meter
or 1 N/m2.
What is Atmospheric Pressure?
Atmospheric pressure is the pressure
at any point in the Earth’s atmosphere.
At Earth’s surface, the atmosphere
exerts a force of about 101,300 N on
every square meter—about the weight
of an elephant.
Atmospheric pressure decreases with
altitude.
Boyle’s Law
Boyle’s law is one of the laws in physics
that concern the behavior of gases.
What happens to the gas pressure if
you decrease the size of the container?
If you squeeze gas into a smaller space,
its particles will strike the walls more
often giving an increased pressure.
The opposite is true, too.
Boyle’s Law (2)
Robert Boyle (1627-1691), a British
scientist, described this property of
gases.
According to Boyle’s law, if you
decrease the volume of a container of
gas and hold the temperature constant,
the pressure of the gas will increase.
An increase in the volume of the
container causes the pressure to drop,
if the temperature remains constant.
Boyle’s Law (3)
 Boyle’s law states
that as pressure is
decreased the
volume increases.
 The opposite also is
true, as shown by
the graph.
 As the pressure is
increased, the
volume will decrease.
Below are the Results of an Experiment
Pressure P
Volume V
P x V
1.1
40
44
1.7
26
2.2
20
2.6
17
Calculate P x V (pressure x volume) for each set of results.
What do you notice?
Boyle’s Law in Action
 When Boyle’s law is applied to a real life
situation, we find that the pressure multiplied
by the volume is always equal to a constant if
the temperature is constant.
 You can use the equations P1V1 = constant =
P2V2 to express this mathematically.
• This shows us that the product of the initial
pressure and volumedesignated with the
subscript 1is equal to the product of the
final pressure and volumedesignated with
the subscript 2.
Problem
 A deep sea diver is
working at a depth
where the pressure is
3.0 atmospheres. He is
breathing out air
bubbles. The volume of
each air bubble is 2 cm3.
At the surface the
pressure is 1
atmosphere. What is
the volume of each
bubble when it reaches
the surface?
Solution
 We assume that the temperature is constant,
so Boyle’s Law applies:
 Formula first:
P1 x V1 = P2 x V2
 Then numbers: = 3.0 atm x 2 cm3 = 1.0 atm x V2
 Now rearrange the numbers so that you have
V2 on one side, and the rest of the numbers
on the other side of the ‘equals’ symbol.
Solution (2)
3
3.0 atm x 2.0 cm
V2 
1.0 atm
Therefore the volume of bubbles = 6.0 cm3.
Note that P1 and P2 have the same units as do V1
and V2.
Pressure and Temperature
Relationship
What happens if you heat an enclosed
gas? The particles of gas will strike the
walls of the canister more often.
If the pressure becomes greater than
the canister can hold, it will explode.
At a constant volume, an increase in
temperature results in an increase in
pressure.
Charles’ Law
 Jacques Charles (17461823) was a French
scientist who studied
gases.
 According to Charles’
law, the volume of a gas
increases with
increasing temperature,
as long as pressure does
not change.
 As shown in the graph,
as with Boyle’s law, the
reverse is also true.
Charles’ Law (2)
 If we place a balloon
in liquid nitrogen, it
shrinks.
 So, gases shrink if
cooled.
Charles’ Law (3)
 A hot air balloon
demonstrates that if
we heat a gas, it
expands.
 Both of these
examples (shrinking
balloon and hot air
balloon) illustrate
Charles’ law.
Charles’ Law (4)
Charles’ law can be explained using the
kinetic theory of matter.
As a gas is heated, its particles move
faster and faster and its temperature
increases.
Because the gas particles move faster,
they begin to strike the walls of their
container more often and with more
force.
Using Charles’ Law
 The formula that
relates the variables
of temperature to
volume shows a
direct relationship,
when temperature is
given in Kelvin.
 When using Charles’
law, the pressure
must be kept
constant.
V1 V2

T1
T2
Using Charles’ Law (2)
• What would be the
resulting volume of a
2.0-L balloon at
25.0C that was
placed in a container
of ice water at
3.0C?
Solution
As Charles’s law predicts, the volume
decreased as the temperature of the
trapped gas decreased.