HF is just that, H-F, or a hydrogen bound to a fluorine. All the other acids in this group - HCl, HBr, HI - are gases. What we call "hydrochloric acid" is HCl in water, "hydrofluoric acid" is HF in water, etc. What's interesting, though, is that HCl, HBr, and HI are all strong acids - if you dissolve some in water, it will completely dissociate into H+ and Cl- (for example). HF doesn't do that, and that makes it unique.
While hydrochloric acid is strong stuff, it's not even on the most-dangerous-compounds-we-use radar. If you spill some on your hands, you wash it away. It's just protons and chloride ion, neither of which are that nasty. Besides, they're ions, which don't penetrate skin well.
HF, however, isn't completely ionized, so it CAN penetrate skin. Once it's inside you, though, it can ionize, and fluorine ion is plenty toxic. It can react with some ions that are important to you, well, staying alive - like Mg2+ and Ca2+. Apparently it has an anaesthetic effect, masking potential toxicity.
HF will dissolve lots of minerals - in fact, it'll dissolve glass; if you have a piece of frosted glass, there's a good chance it was done with HF.
On one episode of Breaking Bad, they try to use it to dissolve a body, but the no-good sidekick uses a ceramic tub, which the HF eats, resulting in the whole mess falling through the floor. I don't know anything about dissolving bodies, but it seems like a weak acid isn't going to do the job. I can, however, believe the HF eating the bathtub.
I am scared of HF. I don't use it. I am scared to handle bottles of trifluoroacetic acid on the off chance that it hydrolyzed. Even people that work in fluorine chemistry are often scared of the stuff. Labs that use it, ever, usually keep a kinda-sorta antidote around.
You can imagine my horror on reading about its use in carwashes (Hey, Walter worked in a carwash! Maybe that's where he swiped the HF! A high school lab wouldn't keep it around...)
Edit: oops, think this is my first duplicate ever.
If you get HF on your skin, it will penetrate and seek out calcium (e.g. bone) to react with.
You won't even feel it until it's too late.
Why again is HF a weaker acid than HCl? After all fluorine is more electronegative than chlorine. I vaguely remember HF actually being a stronger acid than HCl by some other definition of acid.
(The bad thing about this is that I studied chemistry and should know. But all these years of biochemistry/molecular biology have erased the memory.)
A few years ago, I worked as QA in a high-purity chemical factory.
As part of the job, we rebadged ultra high purity acids - Conc. sulphuric, Nitric, TriChlorAcetic, and, of course , HF.
We didn't open the HF. Obviously.
One day, I caught the packers playing catch with bottles of concentrated HF. There followed a somewhat stressed exposition on the dangers therof..
I used to work in the HF section of the BP Alliance refinery. There were some pretty strict safety precautions involved. A thick, heavy 'rain slicker' was required to be worn along with heavy rubber boots, gloves and safety goggles. Each time you exited the HF section of the plant, the suits were washed and the goggles soaked in a solution.
Nasty stuff indeed.
HF is some scary stuff, although I'd like to point out that my high school chem lab had some laying around under lock and key.
I've worked with very low concentrations of HF as a metallurgist... cast aluminum alloys often contain 7% Si, sometimes as high as 15% Si... and to etch the silicon, you pretty much have to use a compound prepared with HF.
Scary, scary stuff... and if you're going to work with it, you really need to make sure you've got the Ca/Mg salts around as an antidote.
A friend of mine, who was working in industry with ceramics, used to say "The industrial use of HF is measured in lives, not dollars."
Lot of folk wind up having to have fingers amputated to stop the rot after HF exposure.
Standard nitrile *sp?) gloves provide insufficient protection.
I'm more afraid of HF than molten aluminum, and I've had that blow up on me.
Scary, scary stuff.
The nice alternative to HF for dissolving silicates is ammonium hydrogen bifluoride, NH_4HF_2. It's nasty but it won't pursue you. High surface tension of the aqueous solution is ameliorated with a drop from a Fluorad surfactant free sample box or a drop of Dawn diswashing liquid (so-so). Forget Silwets, obviously.
I do Boc peptide synthesis, which means cranking open a cylinder of pure HF a couple of times a week. It's like flying by airplane: intrinsically risky, but ends up being safe. When people use HF (at least in the academic setting) they pay a LOT more attention to safety than they do with other procedures. This is especially true after the procedures have become routine: the same guy who regularly overpressurizes silica columns to get them to run faster (despite the chance they might blow up in his face) will still take the time to do leak checks on the HF rig before using it.
Martijn: HF is a weak acid because the HF bond is much stronger than the HCl bond. In principle, HBr and HI are stronger acids than HCl, for the same reason, but you can't tell in aqueous solution because HCl is already effectively 100% ionized. You need to go to nonaqueous solvents to measure the relative strength of HCl and HBr.
I think its because the H-F group is such a righteous hydrogen bond donor that it ends up locked onto water so tightly that the protons can't get up and go for a walk.
I seem to recall that in the gas phase HF is the strongest acid and HI is the weakest.
The local glass man got hold of some to etch glass for church windows. He lost all his fingernails (they grew back) but the reaction stopped before it started on the finger bones. The safest way is to generate it as needed by the reaction of sulfuric acid and calcium fluoride in a lead vessel. Outdoors. Downwind.
Martijn, THe reason HF is a weaker acid than HCl, HBr, etc., is because there is much more orbital overlap between H and F (making a stronger bond) due to being much closer in size than H and Cl, Br, etc. A big giant Br atom and a tiny H s orbital have minimal overlap.