Why do the cans of compressed air get so cold when you spray them? (Example Compressed Gas Duster)?

Question

Serious answers only please : o )

Answer 1

gas cools down if it expands suddenly.

similar principle is used in AC and refrigerators.

Answer 2

Compressed Air Cans

Answer 3

Excellent question! The "compressed" part of compressed air comes from the fact that the air is under much pressure, and Charles' law states that as the pressure of a tank goes down ( in this case, by spraying) the temperature also goes down. This is theoretically caused by a decrease in the frequency with which gas particles hit the sides of the can, transmitiing less kinetic energy.

Answer 4

Compressed Air Canister

Answer 5

Use the ideal gas law: PV = nRT (no, we're not dealing with an ideal gas here, but Van der Waals equation is essentially the same, with 2 added constants to adjust for the gas being non-ideal. We're talking several days of lecture in class, so I'm simplifying for the sake of a brief answer).

P = Pressure, V = Volume, n = moles of a gas, R is the ideal gas constant, and T = Temperature.

When compressed air is sprayed, the large pressure drop results in a large temperature drop.

Answer 6

To understand this fully, and to explain it fully, would require you to have a solid background in thermodynamics. But.....to keep it simple.....
As the molecules separate and spread out, they get more disordered. We physicist call that increasing entopy (increasing the disorder = increasing the entropy). It's one of the basic rules of the universe (thermodynamics) that things like to be disordered and they'll go to that state naturally. It's also a basic rule of the universe (thermo again) that when they do, they will give up kinetic energy in order that energy may be conserved. The opposite occurs as well. When you compress a gas, and the molecules get closer together and more ordered, the gas gets warmer. This is why the cylinders on an air compressor get hot and need cooling fins.

Answer 7

It is not a chemical reaction, it is simple thermodynamics:
The gas in the car is compressed and at room temperature. As the gas expands, it still contains the same amount of specific heat, it is just spread out more. This causes the gas to be colder. Ok, maybe the word simple does not belong here :)

Answer 8

Charles/Boyles Laws

(P1V1)/T1 = (P2V2)/T2

The pressure inside the can is way higher than when the gas comes out. It's volume increases & pressure decreases, so the temperature must go down for the Law to hold true.

There is also some impact caused by the latent heat of evaporation of the propellant.

Answer 9

its because of the chemical reaction inside. The gas( helium, hydrogen, and even oxygen) is compressed at such a high pressure that when it comes out it reacts with the other gases out side. It does not get cold on the inside, it gets cold on the outside.

Answer 10

There isn't any air in canned air. It is a liquid usually some kind of refrigerant, if used upside down the liquid will come out.
http://en.wikipedia.org/wiki/Canned_air

When a liquid boils and turns to a gas it needs heat, in this case the heat comes from the surrounding environment, like your hand making it cold.
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Andre' B.

Answer 11

Which one was is Charle's or Boyle's Law. Anyway the one increased pressure, increased temperature. Well, you're decreasing the pressure inside the container so you're decreasing the temperature.