Last night I had to change propane tanks on the fly, and noticed that the empty tank had some ice formed near its base. This is despite the fact that it was connected to a hot barbecue on a warm summer evening. Here’s what it looked like:
Why does this occur? There’s a vague and incomplete answer referencing the Ideal Gas Law on WikiAnswers:
As you use the tank, the pressure inside goes down. As the pressure drops, the temperature also drops because of something called the Ideal Gas Law. Ice forms on the tank because water moisture in the atmosphere is condensed on the cold tank and then frozen.
Would anybody care to elaborate?
DarrenBarefoot.com 
Sure.
The law is pV=nRT.
Basically, if the amount of gas (n) is constant (i.e. a closed tank), and the pressure (p) goes down, the temperature of the gas (T) goes down as well.
Conversely, if the pressure goes up (i.e. you squeeze more gas into a space), the temperature goes up.
And yes, you’re using up propane more of the liquid propane is converting to gaseous propane due to the lower pressure, so the (n) is basically constant. Therefore the pressure change causes more cooling than the loss of gas causes heating.
The reason the ice forms on the bottom is because of one more thing: some of the liquid propane at the bottom turns into gas and this boiling takes energy, so the liquid gets even more cooled than the gas does.
It’s worth noting that this is how a standard refrigerator works. Gas is expanded/evaporated in tubes inside the unit, causing it to absorb heat and them it is compressed/condensed in tubes at the back of the fridge, causing them to release heat (which is why the back of a fridge is warm).
Energy (heat) is released when you compress a gas. When the gas expands, it absorbs heat.
Same principle as why when you get any of the sprays (Insect Spray, Oil Spray, Deoderant, Canned Air, etc) and spray it, the can gets cold.
Also works from the other side – Cool a gas (i.e remove heat), and it shrinks, heat it and it expands.
You can test this by blowing up two balloons about half way – put one in the freezer, the other in warm water.
The one in the freezer will shrink, the one in the warm water will expand.
I noticed the same thing when I am using a can of compressed air; it gets cold after just a few blasts.
Following up on the first post: if you’re wondering about the mechanics behind this process, consider an “ideal gas” to be a collection of particles that fly around, bouncing off one another and the walls of the container in which they are stored.
If you add energy to the system in the form of heat, the little particles move around faster, which means that more collisions occur between the walls and the particles themselves. This causes the pressure inside the tank to increase.
Conversely, taking energy away by dropping the temperature means that the particles move more slowly, and the pressure drops because there are fewer collisions between the particles and the container walls.
As the first poster indicated, this works from a pressure point of view as well:
Stuff more particles into a given space (increase the pressure), and there will be more collisions and therefore temperature will increase as each collision between wall/particle imparts a bit of energy to the container. Take particles out (decrease pressure) and temperature drops.
Stuff like this (well, only bigger, with weird particles and radiation) is what made me decide to do a Physics degree.
forgot to mention:
“pressure” is defined as the force over a given area, and is just the force of the gas particles slamming against the container walls. The more particles there are & the faster they move, the higher the pressure.
This phenomenon is exploited by the, world famous in New Zealand at least, Jet Engine Beer Cooler;
http://www.asciimation.co.nz/beer/
Wow, thanks for all those thorough explanations. Who knew that there was all that physics knowledge among my readership?
Thermodynamics 101
Commercial pharmacy wart removers use the same principle: you put the swab in a little aerosol can, discharge gas for a little while, and the swab gets very cold. Put that on your wart and it freezes. Not quite as cold as liquid nitrogen like the doctor uses, but pretty good — enough to blister and destroy the wart in many cases.
And no, I don’t have any warts.
Physical property of propane:
When you draw the gaseous fuel out of the container and lower the pressure the liquid will boil into a gas. This boiling point or ‘change of state’ from a liquid to a gas takes place at -44 degrees Fahrenheit.
As EB stated, propane boils at -44 degrees F. The heat needed to boil the liquid is drawn from the air surrounding the cylinder and transferred to the liquid through the metal wall of the container.
The area where the liquid is in contact with the cylinder is called the ‘wetted area’. When the cylinder is full there is a lot of wetted area to transfer the heat needed to vaporize the propane.
As the propane is used, and the liquid level drops, there is less and less wetted area to provide heat transfer. As this happens, the part of the cylinder that is still in contact with the liquid becomes very cold and moisture from the air condenses on the metal. When the temperature of the metal drops below 32 degrees F. the moisture begins to freeze and frost or ice is formed, usually at the bottom, where the remaining propane liquid is in contact with the container.
Disclaimer: I am not a physicist. … that said…
I understand that the -44 degree (F) boiling point is the normal boiling point. This is the temperature at which propane will gasify under normal atmospheric pressure (1 atm). As the pressure is increased, the boiling point increases. So the boiling point for the pressurized propane is higher than -44 degrees (F).
My limited understanding of ‘boiling’ is that when the temperature rises (or gets hotter) for a liquid and reaches its vapourisation point, it will become a gas. Now for propane, when the temperature of it in liquid form reaches -44F, it will boil and become the gas that we use to cook with.
But my misunderstanding is that when the temperature of the liquid decreases to the boiling point, does this not imply that the temperature was already above the boiling point to begin with? And shouldn’t this already cause the propane to boil into a gas if the temperature was originally above -44F? I’m reading everywhere that when the temperature DROPS to -44F, THEN the liquid becomes a gas.
Found my answer here:
http://www.propanecarbs.com/propane.html
Here is the total explanation that will answer everyone’s questions:
A propane tank is a container of propane liquid with propane gas “hovering” above it. The liquid and gas are in equilibrium. This means that if pressure is dropped in the tank, the pressure must be replaced to its original level. This is achieved by the propane boiling. The reason the propane doesn’t boil when the tank is not in use is because at the unused pressure, the boiling point is too high. High pressure above the liquid means high boiling point. So even your hot summer day will not boil the propane. Once you open the valve, the pressure drops. This lowers the boiling point. It is lowered to a point that summer days (or room temp) can now boil the propane. This temp is different for various liquids…the fact that summer days is the right temp for propane is the very reason you are using propane for your bbq (among other obvious benefits).
Now you might ask…but if it’s boiling why does it go cold? Shifting the equilibrium to replace the gas above the liquid and restore the pressure can only happen if there is available energy to allow the boiling to occur. This comes from the room temperature surroundings.
“So energy (heat) from the outside is used up (metal goes cold) to boil the propane and restore the dropped pressure in the gas above to its original value.”
Yes the gas above the liquid goes cold due to the ideal gas law however this has little to do with the ice on the side of the container. The equilibrium shift argument is why you see ice (which is why you only see the ice at the bottom).
You may also notice that expanding gas cools when you blow it on your finger. This is due to the Joules-Thomson effect and is also a consequence of thermodynamics.
Anyway is anyone asks you about the ice…its the equilibrium argument.
Cheers,