A Tale of Two Coke Cans

Here’s a question for all you scientists, engineers and humanities polymaths with enquiring minds.We’ve found two cans of coke lurking at the back of a Careers Service cupboard, with “Best Before” dates of January 2006. They’re unopened, no evidence of leakage – but they’ve only got a bit of liquid left. Any ideas why?

(NB. There’s no tenuous careers link in this post – if that’s what you’re after, just move along to the next post.)

I asked this question on Twitter to both postgrads (from @ManPGCareers) and to my careers contacts (from @eawilkinson). Here’s what the postgrads said:

and here’s what one of my careers contacts added:

So, is it evaporation – and if so how? There’s no evidence of a broken seal, and although one can (the right hand one in the picture – we dented it to test it out) has lost pressure,  the lighter of the two is still pressurised.

Does this mean the water’s got out and left the carbon dioxide (CO2 being a larger molecule than H2O – clutching at straws here…)?

Come on all you researchers – answers, please!

And if you can come up with the answer to this one, have a go at the last summer experiment I did – Why does adding water cooler water to tea makes it go darker?

UPDATE: 28th June

Adolfo (see the comments) has come up with a good suggestion – leakage through a microhole. Even better, he suggested an experiment to test this out. So, this lunchtime, our Careers Service Experimental Officers gathered in the kitchen, to plunge the warmed up “empty” coke cans into a washing-up bowl of hot water to see if we could see any bubbles escaping. Here’s the evidence from the can which had lost pressure (only 5 seconds long):

Definitely lots of bubbles, and eventually we located the tiny hole. However, for the other can, which still seems to be pressurised, we couldn’t see or hear anything. Seems there’s still a mystery – unless you know better …?


5 comments on “A Tale of Two Coke Cans

  1. Leak is the most likely reason, a microhole is enough for the water vapour to get out of the can. Hot summers will expand the aluminium and then the vapour will come out. It should be possible to test this by warming up the can, putting it in a tub of hot water and squeezing them to check if there is any bubble coming out.
    CO2 is almost as likely to come out as H2O vapour, the size difference is not that large between them

    • Thanks Adolfo. I like the idea of further experiments to test out the theory (though a bit scared at the prospect of heating up the still-pressurised coke can – have to go gently there).

      My careers colleagues, scientists and non-scientists alike, have been asking me if we’ve got an explanation yet, and I suspect would love the idea of some more kitchen chemistry experiments. I think you might find us huddled round a hot water bath (ie. washing up bowl) this lunchtime – I’ll let you know the results.

      And glad to see you’re still coming to the rescue with my drinks-based scientific problems 🙂 !

      All the best

    • So, we did your suggested experiment (see video in the updated post) and sure enough, one of the cans showed bubbles clearly escaping from a very small hole – but not the other one. Might just be that the hole is way too small to show very slow leakage, but it has lost more weight than the other can, and still feels very pressurised (no “give” in the can).

      Actually, what’s been even more satisfying though is the interest it has generated in some careers staff, both scientists and non-scientists (all the “Experimental Officers” were pure arts grads, apart from me). I love that this sort of real world phenomenon shows that science doesn’t have to be scary, and is just about what makes everything around us act the way it does.


  2. Really left field ideas here (they could be rubbish), either when it has gone off, water and carbon dioxide will create an equilibrium which generates a small amount of acid which interacts with the internal metallic surface of the can (doesn’t happen before its gone off due to sugars etc in the coke). or ( a smaller possibility) water complexing to the metal on the can and once the coke has gone off, the sugar deposits leaving the water to the surface.

    Mad ideas I know

    • Hi Craig

      Mmm, I wonder …? That microhole (which Adolfo predicted, and which we saw in the video on at least one can) had to start somehow. I guess the pressurised one still remains a mystery though.

      We might only know if we open the cans, but would need to have the ability to test the remaining liquid. If you or any chemists want to volunteer for a full analysis, let me know!


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