Why doesn't acid eat through glass?

IANAChemist (obviously), but I’m guessing that acid works something like water corroding metal. H[sub]2[/sub]O strips metal of its free electrons, eventually causing the metal to be broken down. (right? correct me if I’m wrong…) So, does acid work the same way, but at a faster rate? Why doesn’t it affect glass? Does silicon not have any free electrons to give up, or what? What other materials does acid not eat through?
Enlighten me!

Silicon dioxide, or SiO2, is a complete and stable compound and is not soluable in the acid. It has no free electrons (as you mentioned) and so it’s very hard to get it to react. That’s why glass is used so much in the lab. However, there are some acids, such as HF, or hydrofluoric acid, that will eat glass. HF is nasty stuff though, and I wouldn’t fool around with it.

There are also kits you can buy at craft stores to etch your own glass. I am not sure what acid they use, but I am sure someone can fill it in.

Hope this helps, I am just a student of chemistry myself. Someone more knowledgeable will come along.

Right on, red_dragon60! So, uhh, how would one store hydrofluoric acid?

until they invented very specific HF-resistant modern polymers (plastics), HF had to be stored in glass bottles lined inside with wax.

HF is a very reactive acid. It has a halogen, which are the clingy girls of the periodic table. Very reactive, and never wants to let go once it gets ahold. HCl is pretty damn strong too, but HF and HI are uber-powerful. As Qadgop said, they needed lined bottles to store the stuff. Even today, very stong acids will often eat through the plastic lids on the reagent bottles. In our acid cabinet, we had the plastic lining eaten away in less than a year. Very cool, and very very powerful.

I have a plastic bottle of glass etching cream here. It doesn’t list specific ingredients, but it has lots of dire warnings about not breathing the fumes or getting it on your skin. It contains “Ammonium/Sodium Bifluorides”, and under first aid it says, “Specific hydrofluoric acid antidote treatment may be required.”

The difference between metals and glass is that the silicon in glass is very happy how it is, and each silicon has four very strong bonds to oxygen. Only the silicon - fluoride bond is stronger, which is why HF attacks glass.

most metals are elctron rich, and are more than happy to give some electrons way, in many cases to the H in water or acid to form hydrogen gas. Another way of looking at it is that metals are in a low oxidation state (0) [high electron density], whereas the silicon in glass has already given its electrons away and is in a high oxidation state, so no longer has any electrons to give away

Am I the only one who finds this so down right Funny?

I now have a new word to refer to a ex or two of mine :slight_smile:

Thanks Red_

I don’t think HI is that powerful because I- has a much less electronegativity than Cl-, which has less of an electronegativity than F- (highest of all the elements).

I always learned there were 6 strong acids Nitric Acid, Hydrochloric Acid, Hydrobromic Acid, Sufluric Acid, Hydroiodic Acid, and Phosphoric Acid. By strong acid, I mean that these acids completely dissociate in water compared to all other acids which do not (The Ka is very large on these strong acids). I was also taught that HF is the only somewhat weak halogen acid due to hydrogen bonding. The bond between the H and the F is so strong that they do not dissociate well, and therefore has a low Ka. My book says that the Ka is 7.2 x 10 to the negative 4.

red_dragon60 - I am reading a book that made me think of you, seeing how you are so interested in chemistry, and from what I know, starting at a fairly young age. I don’t know if you’ve heard of it, but its a sort of autobiography by Oliver Sacks (the neurosurgeon) and it’s called Uncle Tungsten - Memoirs of a chemical boyhood. I’m not very far into it, but so far its funny and well-written, with interesting stories about how he first go interested in chemistry, and started playing with things (metals in particular). I recommend it to you, if you haven’t already read it.

As for the OP - I don’t have much to say about it :slight_smile: I was never very good at pure acid/base stuff!

Phosphoric acid is a weak acid. You can add perchloric acid and the less common chloric acid to the strong acid list. Sulfuric acid is strong only for the first proton. Hydrofluoric acid is indeed a weak acid.

BTW, I’ve only read an excerpt of the Oliver Sacks book, but I also found it to be excellent. One thing that sticks in my mind is his chemical laboratory next to a garden. He occasionally had to fling an out-of-control experiment out the door, creating dramatic scorch marks in the garden.

Which begs another thread - “Why doesn’t acid eat through wax?

'cuz wax is nature’s plastic!

Thanks Osip! I try to make chemistry funny… Depending on how smart your exes are, they might take offense at being called that!

mnemosyne, I will definitely look at that book! Chemistry is my life, and it would be great to read about someone else who felt the same way.

While searching around for HF online last night, I found out just how evil it is. When it hits your skin, the H pulls out any water, and rapidly destroys tissue. At the same time, the fluorine will dig deep deep deep into your skin, bonding with metals in your flesh and then breaking away from them, destroying even more tissue. Sometimes it will go all the way down to the bone and burn it too. Symptoms may be delayed for up to 8 hours. DO NOT PLAY WITH THIS STUFF! If you get it on yourself, get thee to an ER post-haste and have calcium gluconate gel applied to the burn. Death can occur with a surface burn as small as 2.5 % of body area! That makes it a really sweetass chemical, but don’t play around with it, and if you are etching glass, you cannot be too safe.

Happy hunting!