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12:36:18 PM Jul 10th 2010
edited by psuliin
Technical note from a chemist: The statements about hydrofluoric acid (HF) are only marginally correct. It is one of the nastier acids, but it doesn't dissolve flesh instantly. The main reason it's worse than most other acids is that it causes nerve damage. Also it's easily absorbed through the skin (further proof that it doesn't dissolve flesh), making its toxicity more of an issue.

The reason HF has such an evil reputation among laypeople is mostly a red herring: unlike other acids, HF dissolves glass. Anything that will eat through glass has to be really powerful, in the eyes of most people. On the other hand, sulfuric or nitric acid will dissolve polyalkene plastics. HF won't. Hence it's stored in plastic bottles, or (in the old days) in glass bottles coated with wax. Dissolving glass is a neat trick, but it doesn't make HF a super-acid.

Another only-sorta-true statement in the main write-up is the assertion that hydrofluoric acid is a gas, unlike other acids. That is true, but only, as Obi-Wan Kenobi said, from a certain point of view. If you see a bottle of liquid labeled "hydroCHLORIC acid," what you are actually looking at is a solution of hydrogen chloride (H Cl) gas dissolved in water. The same is true of hydroFLUORIC acid, except that the gas is hydrogen fluoride.

Heck, as far as that goes the carbon dioxide you exhale will dissolve in water to form an acid. It's called "carbonic acid." So a gas that dissolves in water to form an acid isn't really all that special.

Anyway, the assertion that HF is a gas is true in one sense, but marking it out as different in that way from H Cl is not true, and the whole thing is misleading, since chemists nearly always work with both of them in solution. (And we don't work with HF at all if we can help it. It really is nasty stuff.)

Of course the main write-up doesn't need anything like this level of detail. So I've simply deleted the specific reference to hydrofluoric acid, until and unless we find a reason to mention it specifically.
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