# Why do compressed air cans get cold?

Asked by stephenphl (48) June 16th, 2008

When using a can of compressed air to clean a computer, I notice that it gets extremely cold to the touch. Why is this?

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## 5 Answers

This happens due to the physical characteristics of the can and the liquid inside. Compressed air is actually maintained under pressure primarily in the form of a liquid. However, there is some gas that evaporates “above” the liquid under normal conditions. When you release the valve, that gas escapes. In order to maintain an equilibrium, some of the liquid then boils/evaporates to (in essence) replace the released gas. That evaporation event requires energy, and the liquid draws energy (heat) from the external environment (i.e. the can), thus leading the can to become noticeably colder.

Note: Before I start getting flamed for not invoking the Ideal Gas Equation, I would like to add that I am quite aware of the physical chemistry involved, but figured a simplified explanation would be more suitable.

shilolo (18085)

The temperature of liquid carbon dioxide is -109.3 F or -78.5 C. And thats what they are filled with. To hopefully simplify it more

Stocky (994)

@shilolo: I remember from high school chemistry a formula involving P, V & T; does that have anything to do with this? I’m still wrapping my head around the “adding energy = colder system” part of things.

andrew (16553)

Andrew. You are right. The ideal gas equation is PV = nRT where P stands for pressure, V stands for volume, n stands for number of molecules (or moles), R is Avogadro’s constant and T stands for temperature.

shilolo (18085)

@andrew: Its not adding energy. You’re releasing pressure, so evaporation occurs,cooling the remaining liquid. Its like how you feel cold when you get out of the pool.

charybdys (364)

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