# What are the crystals that have formed around the top of a conc. nitric acid bottle?

I was looking through my chemicals cupboard and found that this decades-old bottle of conc nitric acid had grown a beard. I can't think of any reaction between (presumably) $\ce{NO2}$ and any airborne substance that could produce this. Any ideas?

• You sure it's $\ce{NO2}$? – JSCoder says Reinstate Monica Mar 29 '18 at 19:20
• It might be ammonium nitrate. You could check this by triturating some of the crystals with baking soda. If you smell ammonia, it's ammonium nitrate. – aventurin Mar 29 '18 at 20:06
• ... And please let us know your results, I am curious :) – Archer Mar 30 '18 at 0:13

## 2 Answers

Chemicals cupboard often produce quite extraordinary reactions, much like hospitals are home to some unusual and severe infections.

Nitric acid does not normally exhibit such a bizarre behavior. Yet in your case it did. How so? My bet is on some unknown additional component which came from the same cupboard, but from a different bottle. For instance, it could be some lower aliphatic amine (Upd. or simply ammonia, as was brought to my attention by other answers). They are volatile, and they produce crystalline salts when they meet with strong acids.

A similar thing is known to happen with $\ce{NH3}$ and $\ce{HCl}$: if they are stored nearby, especially in poorly corked bottles, they both (as well as everything around) get covered in a white residue of $\ce{NH4Cl}$. It doesn't form nice crystals, though; it looks more like a layer of hoarfrost. But that doesn't mean much. Related compounds do not necessarily have related crystal structures, let alone related crystal shapes, so it looks perfectly plausible to me that (substituted or not) ammonia nitrate would grow into fine needles.

The prevalence of amine/ammonia in the air within the cupboard (which could happen if the other bottle was corked worse than this one) would naturally lead to $\ce{HNO3}$ molecules fighting this battle at their own doorstep, that is, at the very bottleneck of this bottle and nowhere else. This would explain the accumulation of residue at one spot.

• Ivan can we collect this crystal and do some tests for ions that may be in this salt? – King Tut Mar 30 '18 at 9:21
• Yeah, I guess you can measure the melting point and compare it with the reference value. – Ivan Neretin Mar 30 '18 at 9:37
• Bases and acids should be stored in separate, hood-ventilated cabinets. – user55119 Mar 30 '18 at 21:31
• @user55119 Yes, should be. The reality of many school labs is that there is only one appropriate, heavy duty storage cabinet. This is a pretty common occurence in US schools. – MarsJarsGuitars-n-Chars Mar 31 '18 at 13:50

There are several mentions of this on the interwebs, including this one on a science forum.

The consensus is ammonium nitrate. From working with the stuff and inheriting old bottle in schools, I have seen this a number of times. The fine crystal certainly looks like slow growth from airborne NH3 or amines as the other post said, but NH3 is far more common, depending on which chemicals are stored where in the lab.

Yes, ammonium nitrate is an explosive, but needs a pretty solid activation energy, so you can just slowly carefully rinse this away with lots of water.

You can certainly remove a small amount and test in a beaker with a few drops of water just to be sure.

• Thank you for your comments, I do have ammonia stored in the same place. What would adding the crystals to water tell me? – Will Higgs Mar 29 '18 at 21:56
• A simple test is to wet a piece of red litmus paper and stick it on the bottom of a watch glass. Dissolve the crystals in water in a beaker. Add a couple of NaOH pieces. Put watch glass on top of beaker. The NaOH should release NH3 in air which would turn red litmus blue. – MaxW Mar 30 '18 at 4:18
• BTW, does the place smell of ammonia? – Ivan Neretin Mar 30 '18 at 6:33
• @WillHiggs If there were any concerns about safety of water dilution and cleaning. – MarsJarsGuitars-n-Chars Mar 30 '18 at 12:25
• The cupboard has an odd smell, but not strongly of ammonia. There are other reagents stored there including formaldehyde, HCl and acetic acid. I put some of the crystals on top of my wood-burner, which is probably at least 150', and they instantly melted and bubbled. Visible fumes were given off which may have been NO2. I also tried a lighted match and the crystals stuck to it and bubbled, as on the stove. I don't have any litmus handy just now, but I've ordered some. – Will Higgs Apr 1 '18 at 8:14