Rapid Chilling of an Emptying Carbon Dioxide Canister

Releasing carbon dioxide from a canister will cause the canister to feel colder, even causing frost to form when all of the gas is expelled rapidly. Similar instances of this phenomenon can be observed when using a spray paint can or dusting a computer with compressed air.  The total kinetic energy from motion of particles within a system, such as the canister, is referred to as the thermal energy of the system. When thermal energy is exchanged between particles, such as when carbon dioxide molecules collide with each other or with the walls of the canister, heat is generated. Therefore, when molecules are held under compression, the system heats up—’heat’ referring to the transfer of thermal energy.  When a carbon dioxide canister is filled and the gas is compressed, heat will escape the system by warming up the walls of the canister, which in turn heats up the air around the canister. Filling a canister usually happens relatively slowly compared to how rapidly the compressed gas can escape, so it may feel warm, but usually will not get very hot. The molecules inside the canister will move much more slowly inside of a pressurized canister compared to gas molecules outside of the canister.  When a pressurized canister is punctured, the carbon dioxide can escape very rapidly, significantly decreasing the thermal energy inside the canister. When most of the carbon dioxide molecules have left the canister, only few slow-moving molecules remain, resulting in very low thermal energy of the canister. Then, because far fewer carbon dioxide molecules are hitting the inner walls of the canister, there is very little heat, making the container feel cold.

• How does changing the pressure of a system change the temperature of that system?

• What are some changes in pressure that we experience in our lives?
• When rapidly decompressed, what changes do you notice about the carbon dioxide canister?
• What links do you notice between increasing the number of molecules in a given area, the speed of those molecules, and the total amount of thermal energy?

Students could:

• Participate in a demonstration or hands-on activity where compressed “canned” air is used, and the temperature of the can is measured (with a thermometer, or simply gauged with a volunteer’s hand).
• Interact with online simulation where you increase the pressure of a system, then release the pressure. Students will notice that the temperature of the system drops when pressure is released.

• PhET Simulation | Gas Properties: The emphasis of this simulation for this standard should not be on ideal gas laws, but instead focusing on how a drop of pressure will ultimately lead to fewer particles bumping into each other, thus dropping the temperature of the simulation. Students can pump a gas into a chamber, then watch the gas escape, thus lowering the temperature.
• Minute Physics | Why Do Compressed Air Cans Get Cold?: A video visualization of how and why compressed air cans cool off when depressurized.