# Why are compressed gasses cold? **URL:** \ **Category:** Factual Questions\ **Created:** [December 3, 2003, 2:18am UTC](https://boards.straightdope.com/t/why-are-compressed-gasses-cold/216625 "2003-12-03T02:18:06Z")\ **Posts on this page:** 9\ **Page:** 1
**Author:** ![Mythos45](https://avatars.discourse-cdn.com/v4/letter/m/848f3c/32.png) [@Mythos45](https://boards.straightdope.com/u/Mythos45)\ **Post date:** [December 3, 2003, 2:18am UTC](https://boards.straightdope.com/t/why-are-compressed-gasses-cold/216625/1 "2003-12-03T02:18:06Z")
I’m sure there’s a simple explanation for this, but… When gasses are compressed (such as CO2 and oxygen) they become cold. Or, at least this has been my experience. Why is this? ---
**Author:** ![Joe\_Random](https://avatars.discourse-cdn.com/v4/letter/j/e8c25b/32.png) [@Joe\_Random](https://boards.straightdope.com/u/Joe_Random)\ **Post date:** [December 3, 2003, 2:26am UTC](https://boards.straightdope.com/t/why-are-compressed-gasses-cold/216625/2 "2003-12-03T02:26:35Z")
> [@](#): > > \*Originally posted by Mythos45 \* > **When gasses are compressed (such as CO2 and oxygen) they become cold.** Not quite, they become cold when they’re **de** compresses. It’s the good ol’ Ideal Gas Law, PV = nRT, where P = pressure, V = volume, n = moles of gas, T = temperature, and R is the gas constant. As you can see from the equation, suddenly decreasing the pressure on a gas (such as when releasing it from a pressurized container) will cause its temperature to drop. ---
**Author:** ![Tangent](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tangent/32/3098_2.png) [@Tangent](https://boards.straightdope.com/u/Tangent)\ **Post date:** [December 3, 2003, 2:28am UTC](https://boards.straightdope.com/t/why-are-compressed-gasses-cold/216625/3 "2003-12-03T02:28:56Z")
Actually, increasing the pressure on a gas would tend to raise its temperature. After the gas is compressed and then the gas (and container) are allowed to reach equilibrium with the environment, the gas will be at room temperature. (in the can) When you release the compressed gas–say, into the air–it expands rapidly. This sudden **decrease** in pressure on the gas causes it to drop in temperature. Now, the reason dry ice (frozen CO2), liquid oxygen, and liquid nitrogen are cold it because they _have_ to be so cold to exist in that state at atmospheric pressure. If this helps, the general gas law is (P1_V1)/T1 = (P2_V2)/T2. If volume is constant, then if P2 \> P1 it follows that T2 must be \> T1. ---
**Author:** ![Jake4](https://avatars.discourse-cdn.com/v4/letter/j/bc8723/32.png) [@Jake4](https://boards.straightdope.com/u/Jake4)\ **Post date:** [December 3, 2003, 2:29am UTC](https://boards.straightdope.com/t/why-are-compressed-gasses-cold/216625/4 "2003-12-03T02:29:00Z")
You’ve got it backwards, actually. When the compressed gasses are released (from high pressure to low pressure), they become colder. Google the Joule-Thompson effect for more. ---
**Author:** ![KenGr](https://avatars.discourse-cdn.com/v4/letter/k/ecc23a/32.png) [@KenGr](https://boards.straightdope.com/u/KenGr)\ **Post date:** [December 3, 2003, 4:04am UTC](https://boards.straightdope.com/t/why-are-compressed-gasses-cold/216625/5 "2003-12-03T04:04:31Z")
Maybe you had this impression because sometimes liquified gases are refrigerated for transport and storage. They can be stored a lower pressure when cold, which is sometimes safer or more convenient. Also, once you cool down a tank of gas, it will stay cold for a long time. Propane tanks always seem to feel cold in the daytime because they haven’t warmed back up from overnight. ---
**Author:** ![Bryan\_Ekers](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bryan_ekers/32/183_2.png) [@Bryan\_Ekers](https://boards.straightdope.com/u/Bryan_Ekers)\ **Post date:** [December 3, 2003, 6:16am UTC](https://boards.straightdope.com/t/why-are-compressed-gasses-cold/216625/6 "2003-12-03T06:16:34Z")
An interesting experiment you can perform involves plastic 2-litre soda bottles. Put them in your fridge and put the fridge on a high setting (but not so high that the bottles freeze). Take a bottle out of the fridge, note that the soda is still liquid. Open the cap, hear the hissing of escaping air, and if conditions are just right, ice crystals will only then begin to form in the soda. The rapid expansion of a liquid kept in a (slightly) pressurized container a few degrees above freezing will cause some of the liquid to freeze when the pressure is released. ---
**Author:** ![Finagle](https://avatars.discourse-cdn.com/v4/letter/f/b19c9b/32.png) [@Finagle](https://boards.straightdope.com/u/Finagle)\ **Post date:** [December 3, 2003, 11:07am UTC](https://boards.straightdope.com/t/why-are-compressed-gasses-cold/216625/7 "2003-12-03T11:07:17Z")
> [@](#): > > \*Originally posted by Jake4 \* > \*\*You’ve got it backwards, actually. When the compressed gasses are released (from high pressure to low pressure), they become colder. > > Google the Joule-Thompson effect for more. \*\* Well, not always. The key sentence in the discussion of Joule-Thompson here ([http://www.chem.arizona.edu/~salzmanr/480a/480ants/jadjte/jadjte.html](http://www.chem.arizona.edu/~salzmanr/480a/480ants/jadjte/jadjte.html)) indicates that the behavior of the gas on expansion depends on its inversion temperature. So: "We see that N2 and O2 will cool upon expansion at room temperature, but He and Ne will warm upon expansion at room temperature. " I wonder if this means that He and Ne cool down when compressed at room temperature… ---
**Author:** ![Mythos45](https://avatars.discourse-cdn.com/v4/letter/m/848f3c/32.png) [@Mythos45](https://boards.straightdope.com/u/Mythos45)\ **Post date:** [December 3, 2003, 1:29pm UTC](https://boards.straightdope.com/t/why-are-compressed-gasses-cold/216625/8 "2003-12-03T13:29:07Z")
Had it backwards, I did? I must’ve gotten that impression from the tanks being cold to the touch, but I guess that’s decieving. One more thing, though: When I have a paintball tank filled with C02, the pressure isn’t being increased, is it? The gas/liquid is siphoned from a large tank to a small one, but the pressure wouldn’t change, would it? Yet a newly-filled tank is very cold, often developing white frost. Well, thanks for all the responses, as they didn’t cover this in high school chemistry (and I’m not taking it at university). ---
**Author:** ![CurtC](https://avatars.discourse-cdn.com/v4/letter/c/ce73a5/32.png) [@CurtC](https://boards.straightdope.com/u/CurtC)\ **Post date:** [December 3, 2003, 2:07pm UTC](https://boards.straightdope.com/t/why-are-compressed-gasses-cold/216625/9 "2003-12-03T14:07:15Z")
As the small tank is being filled from the large tank, the pressure is indeed being reduced. The reason that gas flows from the larger tank to the smaller one is that the pressure inside the smaller one is lower. As the smaller tank fills up, the pressure difference will be less, so there won’t be as much cooling action, but by that time the smaller tank has already gotten cold. And I don’t think that PV=nRT explains the effect. The pressure dropping does not mean the temp has to change, to balance the equation, because one other variable is changing: the volume. I could just as easily say that when you decompress a gas, the volume is increasing, so the temp must go up. But both the pressure and volume change, so you can’t use this equation to explain why the temp drops.