I may be missing something here, but I’m not exactly an advanced chemist here, so I was just wondering:
I understand that chemicals are ‘acidic’ because they contain certain ions (OH-, I believe) but why do extremely acidic substances burn human flesh on contact? If their heat is not sufficient to do so, why does this occur?
You are comfusing a chemical burn with a burn caused by radient heat energy. I still don’t have your answer, but needed to make that distinction. My WAG would be that acids & bases are highly reactive and cause chemical breakdown of tissues, leaving scars that resemble burns. I’ve never been “burned” by acid, but I suspect that the sensation is similar, and judging from the faces of some afghani women who have had acid thrown in their faces when they refused to wear their burqa, the scars look like burns from fire.
Yo- can we get a chemist in here?
A mere biologist here, but one with some experience with burns.
Attrayant, I think you’re exactly right. Most acids are characterized by free H+ ions (not OH-, those are bases) which tend to attack and destroy a variety of organic compounds. This destruction is very similar to the destruction caused by heat, and produces similar sensations and scarring. Strong bases also burn skin, for the same reasons.
To confuse the issue, many acids release heat when mixed with water, so may warm up when contacting the fluids in flesh - but I have no idea if enough warming could occur in this sceneario to cause heat burns in addition to chemical burns (and how could you tell?).
Evolution is the answer.
When acid or alkalaii touch the skin, a chemical reaction turns the skin into something else. Let’s call the new substance gloop. Gloop doesn’t have the same properties as skin necessary for survival. Hence, if a lot of skin turns into gloop, you perish. Burning pain is evolutions way of saying, “Hey, go do something else”. Over time, anyone who encountered pain tended to do something else. Those that did not encounter pain tended to die. Evolution.
I got a C in high school chemistry, but perhaps I can enlighten you. Take this with a grain of sodium chloride:
Matter seeks it’s most stable state, and an H+ is less stable than the positive ion attaching to some other chemical, let’s say on your skin. The positive ion leaves your H+ and combines with a some compound on your skin. (Still vamping guys.) This chemical reaction changes what your skin compound was, and because it is more stable, it releases the heat because it was at a higher energy level and that heat cooks you. Repeat zillions of times because it happens with more than one molecule and you get a burn. Essentially it is from a release of energy as the ion finds a more stable orbit around a new molecule.
So chemists, am I completely full of poop?
Mischievous wrote:
Consider it (and me) confused, then. I thought that this is what bases did when mixed with water, no? That’s why you’re not supposed to force fluids on somebody who has consumed an alkaline substance – you’ll compound the problem by burning their insides with heat.
Concentrated acids and bases can both release heat when they combine with water. Sometimes it’s quite a bit of heat. (There might be some acids and bases, like certain salts, that don’t release heat when they’re hydrated, though.)
Ok, this is all a bit less than technical, because the technical details are boring anyway.
Basically, a strong acid is extremely electrophilic, and will grab and bind to any available electrons. A strong enough acid can break bonds–by “taking” the shared electrons. Weaker acids can somtimes still react with a molecule by bonding with unpaired electrons. This means that it will kick the living heck out of just about anything, given enough time.
Your skin has oils on it–made up mostly of long carbon-hydrogen chains. These are fairly unreactive towards acid, so even pretty strong acids won’t burn you at first. Given enough time, however, the acid will either chew through the oils or “slide through”. Then it gets to your juicy bits. Your juicy bits have lots of things that–when protonated–fall apart (they may decompose, rearrange to something else, or react with stuff nearby. In reality, all these things probably happen). Presumably–as already mentioned–the pain is nature’s way of telling you “hey, this stuff is making you fall apart; maybe you should do something about that”.
Now, the stronger the acid, the less time it takes for it to bust up your protective oils (I’m talking about aqueous acids here–anything in water). Organic acids can basically just “slide through” the oils in your skin very quickly, and therefore can be much worse for you than a stronger aqueous acid, because they are quickly absorbed into your skin (once there, good luck getting them out).
Hope that helps.
I’m no chemist (but I did stay at a Holiday Inn Express last night) but doesn’t a Base burn as well if exposed to the skin ? What is Lye ? Isn’t that a base ??
Sorry to Hijack… it will be my last one I promise 
I’m no chemist either, but I never heard of a proton orbiting a molecule. In fact, I never even heard of an electron orbiting a molecule. The standard model is that electrons orbit a nucleus and the sharing of an electron orbit of two atoms compose a molecule. Meanwhile, the protons sit quietly in the nucleus. So, I’ll await a chemist to correct me. 
I’m not sure what you mean by “the positive ion attaching” to some other chemical. As stated above, H+ ions (charged particles) will look around for anything to grab electons from or bond to so that they can share electrons. This breaks the bonds that were held by those electrons that were stolen. The molecules or chains being attacked start to break down. This could include nerve cells once the acid “burns” through the outer layers of skin.
Well, you’re right that the H+ ions are in a higher energy (less stable) state than when they’re combined with something. That’s why this combination reaction happens spontaneously. Things/systems proceed from having more energy to having less if given the chance.
BUT, I don’t know what you mean exactly by “The positive ion leaves your H+ and combines with a some compound on your skin.”
The H+ IS the positive ion. Ion means charged particle, so there’s no other ion we’ve mentioned in connection with an acid, and there’s no sort of “ionic essence” or anything that can leave an H+
Well, I do admit that there is energy released, as I said. I’m willing to bet good money though that the “burn” isn’t from the heat released but from your skin, fat, muscle, and nerve cells being torn up by H+ ions grabbing electrons and breaking bonds left and right.
And barbitu8 is right, I don’t think you meant that the proton is orbiting the molecule. It’s just attached in some way; there are many many possibilities.
To be really picky though, you can have molecules that are just one atom. Molecules are the smallest pieces that still have the characteristics of the substance you’re talking about. So one atom of helium would be a molecule of it, since helium never, ever combines with anything, not even itself, so one atom would be the smallest unit that acts like helium.
Cardinal - An almost HS chem teacher. (I just interviewed for a position.)
There’s a nice discussion of the pathophysiology of chemical burns at this site: http://www.emedicine.com/plastic/topic492.htm
The relevant part might be a little too long to copy and paste since it’s copyrighted. It says that acids mostly produce tissue damage by dehydration of the tissue and denaturation of proteins (which comes from pH changes and dehydration). In addition to those mechanisms, alkalies produce tissue damage by saponifying the fat in the skin (catalyzing the hydrolysis of triglycerides and other lipids). Although a lot of heat can be produced, that isn’t what really does the damage, but when you neutralize the chemical you need to be sure not to do it too exothermically.
There are also acids mentioned that cause additional damage because of the nature of their anions, and special cases like phenol and ammonium hydroxide.
Acids cause tissue damage partly by denaturation of proteins. This does involve “a positive ion attaching to some other chemical” and does not necessarily involve breaking any chemical bonds at all. Most of the electrons in a protein molecule that are going to be “grabbed” by a proton are non-bonding electrons. Protonating a nitrogen atom on the basic side-chain of an amino acid can cause denaturation of a protein without breaking any covalent bonds.
And to be super picky, it should be noted that some (very useful) models of molecules rely on electron delocalization; the electrons (or sometimes only the pi-bonded ones) are viewed as being spread over an area of the molecule…possibly the whole thing.
Ok, I’ll shut up now.
Okay, here is the long and short of it. Acid burns because acid dehydrates things. When you spill aqua regina on you, like I have, your skin dries out. The burning is the acid ripping all the water out of your skin, and in my case, the nitric acid nitrating it.
Bases work in the opposite way. However, I forget how to explain them.