Why does acid burn?

11 September 2018

corrosive

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Question

We say that acids “burn”, but they’re not necessarily hot, so how is this different to something like boiling water?

Answer

Chris Smith put this burning to question from Dee to Chemist, Peter Wothers. Plus, we also heard from Jake on a similar theme, who says, "what naturally occurring substances also have the most extreme pHs?"... 

Peter - So of course the burn that we're talking about here is actually just damage to the tissue. And that's the thing of course that our tissue can be relatively easily damaged. An easy way to do this is to pick up something hot, and that's going to damage that tissue there. But actually you can get burns from, of course, cold things and so we were talking earlier about liquid air being extremely cold. If you were to, certainly, tip your finger into liquid air and I do not recommend this for any period of time, you would destroy all the tissue there but a small splash on you may well cause a burn as well.

But of course, yes, you can indeed also get burnt by acid. Again this is just causing damage to the tissue. We normally have mechanisms to control very precisely the pH of the fluids in our tissue and so on, and adding concentrated acid is absolutely very far away from these normal conditions, which is why the damage is going to take place. But this is rather interesting, so tagging this on to the other question about the extremes of pH, actually it is possible to find incredibly acidic solutions in nature, and this is in certain mines notably ones that have pyrite. So this is iron sulfide, a form of iron sulfide, with the chemical formula FeS2, and this reacts with oxygen and water and can produce incredibly strong powerful solutions of sulfuric acid. And then to tie these two together actually there’s a beautiful story from the Middle Ages in various books on stones and so on. It's called the Fire Stone which probably, is actually because you can use it to start fires by smashing into a flint, but actually there are also descriptions if you squeezed this tight in your hand it will burn it.

Now of course maybe you are thinking that it's going to be burning in terms of because it's really hot and fiery, but actually it could burn your hands because the surface of this, again if there's moisture there and is reactive with the oxygen, could produce acid and so actually, you could get an acid burn from this mineral by holding it very tight. So actually these acids do occur in nature and they can be incredibly strong. In fact some of the record pHs are at minus 3. Now this sounds very odd but a strong sort of laboratory sulfuric acid. You'd get sort of one molar and this would have a pH of what is going to be zero is it? Yes it is, good, and so less dilute is actually 1, but so this is really concentrated when it starts go into negative and so really very strong solutions of sulfuric acid.

Chris - Matt?

Matt - Is there, like a theoretical endpoint for the pH scale, can you just keep on making stronger and stronger acid forever. Is just like a practical limit or is there a theoretical limit to how strong an acid can be?

Chris - That's because you were nasty to him about the universe,  he’s getting his own back.

Peter - Although this quite a good question. So of course, I mean, the pH scale means the, it's the concentration of hydrogen ions per volume and so we are limited here, and so in the same way that actually there is a concentration of pure water. How much water can you fit in a certain volume, unless you start compressing it in a neutron star or something, there’s a limit to this. So pure concentrated sulfuric acid would be, sort of, a limit in some sense but then what really makes an acid, acid, is the water that's also present. So it's a little bit difficult. So absolutely, there is definitely a limit and you can’t put too many protons in a solution of water, and it’s the protons that are making this thing acidic.

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