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Simple answer is Gay-Lussac's law - which simply states that pressure is inversely proportional to temperature (back to the bicycle pump for generating heat).The gas in the bottles is under pressure, but the heat generated when the gas was placed under pressure has long since been dissipated. When you use up the gas, you reduce the pressure in the bottle, which then lowers the temperature, and hence the freezing.
Good question. Nevertheless hydrogen warms up on expansion. The effect is due to non ideal behaviour and G-L's law is to do with ideal gases.http://en.wikipedia.org/wiki/Joule-Thomson_effect
Quote from: Bored chemist on 11/07/2007 20:15:39Good question. Nevertheless hydrogen warms up on expansion. The effect is due to non ideal behaviour and G-L's law is to do with ideal gases.http://en.wikipedia.org/wiki/Joule-Thomson_effectVery interesting. It does not actually contradict what I believed, but merely places limiting parameters around it.What is particularly interesting is that, if understand correctly, gasses (and/or combinations of gasses) can be caused to expand (in the right circumstances), or be compressed, with neither loss nor any gain in energy.
In a butane cylinder where there is a mix of liquid and gas the pressure is approximatelyconstant so there is very little cooling to the cylinder by the gas expanding in the cylinder the main place where the gas expands is in the nozzle of the cooker where it is burnt. As I said before the main cooling to a cylinder of butane while it is being used is the heat flow into the liquid butane to turn it into gas you can see that clearly because it is the part of the cylinder that is covered by liquid not the part that is in contact with the gas that gets cold.
OK then, it's latent heat of vaporisation - the heat is lost at the surface. Heat enters the liquid butane from below - slowly because it's a liquid with poor conductivity(?) and through the steel case - a good conductor with a low specific heat capacity, compared with the butane - so it gets cold.I have noticed that you can 'stimulate' a tired gaz cannister to release its butane by holding it in warm hands and shaking - adding heat / energy to the system.On the other hand - there is a volume change; the volume that goes out into the burner. My equation(delta p X delta v) holds; work is done, one way or another - so energy is lost. The actual pressure won't change much because, as gas is lost through the regulator, it is released from the surface, to maintain equilibrium at the surface.It's probably a bit of both - it's not an ideal gas but, either way, it doesn't contravene any of our beloved laws.
How does it know to do this? who tells the heat not to go in through the side?