Why ARE acids so corrosive?

Added: 2026-05-08

[00:00:00]Why are acids so corrosive? What's happening to this sausage at the molecular level? And what if I told you that it's not really the acid doing the damage, but something else. A molecular shark hiding in plain sight. Keep watching to find out. Now, the Brønsted-Lowry theory of acids and bases says that an acid donates a hydrogen ion to a base, and put the other way, a base receives a hydrogen ion from an acid.

[00:00:32]And because saying hydrogen ion is a mouthful, I'm just going to say proton from now on, because that's what a positive hydrogen ion is. Doesn't sound like much, though, does it? After all, a proton's tiny compared to even a small molecule, and it would barely show up in a picture of anything as big as a protein like collagen. Collagen is the principal protein responsible for the structure of animal tissue like this.

[00:01:03]It's helical shape means it can wrap together with other collagen molecules to make a strong fiber. But in this video, we're going to look at just a single collagen molecule floating around in solution. And this one's in trouble because it's been protonated. Can you see the tiny proton? If you can, it's only because I'm highlighting the electron cloud around it, not the proton itself. But now, let's take a closer look at what that proton is really doing. You see, it's been absorbed into the electron cloud of this collagen molecule, and its high positive charge density is going to change what was a stable arrangement of electrons into an unstable arrangement. In some cases, that might be all it takes to make the molecule fall apart, but usually, it's not the proton doing the damage.

[00:02:00]Something else.

[00:02:01]But before we get to that, I just want to say that this video is produced at Kyushu University. It's one of Japan's top universities, and we've got courses in science and engineering in English. A great alternative in these uncertain times. So, check out the link in the description. So, what kind of thing would take advantage of a molecule in distress?

[00:02:26]Water.

[00:02:27]Water seems safe enough. We drink it, swim in it, make videos in it, but it's surprisingly reactive. It's the molecular equivalent of a reef shark.

[00:02:39]Sharks that cruise round coral reefs looking perfectly harmless, not bothering anyone. But as soon as they sense a fish in trouble, they go in for the kill. So, here's that molecule of collagen again, a protein surrounded by water molecules. It's a long chain of amino acids joined together by peptide links in the same way that long trains are joined by couplings. It's so simple, but so cool. Just 20 different kinds of amino acid being coupled together in different sequences makes an uncountable variety of proteins that form the structure and machinery of all known life.

[00:03:22]But what happens if some extra acid just donates a proton to that peptide link?

[00:03:30]Not much, surely. The peptide link is made of three atoms much larger than the tiny proton. But that tiny size, combined with a full positive charge, means the proton carries a very high charge density, and it sinks into the electron cloud of the carbonyl oxygen, destabilizing the previously stable arrangement of electrons. So, now this carbonyl oxygen is losing electron density to the proton, and oxygen is at the top right of the periodic table, which means it has a very high electronegativity, and that means it hates sharing electron density. So, it sucks extra density away from that central carbon atom, which gives that atom a partial positive charge. And now, the water smells blood.

[00:04:25]The partial positive charge on the carbon leaves it vulnerable to anything carrying a spare pair of electron teeth, and water has two sets of teeth in the form of two lone pairs of electrons. A water molecule in the right position now punches into the carbon, making a new bond with it. But that water oxygen now has so many bonds, it's the one that has the destabilizing positive charge. So, it gets rid of that by dumping one of its own protons back onto a neighboring water molecule. But now, the nitrogen is having its own electronegativity pulled away by two oxygen atoms on the other side of that carbon. Nitrogen is also electronegative and won't stand for that kind of behavior. So, it uses its lone pair of electrons to pick a proton off one of the oxygens, which triggers a break in the peptide link, and the collagen is now cut in half, and the pieces drift apart.

[00:05:25]You might not like that, but if you like this video, clicking the like button will help me make more. Thank you. But we're not finished yet. Remember the proton that was passed back into the solution? When another water molecule picks up that extra proton, it forms a hydronium ion, H3O+.

[00:05:44]The central oxygen doesn't want that positive charge, so it will pass on any one of its protons as soon as it can.

[00:05:52]This effectively means the number of extra protons doesn't change. They just get passed along from one water molecule to another, going round and round, leaving a trail of devastation until they are finally stopped by a base. And now, remember that a single drop of concentrated hydrochloric acid contains over 400 billion billion acidic protons.

[00:06:19]This is one important reason why acids are so corrosive.

[00:06:24]They behave as indiscriminate catalysts, promoting all kinds of reactions. In the case of proteins in water, breaking them down into separate amino acids is the thermodynamically preferred outcome, and the acid just helps that happen faster, much faster. And hydroxide ions do pretty much the same thing. Instead of water attacking here, it's a hydroxide ion punching directly into the peptide link and breaking it up. And once that's happened, we end up with a negatively charged nitrogen, which will steal a proton from a nearby water molecule, regenerating the hydroxide, and around we go again. So, what do you think?

[00:07:09]Are there any other reactions you'd like to see in close-up? Let me know in the comments. And there's so much more to say about acids and bases and how they work, I'm making a whole series about them. So, make sure you subscribe if you don't want to miss them. And a big thank you to my YouTube and Patreon supporters who helped me recruit a professional animator for this video.

[00:07:31]If you want to support me making more videos, too, and get exclusive updates on how things are going, just click the join button below or the Patreon link in the description. Here's a video that explains the wider theory about how acids and bases work, and if you've already seen that one, here's another one that YouTube thinks you'll like. And I'll see you in the next video. Doink.

[00:07:54]Doink. Doink. Doink. Doink. Doink.

[00:07:56]Doink. Doink. Doink. Doink. Doink.

[00:07:56]Doink. Doink. Doink. Doink. Doink.