Is it hydrochloric acid?
I found a webpage which addresses this exact question: http://newton.dep.anl.gov/askasci/chem99/chem99182.htm
From it I quote “Apparently the strongest acid is hydrogen flouride (HF).”
hydrochloric acid isn’t too bad a guess.
http://toxikon.er.uic.edu/toxweek.htm
Hydrofluoric acid is extremely dangerous, but I didn’t think it was actually a strong acid. The above site says it’s a weak acid, but Barthomatus’ link says HF is in fact a strong acid.
If you’re wondering why a weak acid would be so virulent, it’s because its decalcifying action is delayed, and the acid can be penetrate your skin and kill your nerves and dissolve your bones. Strong acids have caustic action on the skin, but don’t penetrate. Not a happy tradeoff. Other weak acids, like Vitamin C and vinegar, are not dangerous at all.
Hydrofluoric acid is only a moderately strong acid, but unlike most acids, it will eat through glass.
According to the Guinness Book of Records 1994, the strongest known acid is an 80% solution of antimony pentafluoride in hydrofluoric acid (Fluoroantimonic acid HF:SbF[sub]5[/sub]).
More about superacids, including HF:FSb[sub]5[/sub]
The strongest of 20 or so common acids seems to perchloric acid (HClO[sub]4[/sub]). There seems to be some dispute about the next-strongest common acid, but the next 3 (in some order) are sulfuric, hydroiodic, and hydrobromic. Hydrochloric is next. Hydrofluoric is near the middle of the pack.
I don’t know about any of that…
When I was in High School, the strongest acid was the purple gel… 
Asking about the strongest acid is a bit of a funny question. It’s a bit like asking “What is the distance between L.A.” between L.A. and WHAT? you might ask. Exactly.
First you need to define what you are dissoling the acid in. Things which are acids in one solute (like water) are not in other solutes (like liquid ammonia)
Acid strength is defined as the hydrogen (or hydronium, depending on which inaccurate theory you subscribe to) ion concentration of the compound in solution at equilibrium. An acid can be said to be a “stong” acid if it produces at least 1 equivalent of hydronium ions for every equivalent of acid. * In water * several acids do this, including the three “mineral acids,” hydrochloric acid (HCl), nitric acid (HNO3), and sulfuric acid (H2SO4). Other strong acids include the perhalic acid series (HClO4, HBrO4, HIO4), possibly HBr and HI as well.
Please note that HF * in water * is NOT a strong acid. Fluorine is so electronegative that water cannot fully deprotonate (take the H+ ion off) it. As a result, while HCl disolved in water yields only H+ ions and Cl- ions, some of the HF is still left as molecular HF. Fluorine will, however, replace oxygen in compounds, turning CaO (in bones) into soluble CaF2, and SiO2 (in glass) into soluble SiF4.
To review, if you are asking about water solutions, then the upper limit of acid strength are those which lose 100% of their hydrogen ion in water, and several acids (but NOT hydrofluoric) qualify.
Now, if you want to get nitpicky, we could say that NOTHING will fully deprotonate, and that all acids in water will have an equilibrium constant. Technically true, which also technically makes HClO4 the strongest acid, but the difference in strength between it and any of the other standard “strong” acids is moot.
I thought it was natural acid. But the book says:
pH values of a normal solution of perchloric acid HCl0 sub-4 tend towards 0, although the most powerful
acid based on its power as a hydrogen-ion donor is a solution of antimony pentafluoride SbF sub-5 in
fluorosulfonic acid FSO sub-3 H. …
What if you don’t dissolve it in anything, and just pour the totally undiluted acid directly on a block of iron or glass or something?
Alien blood.
An acid is something that donates a hydrogen ion. In order to donate an ion, it must be dissolved in a polar liquid, such as water, liquid ammonia, ethanol, etcetera. It’s not an “acid” unless it is dissolved in something else. Hydrogen chloride is a ** gas **, it doesn’t become hydrochloric acid until it is dissolved in something like water. Likewise, hydrogen percholorate is a white powder in pure form that when dissolved in water become perchloric acid. Undiluted acid, by which I take it to mean no water present, doesn’t make any sense.
Most acids, especially sulfuric acid, are what is called * hygroscopic * This means that they will absorb water vapor from the air and form a solution all on their own. Pure liquid dihydrogen sulfate is only isolatable in a completely inert (usually argon) atmosphere.
It’s also important to not that it is hard to isolate the properties of the compound due to its nature as an acid from other properties due to it’s non-hydrogen part. Sulfuric acid will burn you by rapidly dehydrating the compounds in your skin, while nitric acid will leave a painless dead spot on your skin that is stained a bright yellow color. Neither of these properties are due to the acid * per se * rather they are due to the properties of the sulfate or nitrate ion * in the presense of * the hydrogen ion (acid).
Asssuming that we’re talking about saturated aqueous solution here, hydrochloric is the most acidic acid there is, if that’s what you mean by strong. It’s not particularly nasty, though. If you spill even saturated HCl on yourself, you’ve got a couple of minutes to rinse it off before it does any real damage. As far as nastiest acid, HF has to at least come pretty close. You spill some of that stuff on your hand, you don’t have a hand. However, by the chemist’s definition, HF isn’t actually very acidic.
I have been banned however I might be able to help
from beyond the board as it were.
It really depends on what you mean by STRONG acid. In
chemistry we refer to any acid that dissociates 100%
in water as a STRONG acid. A WEAK acid does not
dissociate completely in water.
However, this is decieving as many weak acids cause
alot of damage.
Common STRONG acids include HCL HN03 as well as H2SO4
Also, the most damaging acid I’ve ever used is Glacial
Acetic acid. It melted my gloves and into my hands on
the way to a sink 2 ft away. I was in bandages for
weeks. Nasty stuff.
Also there are some interesting acids. Pycric acid.
I think that is spelled right. Is used in stained
glass window making. It doesn’t damage the skin
because it is weak and cannot ionize the skin. It
does however get into the joints where it can do some
damage it takea a long time to activate. It will
cause lasting damage though.
I hope this has helped. Feel free to post it on the
SDMB for others if it will help.
Have a good one!! OgreFade
Not an acid per se, but I hear if you REALLY want to get the job done sprinkle some pure Bromine on the offending object. Then again I am not a chemist, but I remember that from chem class (from the same instructor who one day brewed up some chlorine gas in lab. Nice of her.)
A good acid is like a good woman. A real good one will eat right through your pants.
I might add, my above quote is from the Worlds Records book, thus, the correct one 
Well, carbolic acid. Itchy poured it on Scratchy, and you don’t just use a regular acids on Scratchy.
An interesting connection: Hydrogen chloride (the gas) is the infamous ‘mustard gas’ of WWI. How can this gas be dangerous, one might ask, if it is not an acid in its gaseous state (as jayron correctly states)?
Because when you breathe in the gas, it dissolves into the mucous lining of your respiratory system, and thus becoming an acid that eats away your lungs. Ow.
Peace. Please.
I wouldn’t put it past either side in WWI to have used HCl gas. But strictly speaking, mustard gas refers to one particular chemical, dichlorodiethyl sulfide, S(ClCH[sub]2[/sub]CH[sub]2[/sub])[sub]2[/sub]. It more often produces blindness than death.
oh i don’t know, i thought the 8 balls in 8th grade were stronger then the purple gels…
but i took 10 hits, thats about the strongest acid IMO.