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English, like many languages, lacks many words for describing smells. There are plenty of words to describe visual and material properties of objects; for example, a balloon is big, red, round, made of rubber, and floats. By contrast, words like 'fragrant' and 'pungent' describe very broad categories of smells, and it's actually quite challenging to tell someone what something they've never smelled smells like without using comparisons to other similar smells.
In this comic, Ponytail explains to Cueball that if he smells sulfides then the chemistry experiment on which they're working has failed. Ponytail then clarifies that sulfides smell like rotten eggs. The main and most distinct chemical rotten eggs emit is hydrogen sulfide, hence most people who smell them will link the chemical with "rotten egg smell".
Cueball replies, however, that he doesn't actually know what rotten eggs smell like, and it's odd that everyone uses that as a comparison. This is a result of changing times — decades ago, when the 'rotten eggs' descriptor became commonplace in chemistry education at high schools and universities, rotten eggs were indeed common enough that cooks avoided adding eggs directly to other ingredients, lest the rotten egg, not detected until after it was too late, force the cook to discard everything and start over. Vastly improved farming, shipping, and marketing practices have made the rotten egg vanishingly rare, at least at supermarkets in the USA. Moreover, much greater recognition of the health hazards of hydrogen sulfide means that, due to various occupational safety precautions, opportunities for sniffing the gas have become scarce, and usually engender swift reactions such as building evacuation.
Thus, the comparison has outlived the circumstances that spawned it, and chemistry teachers parrot a line they learned as students, which is no longer relevant to the students' experience. Given the health hazards of hydrogen sulfide and the regulations now enforced in recognition of those hazards, the chemistry teacher probably doesn't often experience the smell either. Since hydrogen sulfide deadens the sense of smell, taking this smell as a 'baseline' is improbable and potentially dangerous, and it's unfortunate that the title text makes this suggestion.
Cueball then takes the disconnect between the trope and his experience and pushes it for all it's worth. This could be taken as symbolic of people who spot such discordances and blow them out of proportion to troll others, in which case, Cueball has most definitely succeeded, based on how Ponytail reacts — she is clenching her fists in anger as she leaves the conversation, presumably to avoid further irritation.
Some of Cueball's questions suggest that chemists use eggs in place of other items. For example, the superstitious may react to a spilling of salt by picking it up and throwing it over their left shoulder, ostensibly as an attempt to blind the Devil. Another relates to the upcoming night before Halloween event called "Mischief Night", where kids are known to throw eggs at houses.
Even though rotten eggs (and hydrogen sulfide in general) are much less common nowadays, many fuel gases are mixed with odorant compounds to signal that a leak is happening; even if the user might be unfamiliar with "rotten eggs" specifically, a large amount of unpleasant odor still works as an alarm that something bad is happening. People who use natural gas or propane stoves should be familiar with the similarly rotten smell of methanethiol, ethanethiol, and/or tert-butylthiol (the "-SH" thiol group is a common feature of many pungent odors, including garlic and skunk spray). Some mineral springs and other natural water sources also contain sulfides and have a strong sulfide odor and flavor; they are sometimes referred to as "sulfur springs".
- [Cueball and Ponytail face a table with something like a lab stirrer or heater on it, supporting a flat-bottomed and -topped container from which bubbles are rising.]
- Cueball: How will I know if the reaction fails?
- Ponytail: You'll smell the sulfides.
- Cueball: What do those smell like?
- Ponytail: Sulfurous. Rotten eggs.
- [A new panel, the table is gone. Cueball is now facing Ponytail.]
- Cueball: Chemists always compare sulfur to rotten eggs.
- Cueball: But why would I know that smell?
- Ponytail: I dunno, It's a common thing!
- [Ponytail puts her hand out.]
- Cueball: Is it? My kitchen is messy, but there aren't eggs lying around rotting.
- Ponytail: You must have smelled one at some point.
- [Ponytail is now walking right off-panel, away from Cueball. She is clenching her hands and is evidently annoyed]
- Cueball: Are all chemists' houses full of random raw eggs? Do you toss them over your shoulder for good luck?
- Ponytail: My house is not full of eggs!
- Cueball: What do you consider a normal amount of eggs in a house?
- Cueball: If kids egg your house this Halloween, how will you know??
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Chemists get another solid pummeling from xkcd dot com. Also, double question marks, very Ryan North. Lightcaller (talk) 00:41, 17 October 2020 (UTC)
- Well they are just Stamp collectors high on methylacetylate. Kev (talk) 16:15, 17 October 2020 (UTC)
You could disguise the smell of your rotten eggs with selenophenol and thioacetones. Nobody will complain about your mere sulphides then... (ETA: I wonder about selenoacetones?)18.104.22.168 00:53, 17 October 2020 (UTC)
Seperately, I recall the time when I was still at school and we'd made some compound that had produced the distinct smell of almonds. Mentioning this later to my father (a chemist himself) he was initially quite concerned before I clarified whatever-it-was as whatever it actually was and not an actual cyanogen compound. 22.214.171.124 01:03, 17 October 2020 (UTC)
- About that whole almonds and cyanide thing, wild almonds were originally what are typically called "bitter almonds" which have some substantial differences from the "sweet almonds" you usually see (bitter almonds aren't normally sold in the US at all). Thing is, they contain a chemical that when it reacts with water produces hydrogen cyanide (very little of this is in sweet almonds), to the point where eating enough (depending on your weight, possibly as few as a dozen) could produce a lethal dose of cyanide. However this breaks down when cooked, so they are only dangerous raw. It is these bitter almonds that smell somewhat like cyanide in other contexts might, and is quite unlike the normal smell of sweet almonds (though it isn't quite the same as other sources of cyanide, but resembles it much more than whatever almonds you've most likely encountered). Oddly enough, the smell of bitter almonds isn't from the hydrogen cyanide, but another substance the same thing breaks down into called benzaldehyde (which is also poisonous.)--126.96.36.199 09:31, 17 October 2020 (UTC)
I always heave a mental sigh when I see some bit of safety literature informing me that natural gas smells like rotten eggs. First, as Randall points out, that's not a very useful explanation for most people. Second, it isn't the natural gas (i.e. methane) that has that odour, but the thiols that are added to it as a safety feature. BunsenH (talk) 02:42, 17 October 2020 (UTC)
Pretty sure the current version explanation is wildly overthinking this - sometimes a cigar is just a cigar, and sometimes a comic about how weird it is that we reference things smelling like rotten eggs when they’re uncommon is just that. 188.8.131.52 05:17, 17 October 2020 (UTC)
- That was my first impression, too... Elektrizikekswerk (talk) 12:32, 19 October 2020 (UTC)
Actually, sulphides (H2S in particular) smell more like farts that rotten eggs. But science teachers know that if they use the word "fart" in front of a class, there will be no more work done in that lesson, and probably none by that class.
The kids recognise the smell, of course. We used to call it "fartrogen dioxide". Paul Seed 07:43, 17 October 2020 (UTC)
I read that the principal components of flatus ("fart gas") are methyl mercaptan, dimethyl sulfide, and hydrogen sulfide, leading to [ahem] a complex cocktail of odors. https://en.wikipedia.org/wiki/Flatulence#Production,_composition,_and_smell
There was a time, before refrigeration or stock rotation, when bad eggs were much more common. Hence the story of the Curate's egg , updated here 
The Halloween reference could use an explanation. I don't live in the US (or wherever Halloween tradition lives), and connection to eggs is not clear to me. 184.108.40.206 08:21, 17 October 2020 (UTC)
- Kids sometimes throw raw eggs at houses on Halloween just to annoy people. That is all. 220.127.116.11 08:37, 17 October 2020 (UTC)
Rotten eggs were a common feature of the household that had free-range chickens and small children. The hens would lay their eggs in whatever spot pleased them, rather than just in the hen house. Children sent out to collect eggs would delight in finding eggs that had lain hidden for weeks and bring them in to the kitchen where Mother would carefully crack eggs separately, as mentioned in the explanation. Snezzy (talk) 11:42, 17 October 2020 (UTC)
- I think that everyone could find egg which was hidden longer than expected, not just small children. Also, the ratio of households with children didn't changed much, while the ratio of households with free-range chickens dropped massively. -- Hkmaly (talk) 01:42, 18 October 2020 (UTC)
I think that the section at the top seems to fit more in trivia rather than directly as a comic explanation because it doesn't really explain the comic 18.104.22.168 14:16, 17 October 2020 (UTC)Anonymous
Anecdote, and not hydrogen sulfide or (primarily) smell, but related: Scene - a chemistry class being taught by ill-prepared/scatty teacher. Instructions to the class being read out from a book from the front. Flasks on the tripods on desks, ready for the next bit, which was apparently something like "add 2cc of bromine"... Duly added by all the class. Turns page ..."water.". Too much! Boil off a little everybody! Boiling off creates carpet of brown fumes roiling across the floor. Question from a pupil - isn't bromine poisonous? Yes, yes! Deadly (sic) poison! Evacuate the class! - Which, because it was on the top floor and the brown mist was now out of the door and flowing down the stairs meant a far more thorough evacuation of the entire building...
Further anecdote, less related, was when nitration of toluene went a bit far. Instead of being creamy-yellow it was white (or vice-versa - I forget which), so it wasn't nitrotoluene any more. Apparently there was an area in the school fields where they buried (or at least saved for possible later analysis) such things as test-tubes of suspected trinitrotoluene, to which this was (gently!) whisked away. 22.214.171.124 10:26, 20 October 2020 (UTC)
So then, what does sulfur smell like, if not rotten eggs? What's a better descriptor? 126.96.36.199 02:03, 21 October 2020 (UTC)