MadSci Network: Chemistry

Re: Why does canned air get cold when shaken?

Date: Sat Jun 2 15:08:13 2001
Posted By: Allan Harvey, Chemical Engineer
Area of science: Chemistry
ID: 989183866.Ch

I don't know about the shaking part (it is hard for me to see how that would make a difference), but the cooling effect when a gas is expanded through a valve or nozzle is well known as the "Joule-Thomson effect." If you search on that on the Web, you should be able to find plenty of material (much of which will even be correct).

The cause of the J-T effect is the forces between molecules (an ideal gas, with no intermolecular forces, would not change temperatures in such an expansion). Because of the weak attractive interactions between molecules, there will be a particular intermolecular distance where the potential energy for a pair of molecules is a minimum (this would be about the average intermolecular distance in a liquid, about one-and-a-half times the molecular diameter). When the gas is expanded, the molecules move farther apart on average, which (by itself) would give the system a higher energy. But if the expansion is done without any addition or removal of energy, the total energy must remain constant (actually, what stays constant in a J-T expansion is a thermodynamic variable called "enthalpy," but that's a detail you can learn about in college), so the higher potential energy due to the molecules being farther apart is compensated by a lower kinetic energy (which means a lower temperature).

An extreme example of this effect occurs in a vapor-compression refrigeration cycle (like in your refrigerator or auto air conditioner). At some point in those systems the liquid refrigerant is expanded across a valve to such a low pressure that it vaporizes. This is a large increase in potential energy, which is accompanied by a large drop in temperature.

The amount (and even the direction) of the temperature change in J-T expansion depends on the substance being expanded. For typical gases, expanding from a compressed state to atmospheric pressure causes a modest temperature decrease. This would be the case with "canned air" (I think most such products are just compressed air, though I think other chemicals are used sometimes). For a few gases like helium, expansion under some conditions actually produces slight warming. For liquids (water, for example), if you take a compressed liquid and reduce the pressure (but not so much that it vaporizes) you can get slight warming.

Finally, I can mention one other effect that will make the spray from "canned air" feel cold, and that is the heat transfer from your skin due to evaporation into the air (assuming the air is less than 100% relative humidity). This is the same as the "wind chill" effect where you feel colder in wind than in still air at the same temperature.

Dr. Allan H. Harvey, Boulder, Colorado   |
"Any opinions expressed here are mine, and should not be
 attributed to my employer, my wife, or my cats"

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