Gasses exert pressure through the impacts of their molecules on
the walls of their containers.
Although we can’t see any physical evidence of how a gas can exert
a pressure, we certainly can feel that pressure. To understand how a gas exerts
pressure we need to recall the underlying atomic model: a gas is a bunch
of atoms bouncing around like superballs.
When an atom bounces off the walls of its container, the container feels
the impact in the same way you would feel an impact from a ball bouncing off
of a tennis racquet. However, the impact felt by the wall of the container
is extremely small. The bouncing of one atom off of the wall of a container
would be virtually insignificant. It takes millions upon millions of impacts
between atoms and the walls of their containers concentrated on a vary small
area to register a measurable pressure.
There are two primary factors which explain the magnitude of the pressure
exerted by atoms: the frequency of impacts and the force of those impacts.
There are several ways that gas pressure can be increased due to increased
frequency of impacts.
Put more gas in the container.
If you have more gaseous atoms in a container then there will be more
frequent impacts against the walls of the container causing a greater
pressure on its walls.
Make the container smaller. If you
have the same number of atoms crammed into a smaller space, then they
will hit the walls more frequently with more atoms hitting the same
area repeatedly, increasing the pressure on the walls.
Raise the temperature. If you make
the atoms move faster then they will hit the walls more frequently,
increasing the pressure on the walls.
We can also increase gas pressure if each atom hits the wall of its
container with greater force. There is one primary way to make this
happen:
Raise the temperature.
By raising the temperature you add kinetic energy [energy of motion]
to the atoms, therefore, increasing their velocity. If they are moving
faster when they hit the wall of the container, then the impact against
the wall will be greater, increasing the pressure on the walls.
There is an intimate relationship between gas pressure, temperature, volume
of the container, and the number of molecules inside the container.
Boyle's Law: Pressure vs. Volume
If you make the volume of a container smaller the molecules
will hit off the walls more frequently. This means that the pressure will
increase.
If you make the volume of a container larger the molecules will spread
out and hit the walls of the container less frequently, so exerting a
lower pressure on the container.
Charles' Law: Pressure vs. Temperature
If you raise the temperature of the gas molecules then they
will move around faster hitting the walls of the container more often
AND harder. This will increase the pressure felt by the container.
If you lower the temperature of the gas molecules then they will slow
down and the opposite will happen.
Avagadro's Law: Pressure vs. Number of Molecules
If you add more molecules to the same size container then there
will be more impacts on the walls of the container causing a greater pressure.
If you remove molecules from the container then the pressure will decrease.