Vanishing sea shell

On a recent beach trip, my son collected some shells - mainly those from slipper limpets - and we brought a few home.  He enjoyed making imprints with them in play dough, and I thought we could use one for a science experiment!  The shell is made from calcium carbonate, which dissolves in acid, the same reaction as my son saw when he made an egg shell disappear around Easter.

Shell and test tube ready to go

It was a very quick setup, just putting a shell in a container with some vinegar.  We used a test tube because we've got them and my son likes to use them, but we used a jam jar for the egg and that would work just as well here.  I was a bit frugal with the vinegar, and only got my son to squirt in enough to cover the shell - it actually needed more than that for all the calcium carbonate to react, so if you try it, be a bit more generous with the vinegar!

Carbon dioxide bubbles

On adding the vinegar, my son immediately noticed lots of bubbles coming from the shell.  This was carbon dioxide being formed by the chemical reaction between the acid and the shell.  We talked about whether we should put a lid on or not, and decided that we should let it escape after I explained that it might pop lid off or burst the tube if the carbon dioxide had nowhere to go!

Hole in the shell after around an hour

An hour or so later, we took a look at the shell and my son could see a small hole escaping.  We held it up to the light, and it was clearly becoming thinner, although I wished we'd had a look against the light before we started so we had something to compare it to.  We looked again another couple of hours later, and the hole had got a bit bigger, but my son wasn't hugely impressed at the change.  I asked him what he thought would happen by the morning, and he said he thought it would vanish.

Shell has vanished!

I had another look late in the evening and the reaction had clearly slowed a lot, and the hole wasn't much bigger.  I decided not to disappoint him and to change the vinegar as I'd underestimated how much we needed, and this made the reaction go vigorously again.  In hindsight I should have left it overnight so he could have been involved in the change and seen the reaction start again, but maybe we'll return to this with different amounts of acid and some of the remaining shells in the future to demonstrate what happens.  Anyway, this did result in an almost entirely dissolved shell, with just a little bit of the inside where the limpet attaches to its shell remaining, which is presumably not made of calcium carbonate.  He was pleased to find it had vanished, although he did ask if I could make it reappear, and I had to explain that wasn't going to be possible...!