Been meaning to ask this for ages, and feeling I’m going to look a bit of a numpty, but it’s turning into one of those days.
Whilst using a one of those flamethrower things to burn the weeds off the garden path, I noticed, again, the condensation forming on the can and it was getting colder. i know this happens because the can depressurises, but wonder why?
Anyone with a basic school level of physics care to enlighten me?

Cheers

PV=nRT

Pressure Decreases x Volume Constant = n Constant x R Constant x Temperature Change

Scrap that on edit n is changing too

PV=nRT

pressure decreases, but volume is constant, so temperature drops 🙂

As it is Butane (it generally is) that is in the can it isn’t actually depressurising as such. What is happening is that as some of the gas is removed from the top of the can the liquid in can boils to replace it and this causes the temperature of the liquid to drop. As the inside of the can is now a bit colder than the surroundings heat transfers from the surroundings which causes the water to condense.

Edit: The gas laws don’t really apply here as the can will contain a saturated liquid as well as gas. Oh and n isn’t a constant in this situation.

As it is Butane (it generally is) that is in the can it isn’t actually depressurising as such.

It is as the pressure is dropping due to the gas being removed and the volume being constant. My faith in my old physical chem is restored

It is as the pressure is dropping due to the gas being removed and the volume being constant. My faith in my old physical chem is restored

As long as there is some liquid in the can, which there certainly will be to begin with, the pressure will the same as the saturated vapour pressure. For a given composition, this pressure is dictated by temperature and nothing else.

pressure decreases, but volume is constant, so temperature drops

Wrong, volume is increacing (you can’t just ignore the gas once it’s left the can).

GoneFishin has the basic reasoning correct, although it doesn’t explain why a cylinder of compressed gas like ethylene (not boiling liquid like butane/lpg) also cools down, but that’s a lot more complicated, and leads you onto questions about why helium and hydrogen warm up when expanded (so if you did fill tyres with helium, the valve would get cold when you pumped it up and hot when you let it out (the opposite of air!).

this one or that one

PV=nRT

Pressure Decreases x Volume Constant = n Constant x R Constant x Temperature Change
means nothing to me. n Constant could be Fiddler’s Constant

What is happening is that as some of the gas is removed from the top of the can the liquid in can boils to replace it and this causes the temperature of the liquid to drop. As the inside of the can is now a bit colder than the surroundings heat transfers from the surroundings which causes the water to condense.

is an explanation I can get

mikewsmith – that one is the one I understand.

Cheers

although it doesn’t explain why a cylinder of compressed gas like ethylene (not boiling liquid like butane/lpg) also cools down, but that’s a lot more complicated

I really didn’t want to start talking about supercritical fluids, critical temperatures or anything else like that. Mostly because it makes my brain hurt and I have computer software that does these hard sums for me.

Adiabatic expansion= drop in pressure means a drop in temperature

not to disappoint but I’m sure pV=nRT etc was school level…

so was the bleep test, doesn’t mean I could still do it now

As it is Butane (it generally is) that is in the can it isn’t actually depressurising as such. What is happening is that as some of the gas is removed from the top of the can the liquid in can boils to replace it and this causes the temperature of the liquid to drop. As the inside of the can is now a bit colder than the surroundings heat transfers from the surroundings which causes the water to condense.

Which is pretty similar to what happens in a BLEVE…

The answer is latent heat. Similar to why it’s better to put ice into a drink, rather than a cold stone, and it’s how a fridge works.

Heat taken in/given out when changing from solid to liquid, liquid to gas & vice versa.

Which is pretty similar to what happens in a BLEVE…

BLEVE would be throwing the butane can on a bonfire which keeps the volume constant, the increacing pressure stoping the butane boiling (but taking it well above its atmospheric boiling point), once the can ruptures the butane boils and burns almost instantly.

My understanding is that, in a BLEVE, what happens is the liquid boils until the pressure gets too high – then the safety valve opens, releasing the pressure, and allowing more liquid to boil. It’s only when the liquid level drops low enough to allow the walls of the tank to heat up and fail that the explosion occurs.

So a bit like the cooling process in reverse.

Good BLEVE vid