# Are all acids corrosive, or just the strong ones? **URL:** \ **Category:** Factual Questions\ **Created:** [August 30, 2014, 8:49pm UTC](https://boards.straightdope.com/t/are-all-acids-corrosive-or-just-the-strong-ones/697001 "2014-08-30T20:49:19Z")\ **Posts on this page:** 4\ **Page:** 2
**Author:** ![njtt](https://avatars.discourse-cdn.com/v4/letter/n/ecd19e/32.png) [@njtt](https://boards.straightdope.com/u/njtt)\ **Post date:** [September 3, 2014, 4:37am UTC](https://boards.straightdope.com/t/are-all-acids-corrosive-or-just-the-strong-ones/697001/21 "2014-09-03T04:37:40Z")
> [@Nava](#): > > The specific corrosion mechanisms can involve many different reactions: complexations, redox… > > A solution of acid in water has three things which can attack something (let’s say flesh): a concentration of H+ which is higher than for pure water (aka “low pH”), the anion of the acid, and (for weak acids) the undissociated acid. > > Simplifying a lot, Cl- is quite corrosive to some metals, not so much to flesh (if it was, we wouldn’t be able to eat salt in the amounts we do). F- is what makes FH nasty to flesh That is so simplified as to be thoroughly misleading, if not plain wrong. Nitric and sulfuric acids are certainly harmful to flesh, yet sodium sulfate and nitrate (and other sulfate and nitrate salts) are quite as harm **less** to flesh as sodium chloride (or other chloride salts) are. Come to that, sodium fluoride is so harmless that they put it in your toothpaste. It is not the ions as such that are dangerous, it really is the acids as such (or, looking at it another way, it is the H+ ions, found in abundance in strong acid solutions, but in far less quantity in weak acid solutions, and scarcely at all in neutral salt solutions). I also rather think the claim that concentrated HCl is not corrosive to flesh is getting overstated. It will dissolve flesh quite handily, it is just a bit slower than sulfuric and nitric acids to get through the protective layer of oil on the skin and so start burning, so if you can wash it off quickly you will be OK. I think that the reason concentrated sulfuric acid burns so quickly is because it is extremely hygroscopic: that is, it aggressively absorbs water, and, in particular, it starts trying to such the water out of your skin immediately, thus disrupting the protective oils and so that the acid itself can start to burn you. Nitric acid probably acts similarly. I speak as someone who was once spattered with a couple of drops of concentrated sulfuric acid. Luckily, it was very quickly washed off, and I suffered no permanent damage, and very little pain. It did, however, raise puffy welts on my skin, where the skin cells, I believe, had been killed not so much by being burned by the acid as by being dehydrated by it. ---
**Author:** ![Nava](https://avatars.discourse-cdn.com/v4/letter/n/da6949/32.png) [@Nava](https://boards.straightdope.com/u/Nava)\ **Post date:** [September 3, 2014, 4:53am UTC](https://boards.straightdope.com/t/are-all-acids-corrosive-or-just-the-strong-ones/697001/22 "2014-09-03T04:53:35Z")
The post I was responding to indicates “setting aside acidity”. Sulphuric and nitric are corrosive mostly because of the acidity (in certain conditions, and specially when both are combined as aqua regia, nitric can also be an oxidizer). But, on the other hand, and this was the specific question and the specific answer, the difference between ClH and FH is not because of the acidity, it’s because of the anions. And I did not say that HCl was not corrosive to flesh: it is, but because of the acidity that we were setting aside and which is not present in the case of NaCl, whereas Cl- is. Want to swap war stories involving [sulphuric acid](http://boards.straightdope.com/sdmb/showpost.php?p=16858090&postcount=23)? ---
**Author:** ![Busy\_Scissors](https://avatars.discourse-cdn.com/v4/letter/b/d78d45/32.png) [@Busy\_Scissors](https://boards.straightdope.com/u/Busy_Scissors)\ **Post date:** [September 3, 2014, 8:31am UTC](https://boards.straightdope.com/t/are-all-acids-corrosive-or-just-the-strong-ones/697001/23 "2014-09-03T08:31:55Z")
It’s the skin penetrance of HF that causes such bad burns - the undissociated HF molecule is able to get right into the dermal layer and squat down there, where it will then acidify the tissue. An HF spill on the hand is often not immediately painful, the burns come on later. The fluoride is a systemic poison, which interferes with Calcium metabolism, including bone structure, but I wouldn’t have said it is responsible for the corrosive properties of HF. Another acid which gives similar burns is phenol - called carbolic acid in the old days. This is a really weak acid, far weaker than even HF, but again it’s extremely skin penetrant, with anaesthetising properties that mask any initial burning sensation. Its hydrophobicity locks it into the dermal layer where it can cause v serious burns - needs to be handled with care due to these insidious properties. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [September 3, 2014, 3:33pm UTC](https://boards.straightdope.com/t/are-all-acids-corrosive-or-just-the-strong-ones/697001/24 "2014-09-03T15:33:08Z")
> [@](#): > > Quoth **njtt** : > > I also rather think the claim that concentrated HCl is not corrosive to flesh is getting overstated. It will dissolve flesh quite handily, it is just a bit slower than sulfuric and nitric acids to get through the protective layer of oil on the skin and so start burning, so if you can wash it off quickly you will be OK. I think that the reason concentrated sulfuric acid burns so quickly is because it is extremely hygroscopic: that is, it aggressively absorbs water, and, in particular, it starts trying to such the water out of your skin immediately, thus disrupting the protective oils and so that the acid itself can start to burn you. Nitric acid probably acts similarly. OK, I learned something today. My previous statement was made from personal experience: I did in fact once spill concentrated HCl on myself, walked over to the sink and washed it off, and experienced only trivial damage as a result. I don’t know anything about what would have happened had I let the acid just sit there, as that’s not an experiment I’m eager to perform (the other experiment wasn’t one I intended to perform, either, for that matter). So you’re saying that, in the long run, HCl is just as dangerous as H2SO4 or HNO3? OK, now I know. [Previous page](https://boards.straightdope.com/t/are-all-acids-corrosive-or-just-the-strong-ones/697001.md?page=1)