Question: Why is it that when you puncture a CO2 canister, it gets very cold after all of the gas escapes?
Answer: Air canisters can be found everywhere, be it on a scuba mission, a paintball fight, or just around the local home improvement store; but did you know they could teach you a lot about physics? When you puncture a small CO2 canister, there is a violent blast of air, and then afterwards you are left holding a very cold empty cartridge. This phenomenon is a direct result of the first law of thermodynamics.
The first law of thermodynamics is often referred to as conservation of energy. The law states that the total input of energy must be equal to the total output in addition to the change in energy contained in a system. The bottom line is: energy can be transferred, but never destroyed. This relates to the canister in that when the air is released, the energy needed to complete that act is taken from the canister itself.
It can be assumed that the contents of a canister are in the liquid state (very rare of carbon dioxide). When the canister is punctured, the liquid vaporizes quite quickly. In fact, the vaporization occurs at a constant rate of 7-8 MPa (that’s 7-8 mega Pascals or 7-8 million pascals) at a temperature of approximately 20-30ºC. Because of the rapid vaporization, the energy needed will be taken from the canister’s shell (thus dramatically reducing the temperature), but also from the carbon dioxide lowering the vapor’s final pressure. One can see the canister releasing gas into a balloon in the following image sequence:
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