# What would happen to gas in

1. Dec 8, 2007

### yashar_g

what would happen to gas in....

Hi there,
I'm working on some experiments regarding to gases. I want to know what'd happen to gas which is uneder high temprature and high pressure when it enteres into vacuum.

it would be very kind of you to answer me in detail.
Many thanks, Yashar.

2. Dec 8, 2007

### CompuChip

First thing that springs to mind is adiabatic expansion (comparable to what happens in a fire extinguisher)... not entirely sure though.

3. Dec 10, 2007

### yashar_g

what would happen to gas in....

Hi there,
I'm working on some experiments regarding to gases. I want to know what'd happen to gas which is uneder high temprature and high pressure when it enteres into space.
it would be very kind of you to answer me in detail.

4. Dec 13, 2007

### Shooting Star

(Please don't multi-post the same thing.)

If it's a real gas, it will naturally expand and cool down in the process. If it does not radiate heat, the process should be adiabatic. You have to find the adiabatic law for real gases. Van der Waals' eqn of state may be a good approximation.

5. Mar 31, 2008

### Nerd

As I read your question and the reply I thought of something very interesting. If the hot gas is released into space, we would naturally think that the gas should cool down. However, heat is proportional to the kinetic energy of the gas molecules and since there is no surrounding particles to which the gas molecules can loose their kinetic energy, I would think that it would retain it's temperature. Afterall the only way in which anything can loose heat is by either giving it's kinetic energy to surrounding particles or emmiting it through electromagnetic waves. I don't believe that the gas would start emmiting waves as it is released into space, would it?

6. Mar 31, 2008

### dst

Pressure and temperature are inversely proportional. In vacuum pressure roughly = 0. Gases expand to occupy space. You can work out the rest...

Edit: I think what I wrote is bollocks (haven't slept for about 48 hours). So here's a gas law to cover my arse.

Pressure * volume = number of molecules * gas constant * temperature.

Last edited: Mar 31, 2008
7. Mar 31, 2008

### Shooting Star

For an ideal gas in isolation, your argument is quite correct. However, there are various intermolecular forces in a real gas, which are feeble but attractive when the distances between molecules become large. In order to overcome these forces while expanding, the molecules have to lose a part of their KE, and the gas cools down in the process.

About the loss by EM radiation, that is also bound to happen. If two molecules collide, an electron may be pushed to a higher energy state, and it'll come to the ground state subsequently by emitting a photon.

The a/V^2 term in van der Waals’ eqn of state represents the attractive force, which reduces the observed pressure of a real gas, as compared to an ideal gas.

An ideal gas would not cool down while expanding in space, but any real gas would.

Pardon, your **** is still showing...it's not number of molecules, but number of moles.

8. Mar 31, 2008

### Staff: Mentor

That isn't correct, Shooting Star (though the reasoning for what actually happens is correct). An ideal gas undergoing adiabatic expansion cools down, as the ideal gas equations predict: http://en.wikipedia.org/wiki/Ideal_gas_law
http://en.wikipedia.org/wiki/Ideal_gas_law

9. Apr 1, 2008

### Shooting Star

Russ,

An ideal gas expanding in space has no reason to cool down. It does not have any intermolecular force to overcome, nor any work to do by pressing on any outside wall. The average KE per molecule stays constant, since there is no way it can lose heat. (I am, of course, not considering heat loss by radiation, which will happen in a real gas.)

Perhaps I made a mistake by writing the phrase "gas in isolation". I was replyiing to Nerd's comment: "If the hot gas is released into space, we would naturally think that the gas should cool down."

Some further clarification from