Behavior of Gases
Chapter 2 Section 2
Measuring Gases
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When you think of a gas, what kinds of
gases do you think of?
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Helium, oxygen, carbon dioxide
What can affect the volume of a gas such
as helium?
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Temperature
Air pressure
Volume
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Volume is the amount of space that matter
fills.
Volume is measured in cubic centimeters,
milliliters, liters, and other units.
Because a gas fills the space available, the
volume of a gas is the same as the volume
of its container.
Temperature
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Temperature is the measure of the average energy
of motion of the particles of a substance.
The faster the particles are moving, the greater
their energy and the higher the temperature.
You might think of a thermometer as a
speedometer for molecules.
On an ordinary day, the particles in a gas move
very fast.
At room temperature, the particles in a typical gas
travel about 500 meters per second. (1500 feet
per second)
Pressure
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Because gas particles are moving, they constantly
collide with the walls of their container.
As a result, the gas exerts an outward push on the
walls of the container.
The pressure of a gas is the force of its outward
push divided by the area of the walls of container.
Pressure is measured in units of kilopascals (kPa).
Pressure = Force
Area
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About Gases
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The firmness of an object inflated with a gas,
such as a soccer ball, comes from the pressure of
the gas.
If the gas leaks from the ball, the pressure
decreases.
A gas flows from high pressure to low pressure.
The air inside the ball has a higher pressure than
the air outside the ball, so the gas flows out.
The pressure inside the ball drops until it is equal
to the pressure outside of the ball.
Relating Pressure and Volume
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Pressure is also related to the volume of the
container.
In the 1600’s an English scientist, Robert
Boyle was experimenting with ways to
improve air pumps.
He found that gases behave in predictable
ways…
Boyle’s Law
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Boyle found that when the pressure of a
gas is increased at constant temperature,
the volume of the gas decreases.
When the pressure is decreased, the volume
increases.
The relationship between the pressure and
volume of a gas is called Boyle’s Law.
Where does Boyle’s Law Apply?
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Boyle’s Law plays an important role in research done with
high-altitude balloons.
Balloons made of lightweight plastic were filled with small
amounts of helium and released into the atmosphere.
Scientists found that as the balloon rises through the
atmosphere, the air pressure around it decreases steadily,
while the helium inside expands, stretching the balloon to a
greater and greater volume.
If the balloon were full at takeoff, it would have burst from
the change in air pressure before it got very high.
Pressure and Temperature
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Temperature is the measure of the average
speed of the particles of a gas.
The higher the temperature of a gas, the
faster the particles are moving.
When the temperature of a gas at constant
volume (in a closed, rigid container) is
increased, the pressure of the gas increases.
When the temperature is decreased, the
pressure of the gas decreases.
Pressure and Temperature in
Action
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Think about a 18 wheeler. The tires on these
trucks must be very large, heavy, and stiff in
order to support the weight of the truck.
On a long trip, in the summer, a truck’s tires can
get very hot.
As the temperature increases, so does the pressure
inside the tire.
If the pressure becomes higher than the tire can
hold, the tire will burst apart.
Relating Volume and
Temperature
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Gas increases in volume the the temperature
increases.
When the temperature decreases, the volume with
decrease.
This is important, especially to the people who
are in charge of large balloons in parades.
What do the people in charge of the Macy’s Day
Parade need to take into consideration so that
they make sure the balloons are filled properly?
Charles’ Law
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In the late 1700s, a French scientist, Jacques Charles,
examined the relationship between the temperature and
volume of a gas kept at a constant pressure.
He measured the volume of a gas at various temperatures in a
container whose volume could change.
He found that when the temperature of a gas increased at
constant pressure, its volume increases.
When the temperature of a gas decreases, its volume also
decreases.
Because the particles of a gas move fast at higher
temperatures, they collide more often with the walls around
them.
As long as the volume of the container can change, the total
push of the collisions results in the gas taking up more space
Charles Law in Action
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Look on page 55 at Figure 15.
In the photograph on the left, a balloon in a
beaker of water, resting on a tub of ice, shows
how the balloon stays smaller.
In the photograph on the right, the balloon in the
beaker is sitting on a hotplate.
We can see the difference in the balloon where
the increase in temperature results in an increase
in volume.
Drawing a Picture