Wikipedia:Reference desk/Archives/Science/2023 February 23

= February 23 =

When did the Jurassic end??
Old sources generally say 130 MYA; newer sources often say 144-146 MYA. Why?? Georgia guy (talk) 14:27, 23 February 2023 (UTC)
 * Great question! New data occasionally requires re-evaluation of old conclusions. In the case of things like the timing of the Jurassic, it's worth remembering that these aren't arbitrary dates. The Jurassic period is a geologic period, so it is based upon some big change we observe in stratigraphy in rock layers. Those differences are indicative of major events that occurred at that timing. If the timing of these periods changes, it is because we have new data that changes our estimate age for whatever we use to mark between the Jurassic and Cretaceous. Looking that the Jurassic article, it also looks like there is some disagreement about exactly what we want to use as a stratigraphic marker for the end of the Jurassic and the start of the Cretaceous. I wouldn't be surprised to see more changes in the designated timeline as scientists work to figure out and agree upon a specific stratigraphic marker, and then work to most accurately date that marker. --OuroborosCobra (talk) 15:44, 23 February 2023 (UTC)
 * Read the last paragraph in Jurassic. Ruslik_ Zero 11:39, 24 February 2023 (UTC)


 * Stratigraphy has a number of branches and each of these helps define the boundaries between different chunks of geological time. There's lithostratigraphy, which deals with the types of rocks, biostratigraphy, which deals with the fossilised remains of animals, chemostratigraphy, which deals with variations in chemical composition, magnetostratigraphy, which looks at changes in the polarity of Earth's magnetic field, and finally, chronostratigraphy, which attempts to provide absolute dates for the layers in a sequence. Changes in the dates may arise from new understanding of the sequence around the boundary, based on one or more of these approaches, or new radiometric dates, which can sometimes lead to fairly major changes in the numbers. Mikenorton (talk) 12:56, 24 February 2023 (UTC)

Chemistry of detectable odors and smells
Do all substances whose odour or smell is detectable by humans (both those pleasant and unpleasant), have anything common in their chemical structure? That is, what chemical structure produces a detectable scent? 212.180.235.46 (talk) 16:35, 23 February 2023 (UTC)
 * This isn't a question that is simple to answer. Lot's of chemical structures produce scents, and there are tons of different scents. In simple terms, yes, all substances with detectable odors, pleasant or otherwise, have their odors due to their chemical structures. As to what structures give what scents, you'll need to pick out some specific scents. --OuroborosCobra (talk) 17:11, 23 February 2023 (UTC)
 * Quite a few possess an aromatic ring, and then there are the acrid ones like ammonia. It says here that chemical with odors are usually under 300 daltons. Abductive  (reasoning) 17:56, 23 February 2023 (UTC)
 * Yep, the general rule on mass difference is a property of the fact that, to have odor, a compound needs to be able to easily enter the vapor phase at conditions we are trying to smell things (so generally room temperature, 1 atm, pressure, etc). Compounds that are too massive, such as over 300 daltons, generally have intermolecular forces strong enough to prevent much of a sample from entering the vapor phase under those conditions. Basically all compounds that massive will have strong London dispersion force by virtue of physical size and numbers of electrons. Additionally, they may have other forces, such as multiple points for hydrogen bonding. That said, it isn't the mass itself that prevents it from having odor, just that it isn't readily vaporizing due to strong intermolecular forces, and so doesn't reach the nose in the first place. However, you can have small molecules that also lack an odor due to their structures. Methane and ethane have no odor, but methanethiol, which is similar in size, has a very strong odor of rotten eggs, while methanol has an alcohol like odor. Formic acid has a strong odor similar to vinegar. I chose these examples since they are all methane derivatives, but have structural differences that give them their distinctive odors. However, we can also use this to illustrate why the original question is difficult to answer. The -OH (alcohol) group in methanol gave it an alcohol smell, and the -COOH (carboxylic acid) in the formic acid gave it a vinegar odor, but other compounds with those same structural features have very different odors. Menthol has an alcohol group, but smells like mint, not alcohol. Hydrocinnamic acid, on the other hand, has a carboxylic acid group, but smells like cinnamon, not vinegar. This isn't to say that we can't predict odors based on looking at structures, we absolutely can, but it isn't a simple process or easily answered here on the reference desk. --OuroborosCobra (talk) 19:07, 23 February 2023 (UTC)
 * IP editor: You might be interested to take a look at this website (the good scents company) which has an extensive listing of odor/flavor molecules and their categorisation. Mike Turnbull (talk) 22:21, 24 February 2023 (UTC)
 * The short answer is that smelly chemicals have no general structural commonality. The structures of hydrogen sulfide (H2) and sotolon, each having a characteristic smell, are completely different. --Lambiam 08:02, 25 February 2023 (UTC)

Are molecules with nothing but carbon-12 atoms over or under 12 daltons per atom?
From the mass-energy equivalence of the carbon-carbon bonds. Sagittarian Milky Way (talk) 19:12, 23 February 2023 (UTC)


 * A dalton is the mass of an unbound neutral carbon-12 atom. Your molecule with only carbon-12 atoms (like graphite or diamond) is bound, so it has a negative binding energy, so it must be slightly lighter. A back-of-the-envelope calculation shows that it will be on the order a hundred-millionth of a dalton less. PiusImpavidus (talk) 19:59, 23 February 2023 (UTC)
 * So just like nuclear fusion then (hydrogen-1 1.007X, carbon-12 12). I wasn't sure. Sagittarian Milky Way (talk) 20:36, 23 February 2023 (UTC)
 * Chemical bonds release energy when they form; thus bonded atoms are always lighter than the two atoms are separate. No exceptions.  Energy is mass, so the energy released in forming a bond from isolated atoms will always result in a loss of mass.  Perhaps immeasurably smaller, but it will be smaller.  -- Jayron 32 22:11, 23 February 2023 (UTC)
 * That makes sense, you'd have to give an equal amount of energy back to break the bond. Sagittarian Milky Way (talk) 23:26, 23 February 2023 (UTC)

Mold toxicity
Reddit news linked to an article in German saying mold toxins had been found in a certain brand of ketchup. How much should people care about that kind of thing? I figure that mold grows on every kind of food given a chance, and that has been true forever, so any organism alive today must have evolved to deal with a bit of mold in its diet. I won't eat moldy food on purpose but I'm sure I've eaten spots of it here and there by accident.

Should I worry and/or be more careful? Am I in trouble? Is it even surprising that there was some in that ketchup? Question is about mold in general, not particularly about any specific brand of ketchup, though that is where it came up. I had figured mold was like any other microbe, where we encounter zillions of them every day, a few of them are pathogens, and we only get sick from large exposures or especially virulent ones. Maybe I'm naive. Thanks. 2601:648:8200:990:0:0:0:BDFA (talk) 21:57, 23 February 2023 (UTC)
 * The link is dead, but if it's alternariol its no biggie. Aflatoxins, on the other hand, are worrisome. Abductive  (reasoning) 02:03, 24 February 2023 (UTC)
 * It was indeed alternariol, at 47&thinsp;μg per kg, above the EU guideline value of 10&thinsp;μg per kg. --Lambiam 09:29, 24 February 2023 (UTC)
 * I got sick from sealing a dampened bread slice in a ziplock bag then forgetting about it. Instead of coming back in a few days with a magnifying glass and every day after till it was visible if it wasn't then throwing it out it rotted and rotted and rotted till I was a lean, mean nose-leaking machine. When I finally saw it hiding on my high bedroom closet shelf I threw it out and stopped being sick. Are kids' immune systems more sensitive to mold or was it just the fucking large quantity of mold? (it was starting to eat parts of the shelf, nothing an alcoholized paper towel couldn't kill). Sagittarian Milky Way (talk) 03:20, 24 February 2023 (UTC)