Exploding Soda Can Mystery: Uncovering the Truth

[Ntstanch]

Myself and a few friends, back when we were around 12-13 years old, had built a modest fire. Longish story shortish... a diet coke can was the only thing to explode (we weren't dumb enough to try things that were clearly explosive). However the can exploded about thirty minutes after we placed it in the center of the fire on top of the small patch of white hot coals.

Before that, after about five minutes of cooking, we heard it let out the pressure in a tea-pot sort of way. After about eight minutes it had been poked and smacked a few times to where 12-13 year old children had deemed it inert and no fun.

Then, roughly 23 minutes later out of absolutely nowhere the can went off like it was packed with cherry bombs. A serious explosion which sent 1-2kg logs passed our heads, and large chunks of red hot coals as well as flaming chunks of wood at least 5 meters outward. Putting out the fire, and really leaving nothing left in the pit.

What could cause that? This was a tiny bit less explosive than blowing up something like a flammable spray paint can... except there was no fire ball or anything. Just a very loud boom. From a little soda can that fits in your hand. And in every other situation, in my experience at least, soda cans + a fire of any sort makes a somewhat loud and harmless popping sound as the container ruptures. Harmless. However this released pressure audibly, then didn't react to being poked or smacked a bit, and after at least 20 minutes exploded with enough force to have blown your hand clean off (at least) had you been holding it.

Anyway. Input? Ideas? Is diet coke special chemically? I've never really looked into it, but it came to mind just now and after learning a decent amount of college chemistry and physics... all I can think of is, "Weird".

 

[K^2]

Typical coke can is 355ml. It will hold up to about 5-6 atmospheres. That's around 200 Joules of energy. (It works out to gas energy even if it's full of liquid due to flash boiling.) To launch a log 3 meters, you only need 15J of energy. Pretty much any sealed container in a fire is a bomb and can be extremely dangerous. If you need to heat up a sealed container in a fire for a legitimate reason, always use a water bath, which will prevent temperature from rising past 100°C.

Normally, on soda cans, tab fails much earlier, releasing the pressure in a safer way. However, once the soda inside begins to boil, you'll end up with burnt sugar clogging up any openings, which is probably what happened, and what ultimately lead to the explosion.

 

[skeptic2]

you'll end up with burnt sugar clogging up any openings, which is probably what happened, and what ultimately lead to the explosion.

Burnt aspartame?

 

[K^2]

Heh, good point. Well, whatever's there in sufficient concentrations. As water evaporates away, something got to hit solubility limit, so you are bound to end up with something clogging up the holes.

 

[Ntstanch]

Heh, good point. Well, whatever's there in sufficient concentrations. As water evaporates away, something got to hit solubility limit, so you are bound to end up with something clogging up the holes.

Hadn't thought of the clogging factor. What I always wondered was why, especially in the heat, the aluminum didn't just give out quickly. I imagine the pressure building inside and expanding applies for the most part equal force on all parts of the can. Which is why they tended to either rupture at the seal or somewhere on the side where you could see a tear.

 

[Drakkith]

Hadn't thought of the clogging factor. What I always wondered was why, especially in the heat, the aluminum didn't just give out quickly. I imagine the pressure building inside and expanding applies for the most part equal force on all parts of the can. Which is why they tended to either rupture at the seal or somewhere on the side where you could see a tear.

I would guess that the aluminum couldn't rise too far above the temperature of the water inside due to heat transfer.

 

[K^2]

That 5-6 atm figure above I did look up. It can take up to that amount. The temperature is a factor, of course. If you got it close to melting point, which you could easily in the fire. However, the can will be at just 150°C by the time pressure inside gets to 5 atm, so the effect of temperature on structural integrity will be minor.

 

[skeptic2]

I remember reading in a science book when I was a kid that it was possible to boil water in a paper cup on an open flame. I used to have fun on campouts demonstrating that.

 

[Ntstanch]

That 5-6 atm figure above I did look up. It can take up to that amount. The temperature is a factor, of course. If you got it close to melting point, which you could easily in the fire. However, the can will be at just 150°C by the time pressure inside gets to 5 atm, so the effect of temperature on structural integrity will be minor.

I remember reading in a science book when I was a kid that it was possible to boil water in a paper cup on an open flame. I used to have fun on campouts demonstrating that.

I was thinking along the lines of these quotes combined. The can not melting due to the liquid inside not allowing it. Like the plastic bottle not melting on account of the water cooling it too much. So the can probably did clog up the leaks, then the liquid inside kept the aluminum in tact... along with the right set up (incidentally) to let it build up slowly over time until BOOM.