I want to give you an example to my question:

When you spray some pressured gas on a surface, liquid water for example, the surface could freeze. but why..

As far as I know molecules close to each other are on a high energy level, which explains the higher temperature under pressure ..it wants to expand because all molecules prefer a lower energy level.. so when expanding, the gas loses some of it's overall energy.. but wouldn't that also mean the surroundings would be heated instead of being cooled down since the lost energy of the gas would be transferred to it? Is it maybe because the gas takes energy to expand? But then, why does it want to expand if it takes energy to do so?

I know this is a little mess of thoughts and might seem simple to you, but I hope you can find an answer for a non-physicist.

  • $\begingroup$ Maybe this post can help physics.stackexchange.com/questions/14140/… $\endgroup$ – SaudiBombsYemen Jan 24 '16 at 15:25
  • $\begingroup$ Note that temperature doesn't depend on how close the molecules are or how many times they collide, but on the average kinetic energy (gases at high pressure haven't got higher temperatures). If you want to learn more I suggest you take a look to Leonard Susskind's lectures on statistical mechanics, it's a beautiful branch of physics and they are easy to understand. $\endgroup$ – SaudiBombsYemen Jan 24 '16 at 17:30
  • $\begingroup$ Normally to maintain gases under pressure they need to be at very low temperatures, or else the gas bottle would explode. Also, the lower the temperature the lower the volume, which means you can fit more gas in a container. $\endgroup$ – SaudiBombsYemen Jan 24 '16 at 17:33
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    $\begingroup$ Another good hint. I'll have a closer look at it. Thanks again. $\endgroup$ – Lew Pérez Jan 24 '16 at 17:38

The situation I think you're talking about is a hand-held spray can like hair-spray. There is the stuff they're trying to move, the paint or hair gel or whatever, and a carrier-gas. Now in order to save money and not make the can heavy-duty, they choose a gas that is liquid at room temperature at higher-than-air-pressure (but not too high; the can must still be cheap to make.) The liquid and gas co-exist at that temp and pressure. However, if you lower the pressure (by releasing some gas) the liquid starts turning into gas (boiling). The action of boiling requires the input of energy (the heat of vaporization), which comes from the thermal energy of the liquid, that is, it gets cold. Cold liquid gives off cold gas.

Eventually, the liquid gets so cold that it doesn't boil at standard pressure and the can stops working until you let it warm up again.

The heat of vaporization can be very high. Our experience with water shows this. The time it takes a pot of water to reach boiling temperature is so much, but to rise one more degree, to where it's all gas, takes a long time, relative to it. This is the time it takes for the heat source to transfer the heat of vaporization to it.

  • $\begingroup$ That's a late but good answer nonetheless. Thanks alot! $\endgroup$ – Lew Pérez Feb 13 '17 at 17:00
  • $\begingroup$ Well, all the previous answers focused on expanding gas (throttling the gas, is the technical term). But the change of temperature in that case is fairly small. The big change in temperature in the above case is from vaporization, quite a different physical process. Time to refocus the discussion. $\endgroup$ – Andrew Feb 16 '17 at 1:59

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