Charles' Law
What happens when you increase or decrease the temperature
of a gas at constant pressure?
One way that we could find out what happens if we decrease the temperature
of a gas is by pouring liquid nitrogen, a really cold liquid, onto a balloon
filled with air. If you have ever seen this demonstration, you know
that when the liquid comes in contact with the balloon, the balloon shrinks!
Then, after it cools off, it returns to its original size.
Although the French scientist Jacques Charles (1746-1823) did not have liquid nitrogen, he was a pioneer in hot air ballooning. As a result, he investigated how changing the temperature of a fixed amount of gas at constant pressure affected its volume. Charles's law states that at constant pressure, the volume of a gas is directly proportional to the temperature. Therefore, if the temperature doubles, the volume doubles, and if the temperature is halved, the volume is also halved.
To learn how a hot air balloons works,
please click here: How do hot
air balloons work?
Kinetic Explanation of Charles' Law
So why did the air in the balloons contract when it cooled and expand when heated?
When a balloon is heated, the temperature of the air inside increases, and the average kinetic energy of the particles in the air also increases. They exert more force on the balloon, but the pressure inside does not rise above the original pressure because the balloon expands.
When the balloon cools, the temperature of the air inside falls and the average kinetic energy of the particles in the air decreases. The particles move slower and strike the balloon less often and with less force. The balloon contracts and the pressure of the air inside the balloon continues to balance the pressure of the atmosphere.