Need a scientific explanation of how the Confetti Bomb Prank works

February 10, 2008 5:54 PM

I'm looking for a scientific explanation of how this "confetti bomb" prank works. It uses compressed air squirted inside a film canister, then quickly filled with confetti and sealed. After a few seconds, the canister explodes. Why?

I know they use an extremely cold liquid in compressed air containers, and holding the can upside down discharges the liquid. Air contracts when cooled, so the effect is actually technically an "implosion" of the lid being sucked into the film canister. Is this correct?

Also:
- What is the liquid in the compressed air container?
- Why does it squirt out as a liquid when the can is upside down?
- Why is this prank so much fun?

posted by phonepimpster to science & nature (15 answers total)

the compressed air is cold, in the canister it gets warmer, pressure increases, lid blows off.
posted by HuronBob at 5:59 PM on February 10, 2008

The liquid in the compressed air containers isn't cold, but the system cools when pressure is rapidly removed. This is explained by the Ideal Gas Law (Pv = nrt). When pressure drops, and volume stays the same, things on the other side of the equation have to change (like temperature)

In the prank, you fill the film canister with cold air, then seal it. With time, the temperature increases, and so does the pressure (again, reference the ideal gas law). When the pressure gets to be too great, the top pops off.

The liquid in a duster is di, tri, or tetrafluoroethane. They're used because they're inert gases that are easily compressible. (see Gas Duster)
posted by chrisamiller at 6:04 PM on February 10, 2008

1) Usually nitrogen; some use a hydrocarbon (e.g. butane) and are flammable.
2) Because, under pressure, it's liquid at the bottom of the can. For an airduster you want the gas, so the nozzle only draws fron the top of the can (i.e. no tube running down to the bottom like e.g. WD-40.
3) Because bands and flashes are fun ;-)

It's not an implosion, it's an explosion - as the liquid boils into a gas it expands, eventually popping the lid off the container. It's a smaller, different version of sticking dry ice inside a plastic Coke bottle and screwing the lid on.
posted by Pinback at 6:05 PM on February 10, 2008

Here's my take, based on the rudimentary science education I received from the wolves that raised me. The "air" in the compressed air canister is under sufficient pressure that the gas becomes a liquid, which is what you hear in the canister when you shake it. When the valve is released, the liquid becomes a gas again, which, due to magic and/or physics, produces cold. The liquid is not itself cold in the can, it is the rapid depressurization that produces the cold effect.

By inverting the canister, the liquid at the bottom of the canister comes out instead of the gas. The liquid is trapped in the film canister, and as it expands to its gaseous state, it increases the pressure in the film canister until the top (and the confetti) bursts out. Hilarity ensues.

However, I think I would freak the fuck out if someone did that to me. I am, despite my years of service in Her Majesty's Navy, a delicate soul. (Also--obviously be careful about the liquid if you do this, as cold things are cold, and can burn etc.)
posted by Admiral Haddock at 6:05 PM on February 10, 2008

Actually, let me make one correction to myself. The temperature of the compressed air can drops for two reasons. One of them I explained above, the other is that there is a conversion from liquid to gas going on inside the can, which is endothermic.
posted by chrisamiller at 6:07 PM on February 10, 2008

I don't know what the liquid is exactly, it could be CO2 but the can seems a little flaky for that. Anyways, the pressure inside the can is high enough to "squeeze" this substance into a liquid. Under normal atmospheric pressure and room temperature, it would be a gas. It's still at room temperature inside the can, but cools a bit on its way out. When it's let out to regular atmospheric pressure, it quickly boils off, and all the extra gas causes the over-pressure that blows the lid off.

The reason it blows with more power when you turn the film canister upside down is that the liquid seals the lid so no gas can escape before the explosion.
posted by springload at 6:18 PM on February 10, 2008

IANAP (I am not a Physicist) but...
"I know they use an extremely cold liquid in compressed air containers"
This is incorrect. Whenever you relieve some of the pressure within the canister by pressing the trigger, you create an endothermic reaction - As the pressure within the cannister is higher than the pressure outside, when you squeeze the trigger, you create an escape route for the small amount of gas within the cannister (obviously). As the gas is forced out, the corresponding pressure drop within the cannister allows some of the pressurized liquid to evaporate, becoming a gas to equalize the pressure. The energy necessary to evaporate the liquid within the canister is drawn from the surrounding area, which is why the surface of the canister gets cold. I'm sure that the ideal gas law (Pv = nRT) factors into this, but high school physics was a long time ago.

as for why the liquid is forced out when you invert the can, gravity pulls the liquid gas to the bottom of the cannister.
posted by namewithoutwords at 6:19 PM on February 10, 2008

chrisamiller:
They're used because they're inert gases that are easily compressible.

According to the Ideal Gas Law, all gases are equally compressible.

Otherwise, what you described is accurate.
posted by WorkingOnANewMe at 6:26 PM on February 10, 2008

Just tested and yes you can get 'liquid' out of a can of compressed air. Next thing to try on my to do list....
posted by zengargoyle at 7:07 PM on February 10, 2008

I hope you didn't use your tongue.
posted by b1tr0t at 7:41 PM on February 10, 2008

Re: Ideal gas law and compressibility:

What you're looking for in a substance like this is something with the right boiling point. It should be below room temperature (at atmospheric pressure). Then it's going to build up a pressure when you enclose it. If the boiling point is too high (eg water), there's going to be no propellant action - the water will just lie in a pool at the bottom of the can, not boiling and therby not creating any additional pressure. If the boiling point is too low (e.g. nitrogen, which boils at -200 degC), you run into the opposite problem; the pressure gets enormous. You can't lock liquid nitrogen in except in special vessels designed to create huge pressures, which you keep in a separate room under a pile of sandbags when it's in use.
posted by springload at 8:09 PM on February 10, 2008

According to the Ideal Gas Law, all gases are equally compressible.

All ideal gases, yes.

Real gases diverge from ideality; this divergence can be described by a compressibility factor.
posted by mr_roboto at 8:42 PM on February 10, 2008

Springboard says: You can't lock liquid nitrogen in except in special vessels designed to create huge pressures, which you keep in a separate room under a pile of sandbags when it's in use.

In a separate room, under sandbags? Where I'm from, we keep that shit on the street!
posted by Admiral Haddock at 8:56 PM on February 10, 2008

Where I'm from, we keep that shit on the street!

Those tanks are vented.
posted by mr_roboto at 9:34 PM on February 10, 2008

For the record, the gas is: difluroethane (CAS #75-37-6)
posted by TomMelee at 6:36 AM on February 11, 2008