text:
Why is '-ethane' in 'methane'?
Is there a chemical or historical significance in the fact that 'ethane' is just 'methane' without the 'm'?

meth vs eth

[OP] Why is '-ethane' in 'methane'?

This is a coincidence.
Methyl is

ultimately from Greek methy "wine" + hylē "wood.

Source: https://www.etymonline.com.
The terminology was created by Dumas and Péligot in 1834 to distinguish wood alcohol from ethanol, and was published first in the French language:

Source: https://gallica.bnf.fr/ark:/12148/bpt6k6569005x/f15.item
Ethyl is from

Greek aithēr "upper air; bright, purer air; the sky" (opposed to aēr "the lower air"), from aithein "to burn, shine," from PIE *aidh- "to burn"

Source: https://www.etymonline.com.
Liebig had argued in 1834 that what now is known to be $\ce{-C2H5}$ is a "radical", i.e. a reoccuring group in molecules such as diethyl ether, ethanol and ethyl ethanoate, and gave it the name ethyl.

Source: https://babel.hathitrust.org/cgi/pt?id=uva.x002457887&view=1up&seq=30
Using the ending -yl for both the methyl and the ethyl group is a suggestion by Berzelius from 1835 (the correct atomic weight of carbon was not know yet, so the chemical formulas are off by a factor of two):

Source: https://books.google.com/books?id=1DM1AAAAcAAJ&pg=PA376#v=onepage&q&f=false, see Wikipedia entry on Ethyl group.
methane vs ethane
That these both end in -ane is not a coincidence - all alkanes end in -ane. As stated in AChem's answer, this nomenclature was introduced by A. W. Hofmann in 1865.
Acknowledgements
Maurice's answer inspired me to find the first work by Dumas that led to the methyl terminology. I have edited my answer to incorporate information from the excellent answers by AChem and Jan and the insightful comments. In trying to figure out the sequence of events in the 1830s, I made use of this account by Frederick E. Ziegler.
References
Dumas, J.-B, Mémoire sur l'esprit de bois et sur les divers composés éthérés qui en proviennent. lu à l'Académie des Sciences les 27 Octobre et 3 novembre 1834, Paris, 1834
Jean-Baptiste Dumas et Eugène Péligot, Mémoire sur l’Esprit de Bois et sur les divers Composés Ethérés qui en proviennent, Annales de chimie et de physique, 58 (1835) p. 5-74
Justus Liebig (1834) "Ueber die Constitution des Aethers und seiner Verbindungen" (On the composition of ethers and their compounds), Annalen der Pharmacie, 9 : 1–39
Jacob Berzelius, Årsberättelsen om framsteg i fysik och kemi [Annual report on progress in physics and chemistry] (Stockholm, Sweden: P.A. Norstedt & Söner, 1835), p. 376
A. W. Hofmann 1866, in Monatsbericht der Königl. Preuss. Akad. der Wissenschaften zu Berlin, 1865 653

Karsten, You are making it more and more like a Journal of Chemical Education article. Worth thinking about it a formal article.

I certainly enjoyed this account. ;)

It’s worth pointing out how much of a coincidence it is that the modern ethyl and methyl are identical but for one letter. While Karsten already outlined the general bits of etymology behind these two words one can go a little bit deeper and find out how different they are.
Methane’s derivation traces itself back to Greek μέθυ (méthy) and ὕλη (hýlē with a long e) via the intermediate methyl. As the first component included Greek epsilon (ε), it was rendered e in all languages that are based on the Latin alphabet.
Ethane can be traced back directly to ethyl and was introduced to chemistry via German. However, when introduced the German spelling was actually Äthyl, sometimes (especially in the day) rendered Aethyl. The German letter ä represents a sound similar to the vowel in English bear – more open than the e spelling (which corresponds approximately to English here) would suggest. The German spelling changed sometime during the last century and nowadays the pronunciation of Ethyl and Ethan has the closed e-type vowel (while older non-chemists might still write ä and pronounce it with the more open vowel).
The Ä (or Ae) spelling in German was actually systematic, as it traces back to Latin aethēr that already featured ae; practically all Latin ae combinations are pronounced as ä in modern German. Latin borrowed this term from Greek (hence the th) where it was written αἰθήρ (aithḗr). Both the Greek and Latin terms were probably originally pronounced with an /ai/ diphthong (approximately as in high).
The term ethyl may have also entered English more directly, being derived from the already established and borrowed term ether (which came to English via French as a direct descendent of Latin aethēr; French had already dropped the leading a in spelling). The corresponding German term used to be written Äther or Aether exclusively. Nowadays, the chemical term is only Ether while the non-scientific term may be either but is more likely to remain Äther.
Thus, the two terms only began sharing most of their letters by accident and in late stages of their etymology; and had German remained a primary language of chemistry we might still be writing aethane or some derivative.

In the word-origin of methane and ethane, you have to consider both French and German as important languages of chemistry in the 1830s, see the edit to my answer.

@KarstenTheis In all honesty, I just didn’t go back at all with methyl because wherever one would go the form would be written with e. It’s ethyl that’s interesting because it did not develop from Greek epsilon (or eta).

In English, "ethyl" is still pronounced with a sound closer to "ä". (More like an "eh", actually, because most English speakers are also lazy about pronunciation.) It is not the same vowel sound as is used in the English word "here". ("Ether", on the other hand, is pronounced with the same vowel sound as "here".)

@CodyGray Actually, both pronunciations exist according to Wiktionary and I believe I heard the closed e more often. But that’s probably irrelevant for a number of reasons including English spelling and the letter e being a written form for both. I tried not implying that I was talking about the English pronunciation anywhere.

For word-history lovers...It is all gifted from the German language. Hope that satisfies the OP's curiosity.
The suffix -ane, was chosen by a chemist and it was not random or accidental. It was a well thought-out suggestion by Hofmann.
Current sense.

In sense 2 after German -an (A. W. Hofmann 1866, in Monatsbericht der
Königl. Preuss. Akad. der Wissenschaften zu Berlin, 1865 653,
translated by the author in quot. 1867). In this passage, Hofmann
proposes a series of five such suffixes, one with each vowel (German
-an , -en , -in , -on , -un ), and the systematic application of these to specific types of hydrocarbons, or their analogues. Of these (and
their English equivalents), only -an -ane suffix2, -en -ene comb.
form, and -in -ine suffix5 were adopted by others. As influences on
his nomenclature, Hofmann cites (in the same paper) A. Laurent
(compare -ene comb. form) and C. Gerhard (compare discussion at -ol
suffix and -one suffix).

Methane

Origin: Formed within English, by derivation; modelled on a German lexical item. Etymons: meth- comb. form, -ane suffix.
Etymology: < meth- comb. form + -ane suffix, after German Methan (A. W. Hofmann 1866, in Monatsbericht der Königl. Preuss. Akad. der Wissenschaften zu Berlin, 1865 653).

Ethane

Etymology: < eth- comb. form + -ane, after German Aethan (A. W.
Hofmann 1866, in Monatsbericht der Königl. Preuss. Akad. der
Wissenschaften zu Berlin, 1865 653; also Äthan and (now usually)
Ethan).

Ref: Oxford's Unabridged Dictionary (behind subscription).

@Farook. Your text does not give the Greek or Latin meaning of "meth" or "methy" – Maurice 1 min ago    Edit

@Farook While the -ane suffix seems to be a German language origin (and published in German), the stem meth- has French language origin (and was published in French), see the edit to my answer.

@KarstenTheis, OED traces back to Swedish work (cf 1835 French), same year. Origin: A borrowing from German. Etymon: German Methyl.
Etymology: < German Methyl (F. Wöhler 1836, in Jahres-Bericht über die Fortschritte der physischen Wissensch. 15 381), shortened < Methylen methylene n., after Swedish metyl (Berzelius 1835, in Årsberättelser om vetenskapernas framsteg, afgifne af Kongl. Vetenskaps-Academiens embetsmän 376), itself shortened < metylen methylene n., after etyl ethyl n. (compare -yl suffix). Compare French méthyle ( A. J. L. Jourdan tr. Berzelius Traité de chim. (1839) III. 403).

Your reference is from 1835. Not sure who published first.

Dumas gave two talks at the French Academy of Sciences in 1834, and subsequently published with Péligot in 1835. I think the work by Berzelius is an annual report of progress in science to the Swedish crown. I added more references to my answer.

Thanks Karsten. That is really deep scholarship.

This seems like only a partial answer (and arguably the less important part, at that). Good answers on Stack Exchange are generally meant to be complete, or at least to explicitly acknowledge any intentional omissions. I feel this answer would be significantly improved if you added at least a brief mention of the origins of the meth- and eth- roots, even if you choose not to go into as much detail about them as some of the other answers have.

@IlmariKaronen I think you are mainly criticizing Oxford's dictionary for not digging deeper. I am glad this answer exists because it contains the origin of the suffixes -ane and friends, something lacking from other answers.

@IlmariKaronen, Thanks for suggestions. Remember that we are volunteers not paid workers of SE or any other organization. This is not my research assignment either. Other answers have already addressed the origins. You are always free to write another answer. This is the beauty of public forums.

I found the Berzelius reference online (via Ethyl group in Wikipedia): https://books.google.com/books?id=1DM1AAAAcAAJ&pg=PA376#v=onepage&q&f=false. So Berzelius took the methylene nomenclature from Dumas and the Aether nomenclature floating around and suggested "methyl" and "ethyl" for the respective groups.

Yes. It is a prefix and a way to make a distinction for the number of carbon atoms in a molecule as such Meth: 1 carbon, Eth: 2 carbons, Prop: 3 carbons, Βut: 4 carbons, Pent: 5 carbons, Hex: 6 carbons, etc
The next part relates to the number of links between those carbon atoms as such: -an- single link, -en- one double link, -in- one tripple link, -dien- two double links, etc
Finally the suffix goes like this: -ic acid COOH, -cn nitrile, -ale CH=O, -one CO, -ol OH, etc
CΗ3 ― CΗ = CΗ2 :  προπ-έν-ιο propane
CΗ3 ― CΗ2― CΗ2 ― CΗ3 :  βουτ-άν-ιο butane
CH ≡ C―CH=O:  προπ-ινά-λη propinal
Η―CH=O:  μεθ-αν-άλη methanol
Η―CΟOΗ:  μεθ-αν-ικό οξύ methanic acid


text:
Does the level of water go down when you add salt to it?
My textbook states: Did you know that if you pour a handful of salt into a glass of water the water level will actually go down rather than overflowing the glass?
I tried this experiment and the glass overflowed.
I did a bit of research online and I think what the writer was referring to is that the volume of the salt solution is less than the volume of the sum of the water initially and the salt initially, not that the volume of the salt and water is less than the volume of the water alone.
Is my understanding correct here?
I'm looking for a yes/no answer if possible.

Search for density of salt salution for particular salt:water ratio. You can then  check yourself if the volume is smaller than volume of original pure water.

Providing yes/no answers does not fit well the site policy, focusing rather on principles than particular results.

You obviously can't do the experiment with a full glass of water. The salt doesn't completely dissolve instantly. So initially you essentially have the volume of salt plus the volume of the salt. You could somewhat mitigate this by adding the salt slowly but you'll also need to stir somehow. // All in all this is an interesting question for you to research as Poutnik has pointed out. Frankly I'd expect some contraction at low concentrations of salt, but I'd expect that a saturated solution of sodium chloride would have a greater volume than the initial volume of water.

https://chemistry.stackexchange.com/questions/33058/why-there-is-no-change-in-water-level-when-salt-is-added

In other words: Plot the density of water + 2.16 g/ccm vs. salt content/(g/ccm) , and check if the line ever goes above one. I've never actually seen that curve! +1

source: Stack Exchange

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text:
Organic-catalysis microwaves
When we perform a organic reaction without any addtional catalyst except that the reaction is carried out under microwave condition we can accelerate this reaction in comparison with one under heating only. So, microwaves can be considered a form of catalysis according to the concept of reaction kinetics?

A number of people, scientists, have claimed that. What convinced you?

There are several factors affecting microwave synthesis. Can you elaborate the method convinced you as @Karl's comment?

Apparently, much literature has been contributed in this area. See my answer and cited references therein.

If the question is "can microwave speed reactions?" then the answer is yes, it also has a large literature. If the question is "can we call this catalysis?" then I would say, I do not think so, we generally call something else catalysis, but one can argue that it is a similar scenario to photocatalysis or electrocatalysis.

Microwaving  causes thermodynamical disbalance with molecules not being in mutual thermal equilbrium. The ones with active MW absoption are "warmer" and more energetic than the others and react faster than they should at the otherwise measured temperature. So activation energy was not decreased as would be at catalyses, but extra energy is being provided like if it was warmer than it is.

@Poutnik Beware, microwave heating does not work via resonant absorption.

@Karl It works by non-even providing of kinetic energy to various molecules or bonds by Dielectric heating, acting on structures with a dipole moment, in contrary to classical heating.

Here is an example of an entire online GoogleBook dedicated to the topic Microwave Assisted Organic Synthesis, apparently based on some 2,000 papers.
On page 3 of the cited reference, notes to quote:

Interaction between microwave radiation and solutions of polar molecule polar molecules may be adequately described using classical models, which may be derived from Maxwell's equations.

And further:

Although this analysis has concentrated on the oscillating components of the electromagnetic field of the microwave radiation,  it is also possible to get interaction between the oscillating magnetic field and the magnetic dipoles in the sample. The high-heating rates for Fe304 may be attributed to such interactions.

With respect to benefits, per page 23 to quote:

Reaction rates are generally very high and yields in many cases can be greatly improved, as competing side reactions can be minimized...

I would also add that the use of microwave-based dielectric heating is, at times, more than just fast and efficient and heating, as it can, in the presence of activated carbon, for example, introduce surface-based radicals into the reaction mix. See, for example, Generation of hydroxyl radical in aqueous solution by microwave energy using activated carbon as catalyst and its potential in removal of persistent organic substances.
As such, I described the general use of this new heating technology as microwave-assisted synthesis, and not per se, as catalytic, albeit depending on the reaction mix, it can be (per last cited reference).

Can I (genuinely) ask why you mentioned EM fields and Maxwell equations in respect to answering the question?

Yes, its relates to the underlying mathematics used in employing microwaves to achieve max efficiency in likely a commercial synthesis application. Read the first 26 available pages of the ebook. A question you should ask yourself is why the book mentions it and you apparently have an issue.

I think your misinterpreting my intentions. Don’t confuse my inquiry about your answer with a challenge to you intellectual abilities. I was just curious.

Additionally, the full book is not accessible on google for me.

In other words, any effect except the purely thermal occurs under very special circumstances and/or is highly disputed.

Karl: Please read the available 20 plus page ebook preview. The cover material based on a mass of over 2,000 papers on the employment of microwaves hardly warrants the use of the word 'disputed'. I have significant favorable experience using microwave heating to date.

If you look at the Arrhenius equation for reaction rates,
$k=Ae^{\frac{-E_a}{RT}}$
we see that the rate is dependent on collision frequency (A) and the portion of molecules with sufficient energy to overcome the activation energy barrier ($E_a$) [1].
By heating the solution with microwaves, we are increasing T, therefore, increasing the portion of molecules with enough energy to overcome the $E_a$.
Catalysts lower the $E_a$ or provide a different mechanism with a lower $E_a$. This is not how microwaves increase reaction rate as irradiation does not change the activation energy.
Edit
Some studies suggest that non-thermal effects are present (i.e. interactions between the EMR field and dipole moment of molecules) however it is still under debate whether they actually exist. A recent article publish in 2018 suggests that a majority of research disputes the existence of non thermal effects or suggest that non thermal effects arise from enhanced diffusion.
Either way, microwaves aren’t a catalyst.

I feel it is not the effect of temperature only.

@M.Farooq what did you have on mind?

See microwave effects https://en.wikipedia.org/wiki/Microwave_chemistry#:~:text=Microwave%20chemistry%20is%20the%20science,conducting%20ions%20in%20a%20solid.

@M.Farooq The reference is much appreciated . It said somewhere that non-thermal effects aren’t present in liquids. I just thought that was interesting.

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text:
Is it possible to calculate a maximum pressure attainable with a soda vinegar reaction?
Given Le Chatelier's principle, there must be a peak pressure of $\ce{CO2}$, at which, under room temperature, the reaction of soda with vinegar would stop?
$$\ce{Na2CO3 + 2 CH3COOH -> CO2 + 2 CH3COONa + H2O}$$
Is my reasoning correct and if so what formula to use to calculate it?

You're right, and there is no simple formula. Much too complex, the problem. ;)

It's not just a matter of pressure, but pH and it's dependence on CO2 solubility, and acetate's too.

Take CO2 solubility and all respective dissociation constants and calculate the equilibrium. There is no simple formula for it, at least not without significant problem simplification. It leads to a set of several nonlinear equations for several variables.

Note that very high activity of $\ce{CO2}$ may replace reaction with acetic acid by its own reaction $\ce{Na2CO3(aq) + CO2(aq) + H2O -> 2 NaHCO3(aq)}$

I’m voting to close this question because you need to add details (physical state of species, type of container) at least to the extent that you could imagine doing an experiment to measure the equilibrium pressure.


text:
Which of the following octahedral complexes absorbs the photon with the longest wavelength, λ?
I wonder which of the following absorbs photons with the longest wavelength, $\ce{[Cr(CN)6]^3-}$, $\ce{[Cr(SCN)6]^3-}$ and $\ce{[Cr(H2O)6]^3+}$.
I know the one to absorb the highest wavelength should also have the highest spin. Since all of these are coordination isomers, I think the strong-field ligands should have higher energies (making the answer $\ce{[Cr(CN)6]^3-}$).
I hope someone can clarify the relation between ligand field strength and energy. Any help is much appreciated!

Look up the spectrochemical series, although empirical this is good at predicting the d orbital splitting and so absorption band frequencies for metals in 'normal' oxidation states.

Since CN- is a strong field /pi-acceptor ligand, therefore, it will create larger splitting. So for transition higher energy will be needed. It means that in case of Complexes with CN- higher energied photons i.e. photons with shorter wave length will be absorbed.


text:
What's the least common element in the universe?
What's the least common element in the universe? And what is its frequency measured in Hertz?

What do you mean by frequency?

Any element above 118, oganesson, in the periodic table. It's frequency is zero Hz, since it isn't here, yet. And there may be many others that haven' been discahvahd. (T, Lehrer)

Probably astatine.

Questions without elaboration effort are not very welcome, and may be closed. Always think about and search for possible answers thoroughly before posting a question.
Written effort attracts elaborated answers.
Include in the question what you already know/thought about, what you found in textbook or online resources, what you have understood or not/tried/failed.
Always add eventually relevant  circumstances and background to prevent wrong assumptions and requests for clarifications.

There are 2 elements in universe that are so rare they have not been observed in the Universe yet. Which of them is less common ?

If the question is raised in context of your other question about Ca+ and Fe+ frequences, the mentioned frequencies are frequencies of molecular transitions.  Single atoms or ions do not have such frequencies.

Excluding the artificial ones, I've been often told that the less abundant element in the universe is Astatine.

Uhhh.. who closed this question and why?  It's a good one.

(edited based on @JonCuster's comment) The rarest known elements are most probably the man-made heavy elements. These isotopes (elements) are very unstable. They most probably also formed in supernovas, but their natural formation is minimal, and due to their short lifetime, they do not accumulate either.
As Jon added, most probably heavier elements are also formed in supernovas, however, I am not aware of any of them observed. The reason for that is 1) again, short lifetime prevents any large accumulation, 2) under the formation conditions they are not atoms, but bare nuclei /plasma, so one cannot observe their atomic spectrum.

One might suspect there are a number we haven’t made, needing supernova conditions to appear even briefly...

@JonCuster Yes, I guess. Why do not you make it an answer?

I'd just add it to your answer - as additional info around the 'formation in stars is minimal' and call it good.


text:
Which metal bicarbonates do not exist in solid state?
$$\ce{(i) LiHCO_3       (ii) Ca(HCO_3)_2       (iii) Zn(HCO_3)_2      (iv) AgHCO_3}$$
From my prior knowledge compounds i and ii are not solids because of the high hydration energy of Li and the high polarizing power of Ca respectively. But the answer to this question includes both options iii and iv. Which property makes them not solid/exist in solution state?

Regarding LiHCO3, see these questions: https://chemistry.stackexchange.com/questions/73575/is-lithium-bicarbonate-an-aqueous-solution-of-lithium-carbonate .... https://chemistry.stackexchange.com/questions/69331/why-doesnt-lihco3-exist-in-solid-state/69556

Too bad your teacher forgot to give you his list that you can learn by heart. What a moronic multiple choice question.

The only existing carbonate at the solid state is LiHCO3.

@Maurice LiHCO3 does not exist in solid state

@Singularity. The Handbook of Chemistry and Physics, 43 rd Ed, p. 597, informs that LiHCO3 is white and its solubility is 5.5 g in 100 mL water at 13°C.

@Maurice for some reason the solution to this question states all of these don't exist in solid form and some of my friends (who didn't give me a reason) implied the same. Ps- Maybe i got bad friends Xd Thanks for the help

@Singuilarity. Or maybe my Handbook was wrong. Who knows ?

@Maurice I didn't mean to imply that, sorry, i was just saying what i knew.Btw thanks for that reference book that seems quite useful for my purposes.

@Singularity. Well. The Handbook of Chemistry and Physics is a must for all students and professionals in chemistry. It has more than 2000 pages. And it is not too expensive, because it is reprinted every year, and so many people are buying it. It can also be found on the net.


text:
How to derive the Henderson equation for liquid junction potentials given in Bard and Falkner?
The general equation for liquid junction potential is given by $$E = {-RT\over F}\sum_i\int_a^b{t_i\over z_i}d ln(a_i)$$
Where a and b are the two phases, $t_i$ is the transport number of the ions and $z_i$ is the charge on the ions.
When making the additional assumptions that (1) the concentrations are equivalent to the activities everywhere in the junction and that (2) the concentration of each ion follows a linear transition between the two phases, the above equation becomes the Henderson equation:$$E = {\sum_i{|Z_i| \over z_i}u_i[c_i(b)-c_i(a)]\over \sum_i|z_i|u_i[c_i(b)-c_i(a)]}{RT\over F}{ln{\sum_iz_iu_ic_i(a)\over \sum_iz_iu_ic_i(b)}}$$But when I tried to evaluate the integral by my self by substituting the equation of transfer number $t_i = {z_ic_iu_i\over \sum_iz_ic_iu_i}$ and putting $a_i = c_i$, I'm stuck with the following: $$E = {-RT\over F}\sum_i \int_a^b{{|z_i|\over zi}u_i\over \sum_i|z_i|c_iu_i}dc_i$$ The  integral is of the form $\int{1\over {ax + b}}dx$ where $a = |z_i|u_i$ and $b = \sum_{j \ne i}|z_j|c_ju_j$ $$E = {-RT\over F}\sum_i{1\over z_i}ln\left( {|z_i|u_ic_i(b)+\sum_{j \ne i}|z_j|c_ju_j\over |z_i|u_ic_i(a)+\sum_{j \ne i}|z_j|c_ju_j}\right)$$Which is clearly not the Henderson equation given in Bard and Falkner, and I couldn't figure out where to use the second assumption. Where was  I wrong and how to proceed?


text:
Parallel reactions for one product
For me it is clear how I could solved the kinetic of a parallel reaction like this

But, how can I solve if this would be the case:

Thanks in advance!

As two separate first order reactions and then sum to get total A.

If it is clear for you how to solve the former, it must be clear for you how to solve the latter.


text:
Excitation states of neon (or other gas) inside neon lights
I'm interested in the spectra of gas discharge tubes.
Taking neon as an example, when I look up on NIST the spectra for neon, I find excitation states up to Ne IX. Where do I find/how do I work out which excitation states are predominate in a neon light? Is it voltage- or current-dependent, or depending on transition probabilities somehow? For context, I studied physics but lack some of the finer details that chemists have when it comes to electron configurations.
Ideally I'd like an answer that applies to elements other than neon, as all of the noble gases and a couple of others are common inside gas discharge tubes.

It is a good question but horribly difficult. I have asked plenty of spectroscopists (online) and via email about line assignments but did not get any useful reply

The lines are narrow and the frequencies specific to particular excitations. Match the observed frequencies from the discharge tube to the known transitions and you have the answer.

At least if the gas is low pressure. High pressure tubes have much broader lines but most simple display tubes are low pressure. And don't forget most also contain not just the pure gas but also some other components.

A @matt_black, states, pressure is an influence. Some spectral lines in aurorae could not be reproduced on earth until high-vacuum technology became available. If the mean free path of the atom is short, some excited states may not have a chance to radiate before a collision. As for higher states, and higher level of ionization, an electron beam or high-energy photons may be needed, rather than a glow discharge.

@matt-black, once I have this sorted I do plan to use spectra of common penning mixtures (ie doping neon with argon) and mix in some of the doping elements' spectra.
I am also unfortunately going from the theoretical into the observed side here, ie I want to create the RGB values out of spectra to match the chemistry of the gases inside rather than to match an observed tube (allowing me to generate light colours based on their gas composition once I load up the different spectra).

The intentionally addition of some other gas (correctly identified by you as Penning mixture) was subject of an earlier answer here).


text:
Does the volume of water involved affect rate of electrolysis?
I am working on the design for a homemade PEM electrolyzer, and my goal is to make it as compact as possible, so as to allow easy transport. My question, as it relates to that, is whether - with the same applied current - the volume of water inside the electrolyzer would influence the rate of decomposition into H2 and O2.

There are many factors that would influence the rate, e.g. area, spacing and type of electrodes, electrolyte, current and current density, etc. However, if the only factor changed was volume (i.e. electrodes are fully immersed and always at the top of the solution on a float, not at the bottom where pressure might change), volume would have no effect at first.
However, as the reaction proceeds and the concentration of the electrolyte starts to change, due to water loss and possibly due to dissolution of electrodes, the concentration would change more slowly with a large volume of electrolyte. Therefore, there might be a second-order difference as electrolysis proceeds.

Thanks for the feedback, however I'm a little unclear what you mean by there being a "second-order" difference. And my electrodes will be made from graphene oxide, which is not easy to corrode as it transfers current with a high degree of efficiency without breaking down.


text:
Guidelines for home generation of hypochlorous acid disinfectant from salt water
I bought a home generator for making hypochlorous acid by electrolysis of a water and salt solution.  I would like to figure out guidelines for safe and effective use of my generator.
I have found multiple resources saying that hypochlorous acid solution is not harmful to people or animals and yet very effective as a disinfectant.  A lab made mice drink it instead of water for weeks with no observed ill effects; the FDA allows it to be used as a no-rinse disinfectant in food prep areas; and people routinely put it into "foggers" to disinfect areas where people might go.  I just want something to put into a spray bottle and disinfect the toilet or whatever, so this seems like one of the safest possible disinfectants; and now that I have a home generator I can make it by the litre anytime I need more.
It breaks down quickly, but I believe it will still be effective for up to a week, and it's just a few pennies worth of ingredients and electricity to make another batch.
The device came with a terse instruction booklet, which is written in reasonable English but nonetheless leaves me with questions.
The booklet says to mix 2 litres of water with 12 grams of pure salt (I'm planning to use kosher salt).  Then activate the electrolysis and let it run.  Then dilute the solution 4:1 before using.  Here is a table of suggested times for electrolysis:

5 minutes to make 200 PPM
15 minutes to make 500 PPM
30 minutes to make 1000 PPM

The device has a built-in timer that can be set for these three time intervals, and then stops the electrolysis automatically.
When I bought the device, the Internet retailer web site showed me that often people buy test strips with it.  I thought that was a good idea so I bought both pH test strips and chlorine PPM test strips.
The booklet does not discuss this, but I have read that it is a good idea to add vinegar to the water as well as salt, because there are multiple possible reactions when you electrolyze the saline solution, and you want the hypochlorous acid solution and not sodium hypochlorite.  And in fact there is one brand of home hypochlorous acid generator that sells small plastic capsules containing a pre-measured mixture of vinegar and salt.  (Their web site claims the FDA requires them to sell the pre-measured capsules to make sure the end user gets the recipe correct.)
I searched the Internet for a set of guidelines for the home user, and found a discussion on another site that urged adding a base rather than an acid to the salt solution before electrolysis.  I've only seen that one place; the suggestion to add vinegar seems to be the common advice.
The booklet says the reaction is:
NaCl + H20 -> NaClO + H2^
I've seen videos of devices like this in operation and bubbles appear in the water; presumably that's the H2.  Is there any worry it might generate chlorine gas or anything else bad?  I'm planning to use this in a well-ventilated area.
We have a large supply of powdered citric acid, so I would like to work out a recipe that uses citric acid and salt instead of vinegar and salt.  According to a video I found, ideally the salt solution should have a pH of "between 4 and 5".  So presumably I can just start mixing citric acid powder with my tap water, and using my pH test strips, until I find the correct amount of citric acid powder to get a 4.5 pH solution.  But I thought I should ask here first before I try it.
And I would like to know what I should expect to see on the test strips if the reaction was the desired one.  What should I expect the final pH to be?  Presumably the PPM test just shows PPM directly and should match what I'm trying to make by setting the timer.
I'd like a recipe to make 1 litre at a time.  And ideally I'd like to work out a recipe I can use without diluting it... one litre of spray disinfectant easily should last me a week; I don't need to make 4 litres at a time.
References:
The generator I bought:

https://www.gosoitwater.com/all-purpose-cleaner-maker-machine-disinfectant-sanitizer-cleaner-p2580977.html
https://amazon.com/GOSOIT-Multi-Purpose-Detergent-Deodorant-Personal/dp/B085TFB5Z2

Article suggesting that hypochlorous acid is a safe and effective disinfectant:

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7315945/

EPA listing of disinfectants known to be effective against SARS-CoV-2, which includes hypochlorous acid:

https://www.epa.gov/pesticide-registration/list-n-disinfectants-coronavirus-covid-19

FDA allows it for food prep with no rinse:

https://www.hypochlorousacid.com/regulation

The generator that sells pre-measured capsules of vinegar and salt:

https://www.forceofnatureclean.com/natural-cleaner-and-disinfectant/

Discussion on another site suggesting that one should add a base, not an acid, to the water:

https://communities.acs.org/thread/10731-need-help-understanding-the-production-of-hypochlorous-acid-and-byproducts

One of the many videos I found about making hypochlorous acid:

https://www.youtube.com/watch?v=te8C0bo_Zrc

A video showing a recipe to use instead of the pre-mixed plastic capsules:

https://www.youtube.com/watch?v=5sNlwO6_gco

In short, hypochlorite ion is nothing but chlorine bleach used un laundry. You should not follow any scam or disinfectant generators for home. These are not safe products. Do not add vinegar with salt during electrolysis. Most likely you might form cancer causing substances. Alternatively, citric acid or vinegar and salt are good for nothing as disinfectants.

The only reasonable use for hypochlorite at home is in a pool. Don't go disinfecting stuff unless your doctor urges you to, usually because you have a norovirus or similar at home.

@M.Farooq I gave you references explaining why I thought this is a good idea.  You haven't given me anything other than what appear to be your personal opinions.  Please provide references if you wish to persuade me.  "These are not safe products"  Okay, here's the Safety Data Sheet for Force of Nature.  Please tell me what you consider most unsafe part: https://www.forceofnatureclean.com/force-nature-material-safety-data-sheet-2/  If you think Force of Nature is safe and the gadget I bought isn't... why?

@steveha, If someone tells you that playing and splashing hot boiling water may cause injury or burn your skin, will you call it a "personal opinion" and ask for references? Sorry it is not anyone's job to persuade you not using a commercial dubious gadget. All that was said was that hypochlorite disinfectant is nothing but a laundry bleach. You do not need to prepare bleach at home, which is prepared commercially with full safety. And your idea of adding vinegar and salt in the electrolysis cell can potentially generate carcinogens. Do you see close votes by SE readers?

Head over to https://www.sciencemadness.org/whisper/ and post this in the Technochemistry forum! There are a lot of people interested in this.

I hope the well-sourced answer provided below is of assistance.  The closing of this question (on which I have voted to reopen) and my unwarranted down vote, is more of a forum policy issue to avoid giving advice on health issues (aka, fear of liability issues) and should not be construed of any lack of accuracy per my many government-based sources or the significance or quality of the presentation of your question. I do agree with the opinion, however, that adding vinegar is a potentially bad idea, and explaining why is also equally important.

@M.Farooq Various branches of the US government recommend hypochlorous acid solution as a safe and effective disinfectant.  I provided you with a Safety Data Sheet for a commercial gadget similar to the one I bought, and if you actually read it you will see that it contains statements like "c. Carcinogenic Effects: Negative  d. Mutagenic Effects: Negative"  In return you provided unsourced opinions such as that it "can potentially generate carcinogens."  Don't worry, I see that this question was unwelcome in this community; and indeed I feel unwelcome here personally, so I'll be gone soon.

@John I visited the site you suggested, and tried to register as a user.  I got an error message that the function I attempted to use is disabled at this time.  I looked to see if I could post a new topic without being a user, and didn't see any way to do it.  So, I posted my question on Quora.  Maybe next week I'll try the site you suggested again.  If you are able to post there, I'd welcome you posting this question there; I don't care if I can join the site, I just want an answer.

Here's my last word on the question.  If you want to make HOCl solution for use as a cleaner and disinfectant, I recommend you buy the "Force of Nature" (FoN) product.  FoN is an "electrolyzer" which you fill with water to a line and then you add a pre-measured pouch (which actually contains only a small amount of vinegar and salt).  Plug it in, push the button, bubbles fizz and in five minutes you have 16 ounces of HOCl solution.  It's safe enough to spray in your eyes yet the US government lists it as an effective sanitizer.  I have no connection with FoN.

I bought a total of three gadgets.  The first one was a total mistake.  The second one was an Ecolox Eco One, which works okay but we found less convenient than the third one, "Force of Nature" (FoN).  FoN sells little plastic pouches with pre-measured portions of salt and vinegar; the Eco One just has a recipe and a measuring spoon.  It would be possible to make your own FoN mix if you wished and people do that, but I wouldn't dream of suggesting something like that here because someone would accuse me of creating mutagens or something.

As for the actual answer to my question: I found a description of the reaction these perform.  Visit this page to see a graph: https://hocl.com/hocl-chemistry  By reading the graph you can plainly see that if the pH of the solution is between 4 and 5 the reaction will make HOCl and essentially no free chlorine.  If you allow the pH to get too low, free chlorine would be produced.  I bought test strips and made sure that the recipes I personally made had the proper pH to avoid producing free Cl2.

Here are my comments having extensively studied (pending patents) and produced (albeit not via electrolysis) hypochlorous acid ($\ce{HOCl}$) for various applications.
First, a possible issue I have is the mentioned use of vinegar, an organic acid, to the mix. The reason arises if not employing distilled water (meaning you may be using tap water rich in transition metal ions). The latter can react with $\ce{HOCl}$ in a Fenton-type reaction (detailed below) introducing radicals. Depending on whether the pH is greater or less than 5, you obtain either the more powerful hydroxyl radical ($\ce{.OH}$) or the chlorine radical ($\ce{.Cl}$), so a pH of 5 to 7 is preferable. The introduction of the chlorine radical allows the potential attack of organic compounds providing a path to chloro-organics, a potential health issue if consumed internally (externally, for surface disinfecting, a lesser concern with proper ventilation). Note, the action of the $\ce{.OH}$ radical on aqueous chloride ($\ce{Cl-}$ from salt) can also be a source of the chlorine radical (see chemistry detailed below).
Note, the action of even a weak acid on Sodium hypochlorite ($\ce{NaOCl}$), like dilute acetic acid found in vinegar, will form $\ce{HOCl}$ (including also very dilute mineral acids at pH 6). Hypochlorous acid is cited to be nearly 100 times more powerful in disinfecting than $\ce{NaOCl}$ (see related comments here and elsewhere). As such, one could be faced with a decision in the presence of transition metal ions to balance the immediate dangers of drinking a water source which may lead to a rapid onset of a fatal disease with choice of either weaker $\ce{NaOCl}$ or more powerful $\ce{HOCl}$. Interestingly, one can apply carbon dioxide, as an acid gas, to $\ce{NaOCl}$ as a path to hypochlorous acid, and avoid the chloro-organics issue. The associated radical formed than becomes the less powerful (but much-prolonged life span) carbonate radical anion ($\ce{.CO3-}$ formation detailed below) in place of the more reactive, but transient, hydroxyl radical.
Also, iron and copper are transition metals, so avoid using as electrodes in an electrolysis path to hypochlorite/hypochlorous acid if you want a product with a longer shelf-life. Hypochlorous is itself, with exposure to sunlight or even air rich in dust particles, or $\ce{HOCl}$ in contact with an anodic metal source (like stainless steel creating a battery cell) should not be expected to have a long shelf life (more like hours).
Some details on the cited chemistry with sources, in the presence of iron or copper ions:
First, from "Fenton chemistry in biology and medicine" by Josef Prousek, to quote reaction (15) on page 2330, a general depiction of Fenton-type reactions, to quote:
"For Fe(II) and Cu(I), this situation can be generally depicted as follows [20,39],
Fe2+/Cu+ + HOX → Fe3+/Cu2+ + ·OH + X- (15)
where X = Cl, ONO, and SCN. "
In a possible presence of copper ions:
$\ce{Cu -> Cu+ + e-}$
$\ce{Cu+ -> Cu++ + e-}$
$\ce{HOCl + e- <-> •ClOH-}$   (see, page 13, Table 1.2a here and Supplement Table S1, link below, Reaction [1] )
$\ce{•ClOH- <-> Cl- + •OH}$  (pH > 5, see Eq(8), page 8, same source as above)
Also, different radicals and products at low pH:
$\ce{•ClOH- + H+ -> H2O + •Cl}$   (pH < 5, k = 2.1×10^10 Source: Eq (7) in Supplement Table S1 available here and click on provided Supplement File 1 to download)
And, with a bicarbonate or chloride presence (likely in the electrolysis of NaCl):
$\ce{HCO3- + .OH -> H2O + .CO3-}$  (see Carbonate Radical in Natural Waters)
$\ce{Cl- + •OH -> •Cl + OH-}$
$\ce{Cl- + •Cl -> •Cl2-}$  (Eq(12) in Supplement S1 file link)
where radicals, especially $\ce{•OH}$ can attack and breakdown organic compounds.

Where is Eq($8$) above ? There is nowhere an Eq($8$).  And also, please, don't oblige us to read the $294$ pages of the" table available here"

Maurice: Yes, a little confusing. I did an edit to more clearly note that it is a reference to the Supplement Table S1 (look for Supplement File material section and click link). Note, Equation  (12) in the cited work. – AJKOER 4 mins ago

Thank you for providing a thorough answer.  Since my question was closed yours might be the only answer I will get here.  I don't have a sufficient understanding of chemistry to fully engage with your answer but I'll try to comment usefully.

First, in re: to the vinegar: it's not my idea but seems to be common in this area, and is standard in the Force of Nature capsules.  Force of Nature got an EPA certification and they recommend tap water.  However, the one I bought (the Gosoit device) does not recommend vinegar.  The second YouTube video I linked has a person recommending the use of vinegar to make sure the reaction primarily produces hyopchlorous acid, along with a chart of unknown source showing that a slightly acid pH is ideal.  My tap water is neutral or very slightly basic.

Second, the Gosoit product page, which I linked, includes this claim: "...with latest Platinum coated Titanium (SPE) electrolysis Plates and the long lifetime Proton Membrane (PEM) made by DuPont in the USA."  I was interested to read your concern about the use of iron in an electrode; on this point I think you can be reassured.  That page further says: "Only electrolysis water and salt into sodium hypochlorite disinfectant"  So your anti-recommendation for vinegar matches with the official product advice.

Third, as for shelf life, I have seen a wide variety of claims for shelf life.  There are numerous bottled products that claim to be stabilized hypochlorous acid; some people claim the homemade stuff is good for two weeks; some claim one week; and Gosoit recommends using the solution within 6 hours!  On balance I was assuming that I could probably keep a bottle of solution for a week.  I expect to usually use the solution right after making it.

Okay, I have read through your answer a few more times and I think I am understanding it a little better.  It sounds like ideally I should use distilled water rather than tap water, but it also sounds like you aren't super-concerned about it.  pH between 5 and 7 is recommended, which matches other advice I have seen.  On my first reading I thought you were recommending against the vinegar but now I think you aren't.  In any event I was planning to use citric acid powder.  I've upvoted and accepted your answer.  Thank you.


text:
Is a Methoxy group meta directing in this problem?
The following question was given in chemistry today , August 2020 edition , page 47 , problem 9. The answer to this problem is (a).

My Attempt

In the third step , the carbon electrophile can attack at ortho position to carbon substituent to give a 6 membered ring.
But , the O-Methyl group is ortho para directing. The resonance structures are as follows.

The given answer is , a , is possible only if we consider O-Methyl group on Benzene as meta directing group.
My Question:
Is methoxy group on ANISOLE Meta directing ?

The presence of the OMe activates the whole ring to electrophilic attack. The o & p positions are most activated but m is also activated. Your attempted mechanism is correct

Anisole is a whole. The question in the last line is wrong in terms, Even if we know what you mean.

My question is how did the CH2 group para to the methoxy shift to a meta position? Unless this question represents a surprising mechanism, answer d seems to make the most sense.

I agree, d is the product

@Waylander , understood that Methoxy group activates ring. But electrophillic attack at meta gives an intermediate that is relatively unstable. Since intermediate is unstable , can EAS occur ?

It is possible for the electrophile to attack at the para position, followed by an alkyl shift to the meta position. The best answer would still be (d), I think, although it depends on which group migrates. (a) makes no sense, I'm afraid. There's no way anything can get ortho to the methoxy.

@ChakravarthyKalyan   Ortho attack is impossible as the electrophilic centre simply cannot reach the ortho position. Once that is ruled out you have to ask "what is next most favourable reaction pathway?"

Are you not concerned how an E-double bond becomes Z in the cyclized product?

The saturated acid is known to undergo FC cyclization to 7-methoxytetralone.

The double bond geometry is not specified in the original question.

Now that the general consensus of the Commenters is that meta cyclization is viable and that the answer is (d) and not (a), the OP and other readers should be concerned as to how an (E)-double bond can cyclize in a Friedel-Crafts (F-C) reaction. It doesn't! Prior isomerization to the (Z)-configuration is required.
(E)-Acyl chloride 1 must rotate about the red β-γ single bond and $\ce{AlCl3}$ must complex with the carbonyl oxygen forming species 2. [One may also invoke $\ce{AlCl3}$ complexation with the chlorine and rationalize the subsequent steps via an acylium ion and conjugated ketenes. The principle is the same.] Loss of a proton at the γ-position of conformation 2 provides the β,γ-(Z)-double bond of 3. Now deconjugated acyl chloride 4 is ready for F-C cyclization to intermediate 7 and onto 7-methoxynaphthalen-1-ol (9), aka (d). Alternatively, rotation about the red bond in 3 leads to conformation 5, which, upon γ-protonation leads to α,β-(Z)-conjugated acyl chloride 6. Subsequent F-C cyclization via intermediate 8 affords 9. Incidently, base is not required to produce 9 from 7 or 8. F-C conditions are sufficient.


text:
What is intermolecular distance in water?
I am wondering what is an average intermolecular distance in liquid water (say under normal pressure and room temperature).
I need just order of magnitude. A reference would be helpful.

Questions without  explicit particular effort are not very welcome, and may be closed.  Think  thoroughly about  possible answers to your questions and search for them before asking.   Provide in the question elaboration what you already know, what you have thought about, what you tried to search for in textbooks and online resources, what you found, understood or not, where you failed. Elaborated questions attract elaborated answers and vice versa.  Include in  questions all eventual relevant circumstances to prevent wrong assumptions and requests for clarifications.

You could actually calculate it yourself using the density of water, its molecular weight, and Avogadro's constant.   I'd suggest giving it a shot and showing us your work.  [Hint: the first part is to determine the number of water molecules per cm^3.] Forum members can help you if you get stuck.

@Theorist I was just going to provide Wikipedia references to water molar mass, water density and Avogadro' constant. But the OP can easily find it.

@Poutnik : I tried to find in internet of course, but could not.  I am not a specialist in the field (I am a mathematician and need this data for some course I will teach, as an illustration). Thus I am wondering if there are any tables in chemical literature containing all such data for all kind of materials.

@makt: You can easily find the density of water, the molecular weight of water, and Avogadro's constant on the internet.  So I assume you are instead saying that you have no idea how to use these to calculate the average intermolecular distance, i.e., that you have no idea how to even start the calculation.  Is that correct?   If so, then I suppose I would need to just give you the formula.[Indeed, you could easily find this data for all sorts of substances, and just do the calculation for all of them at once using an Excel spreadsheet.]

As an order of magnitude it is the same as in solid water, just a tip of an iceberg shorter. The structure of ice is known. Do you want center to center distance, closest neighboring atom distance, or vanderWaals radio overlap?

Which intermolecular distance?

Not all data are provided in explicit form. They are often not frequently needed and can be often deduced from other data with using of high school knowledge.
You can calculate the volume occupied by 1 water molecule ( in average ) from the water molar mass $\pu{M}$, water density $\pu{\rho}$ and the Avogadro constant $N_\mathrm{A}$ as
$$V=\dfrac{M}{\rho \cdot N_\mathrm{A}} $$
Then you can calculate the side length $a$ of a cube of this volume:
$$a={\left( \dfrac{M}{\rho \cdot N_\mathrm{A}}  \right)}^{1/3}  $$
As a rough number, is is equal to
$a  \approx { \left( \dfrac{\pu{0.018 kgmol^-1}}{\pu{1000 kgm^-3} \cdot \pu{6.022e23 mol^-1}}  \right)}^{1/3}  \approx \pu{0.31 nm}$
what can be taken as the typical "center to center" distance of water molecules within the order-like rough precision.
The more precise average distance would have to calculate with the size and shape of the water molecule ( $\ce{O-H}$ bond length about 0.096 nm, the bond angle 104.5 degrees) and to make the integral average over the directions. But it would be extremely complicated, considering also difficulties with molecule borders and hydrogen bonds. In some sense can be said the distance is zero or even "negative", as the molecules form bonds ands overlaps.

For an order of magnitude estimate, look at the structure of ice. The density of liquid water under ambient conditions is higher, so water molecules would be closer, but the difference is less than 10% (in terms of volume, so less in terms of distance).

In ice (hexagonal ice, to be specific), each water has 4 waters as closest neighbors. The oxygen atoms (i.e. roughly the centers of mass) are 277 nm apart, the closest atoms are 177 nm apart (and form hydrogen bonds), and the van der Waals radii of closest atoms overlap (as seen in the red-yellow "pillow"). The coordinates for this image are from ChemTube3d.


text:
Why Ammonium bifluoride is used in the iodometric estimation of copper in brass?
When brass solution (mixed with nitric acid and urea) is to be estimated for amount of copper present, ammonia is added to neutralize the solution. This gives a turbidity which can be dissolve by adding ammonium bifluoride (NH4HF2).
What exactly causes the turbidity? Which compound formation is responsible for the turbid appearance?

The turbidity is possibly traces of iron, ferric hydroxide, and possibly aluminum that was in the brass. Fluoride complexes with both.  Ferric ion interferes with the iodine titration method, lowering its activity reduces the interference. The word estimation is bothersome; this method should be reasonably quantitative, what shortcuts are being taken?


text:
Mechanism of Birnbaum–Simonini reaction
When silver acetate reacts with iodine and $\ce{CCl4}$, it gives esters and 2 moles of $\ce{AgI}.$ But when it reacts with iodine  and lead acetate, why does it give alkyl halide, $\ce{AgI}$ and $\ce{CO2}?$
$$
\begin{align}
\ce{2CH3COOAg + I2 &->[CCl4][\phantom{(CH3COO)2Pb}] CH3COOCH3 + 2AgI + CO2}\tag{R1}\\
\ce{CH3COOAg + I2 &->[(CH3COO)2Pb] CH3I + AgI + CO2}\tag{R2}
\end{align}
$$
Why doesn't the alkyl-iodide bond break similar to when $\ce{CCl4}$ was used as solvent? What exactly lead acetate do such that alkyl-iodide bond don't break and we get alkyl iodide as product?
Also, my lecturer told that the alkyl iodide bond wont break since lead will form the complex with acetate which is not so clear. I tried to get the answer on Google but no where it shows the explanation for it.

https://onlinelibrary.wiley.com/doi/full/10.1002/9780470638859.conrr086 Here's a link from Wiley library...that says the Birnbaum Semonini (first reaction in your attachment) has been modified for preparing esters from a reaction between lead tetra‐salts of carboxylic acids and iodine.(I am not allowed to comment..so this answer).

Compiling my comments(now deleted) into an answer.

TL;DR: the products are different due to different stoichiometric ratios of the reactants and is not dependent on the solvent. You can use different solvents so that you can increase the yields.
Hunsdiecker reaction
The Hunsdiecker reaction is a reaction whereby silver salts of carboxylic acids react with a halogen to produce an organic halide.

The reaction was first demonstrated by Alexander Borodin in 1861 by preparing methyl bromide ($\ce{CH3Br}$) from silver acetate ($\ce{CH3CO2Ag}$). Shortly after, the approach was applied to the degradation of fatty acids in the laboratory of Adolf Lieben. However, it is named for Cläre Hunsdiecker and her husband Heinz Hunsdiecker, whose work in the 1930s developed it into a general method and thus the reaction was renamed to Hunsdiecker reaction. The mechanism is given below:

Birnbaum-Simonini reaction
During the discovery of this reaction by Borodin, Angelo Simonini who was working as a student of Adolf Lieben at the University of Vienna, investigating the reactions of silver carboxylates with iodine. They found that the products formed are determined by the stoichiometry within the reaction mixture. Using a carboxylate-to-iodine ratio of 1:1 leads to an alkyl iodide product, in line with Borodin's findings and the modern understanding of the Hunsdiecker reaction. However, a 2:1 ratio favours the formation of an ester product that arises from decarboxylation of one carboxylate and coupling the resulting alkyl chain with the other. It was named Birnbaum-Simonini reaction.

The mechanism is given below:

Simonini reaction
Using a 3:2 ratio of reactants leads to the formation of a 1:1 mixture of both products. This reaction is called Simonini reaction.
$$\ce{3RCO2Ag  +  2I2 -> RI + RCO2R  + 2 CO2  + 3 AgI}$$

What's the matter with different solvents?

The reaction was initially done using carbon tetrachloride. But it was seen that when a different solvent was used, the yield varied slightly. It was seen that the yield of the latter reaction improved when lead tetraacetate was used. So, this solvent was used for this reaction. (See here)

The Hunsdiecker is likely a radical chain reaction $$\ce{R^. + RCO2Br -> RBr + CO2 + R^.}.$$


text:
When blue vitriol is dissolved in water
when CuSO4.5H2O is dissolved in water, what physical and chemical effects happens to its water of crystallization?
While solving a stoichiometric problem the ques was that 1litre of CuSO4.5H2O was dissolved in 1 liter water and the concentration of Cu2+ ions were asked. When calculating molarity the volume which those 5 water of crystallization were also taken along with the solvent water.
I had the doubt that if the 5H2O is attached to CuSO4.5H2O then how can we include that also while calculating volume of solvent (for denominator of molarity). Those 5 water molecules are not free, they are bonded then why do we consider it?

The waters of crystallisation simply mix with the rest of the water in the solution. The $\mathrm{ Cu^{2+}}$ is, however, solvated and forms $\mathrm{Cu(H_2O)_6^{2+}}$ with two of the water ligands further away than the others. One litre of CuSO4.5H2O does not makes sense as it is a solid.

"blue vitriol"? Are you learning from a century old book?

When a sample of $\ce{CuSO4⋅5H2O}$ is dissolved in water, the $5$ molecules water are mixed with the water used as solvant, and cannot be distinguished any more from the other molecules of water. It does not really matter, as if you want to calculate the molarity, you never dissolve your sample of $\ce{CuSO4·5H2O}$ in exactly $1$ litre water. No ! You dissolve it in a volume of water big enough to get one liter solution, whatever the exact amount of water used for this dissolution. It should be a little less than one liter water. The only important parameter is the final volume of the solution, namely one liter.


text:
Carbon Sequestration For Indoor Farming
First the backstory:
We've begun growing mushrooms (currently Enoki, Lion's Mane and Blue Oysters if anyone is curious) for the sake of eating and maybe potentially selling someday soon. My partner and I have a combined diplomas and degrees in Biotechnology, so we know about all the rigors of keeping things clean and contamination free, figured it was time to try our hand at what we've learned and see how well we can do it.
Anyways I got to thinking, what if I grew them in a more oxygen rich environment? I've made the assumption that, like the dinosaurs, an oxygen-rich environment would allow the fungi to grow larger than they would without extra oxygen.
Here's the idea:
At a job I used to work at, we would sequester CO2 in about 0.5M NaOH in order to measure methane gas composition percentages in digesters. The gas was composed of mostly CO2 and CH4. By sequestering one, we can roughly calculate the other. For volumes we'd use about half of the volume of gas in 0.5M NaOH to sequester (not sure if that volume/concentration is the most effective). Shake the solution in the gas and equalize the gas to find out your CH4 percentage.
I'm wondering if I could build, in the most basic sense, a vacuum with an internal NaOH atomizer that pulls outside air through, sequesters the CO2 in liquid, and allows higher concentrations of oxygen to pass into the grow room. As for actually capturing and storing the CO2, I'm thinking I could pass the NaOH through activated biochar and reuse the NaOH or maybe there's a way to precipitate the CO2?
I understand the materials required all need to be compatible with NaOH, especially the atomizer, so it's a bit of a pipe dream. But have I just thought up an effective way to sequester CO2? My minds racing that this may actually be a good idea!
Open to all suggestions and recommendations. I'm not flying forward with this idea at all, so don't worry about me dunking an atomizer in a  bath of NaOH (which no one should !EVER! do unless in an incredibly well-ventilated area, and the equipment is compatible).
First time using this site, hopefully this wasn't too long winded, apologies if I've taken up your lunch break.

Would it be simpler to buy tanks of compressed Oxygen gas and just pump that into the grow room?

User $137$ is right. Furthermore, sequestering $\ce{CO2}$ from air will not concentrate the remaining oxygen amount in air. There is about $20$% $ \ce{O2}$  in air, and this proportion will not change if you remove $\ce{CO2}$ from air.

I agree with user137 and Maurice, at 140 ppm or so of carbon dioxide, unless it inhibits growth, taking it out of the air may not do much.  How about generating oxygen!  Electrolysis?  There seem to be a number of DIY approaches for oxygen generation on youtube - which I must admit I have not screened or viewed for the best approach.

You need to check back to your biology books about the dinosaurs. What you write about their size and oxygen is just pure nonsense.


text:
Fluorometer for measuring fluorescence in a sheet of water
Does anyone know of a fluorometer that I could use to measure fluorescence in a thin sheet of water.  So it would be like holding a piece of writing or printer paper vertically and trying to get a reading of the fluorescence at various locations around an 8.5 by 11 area of the paper sheet (although the sheet of water won't be nearly that big - more like 3x5).  A small probe so that multiple locations could be measured by simply moving the probe would be great. A submersible probe would not be good as it would disrupt the sheet.  The sheet will contain quinine at varying amounts depending upon how far a quenching reaction with chloride ion has progressed.
Thanks.

I don’t know of a fluorometer, per se, that would directly do what you want. But the fluorescence maximum is about 460 nm, so a glass or plastic fiber optic, photodiode, transimpedance amplifier and ADC (assuming digital output is required) would work fine. One end of the fiber optic would be movable and placed wherever needed and the other would go directly to the photodiode. You would also need a non-fluorescent long pass optical filter to avoid the excitation light reaching the photodiode. Sounds like a nice project: best of success with it!

Thanks Ed V.    I have never used a fluorometer.  Can you get me started?

Do you know the units associated with your length? 8.5 x 11 ad 3 x 5 ? If you take pictures of your sheet with a right cut-off filter in front of your camera, you may use Image-J with light intensity. You will have to illuminate uniformly.

@M. Farooq:  The lengths I quoted were for illustration only; they were in inches.  The sheet will have different lengths depending upon velocity and nozzle size.  I never heard of ImageJ.  I googled it and it looks like it is a Java-based image processing program.  I'm trying to figure out what it does and how it can help me get quantitative readings of either fluorescence or color.

Thanks, Ed!  M. Farooq's idea has a certain simplicity to it (for me at least), so I'll probably give that a try, first. So, will any blacklight from Amazon work, or do I need to try and find something that is about 460 nm?  For uniformity of lighting, I would buy something that has the UV lamps in a rectangular enclosure that is significantly larger than the sheet - something like a floodlight.  Does this seem a reasonable approach?

As I state here, the optimum excitation wavelength is about 365 nm, i.e., near UV or so-called “long wave UV”. The peak emission wavelength is 460 nm. A blacklight should work well, assuming uniform illumination. I would try that.

@rdemyan Please be careful with UV light. It can harm your skin and eyes if not used properly. Do you have an academic supervisor who is guiding you? What is this project about?

Silly idea: Switch on UV, take photo, switch off. Build a nice cardboard box so that sheet and light are always in the right place, with a hole on the top to put your camera/phone on. Use ImageJ to evaluate the picture, or Matlab/Octave, gnuplot, ...

@Karl Good idea, actually, especially given the OP’s stated experience level. Minor variant: record a video and use simple frame grabbing software/app. Then time evolution is easy to see, etc.

@M.Farooq:  I had an advisor 30+ years ago :). The plan is to focus the camera without the UV light on and then once it is focused, turn on the UV light and take a few pictures.  Then turn off the light and examine the pictures on a large screen. If not good, repeat, otherwise change the pressure drop and take more pictures. I have goggles that provide UV400 protection.  But I didn't really consider the skin too much, so I'll add in some precautions (like maybe a face shield and gloves).  Thanks for the advice!

@rdemyan, This is a problem with anonymous public forums. We have no idea whether the OP is a teenager, adult or a very experienced person. In any case, the context is missing as to what you are trying to do.

@Ed V.  I'm not sure video will work, because the whole process will be at a steady state.  If the video sees the whole sheet, I think every frame will look the same during this steady state process.  So if the fluorescence disappears after 1 cm of flow, that's pretty much what will always be seen while conducting a run. Digital photography should work better.  To get sharp lines, I guess I will want to reduce the shutter speed substantially.  It will be interesting to see how low I can set the value (with respect to ISO and F stop).

@M. Faroog:  I appreciate your replies.  The Image-J idea sounds intriguing and could be exactly what I need.  However, I suspect I will need to do a fair amount of calibration work. I hope there will be some leeway with respect to the distance the UV light source is from the sheets.  Unfortunately, I have to take down my equipment after every session. So the UV light will have to be mounted every time.  I will measure the distance between source and sheets and try to keep that as constant as possible.  I'll have to make some standardize solutions of quinin for calibration, I think.

Forget about skin protection. You're not going to have your hand under there for more than a minute at a time, are you? I personally also woudn't bother with UV safety glasses, make sure the lamp is mounted in a way nobody can look directly into it. Build a cardboard box for it.

@M. Farooq:  I've googled "right cut off filter" but can't seem to find what it is or means. Is this used with regular light to get just the UV into the camera.  Is it used in addition to a UV source such as a black light?

@rdemyan, I have no idea of cameras and their filters but a cut-off filter is a generic term in optics.  Search Edmund Optics. https://www.edmundoptics.com/knowledge-center/application-notes/optics/optical-filters/

@M. Farooq:  I've been investigating Image-J and even left a rather long post at the main help forum.  My biggest question right now is how to determine the concentration of quinine.  I will know the quinine concentration in the sheet that contains quinine and acid.  Once that sheet impinges with the aq. chloride sheet, quenching will start to occur.  I'll be able to get pixel readings from the photographs, but I'm not sure how I convert those readings into concentrations of quinine.

@M. Farooq: Also, if I get a reading of say 100 and areas of the photograph that are black and not part of the sheets have a reading of 25 (so a background reading if you will since these areas have no fluorescence), would I need to subtract the background reading from the 100 reading yielding a value of 75.  Or how do I account for the fact that the lowest possible reading is 25 and not zero.  Thanks!

@rdemyan, What you are trying to do is a non-trivial exercise which may require some time to optimize. I have never done this type of flow analysis on a sheet. Until and unless your post is clear, with schematics, with all experimental variables no reader can make any suggestion. This also means you explain the purpose of this experiment. Quinine quenching by chloride follows a named law, which I am forgetting now.

Hi M. Farooq.  I have posted a pretty detailed discussion of the issues along with a fluorescence photo from 30 years ago (using coumaric acid).  It doesn't seem to me that this forum is the place to post such a detailed post (but maybe I'm wrong).  Anyway, if you get the chance and have any interest, the post is located at forum.image.sc.   Just enter "impinging" into the search box and the post will appear.  Thanks for any help or guidance you can provide.

Well, I tried using quinine and spraying it out into a thin liquid sheet.  No difference in fluorescence relative to spraying water only was detected.  Based on literature I tried using 2 liters of tonic water diluted about ten times.  This would give a concentration of less than 10 ppm. Sulfuric acid was added to the tank to provide a molarity of 0.05.  No fluorescence was observed in the sheet, although a little could be observed in the edges of the sheet which are significantly thicker due to surface tension. I used a 365 nm UV light for excitation.  Comment continued below.

I'm guessing I need a much higher concentration of quinine.  Quinine is quite expensive when dealing with liters per minute of flowrate.  I guess I'll try fluorescein instead unless someone can point to something I'm not doing correctly.  For fluorescein it looks like I should get a UV light source with a longer wavelength.


text:
Stability of pyridine derivatives

Arrange the following pyridine derivatives in the order of increasing stability
of adduct formed with $\ce{BBr3}.$ Explain briefly.

I'm pretty confused about which structure is the most stable. I'd say structure III is the most stable as it has two methyl groups to donate electrons to nitrogen and increase its Lewis basicity.
However, I think III may not be the most stable because of steric hindrance. In fact, structure II could be the most stable as the methyl group is closer to nitrogen and there's no steric hindrance. Therefore, the answer should be III < I < II.
Is my reasoning correct? Are there any factors to consider for stability?

I believe that the order of stability is II > III > I. The adduct should be formed between the lone pair on nitrogen and the Lewis acid. From there, I would apply the M+, I+ effects of the methyl groups as are known from benzene derivatives (although the substitution chemistry of pyridine is different from benzene).

I see. Just clarifying, does the distance between N and the methyl group play a part in stability? If the methyl group is closer to N, it'd be easier for N to receive electrons from the methyl group, hence higher lewis basicity. I'm not sure if it's a misconception.

I think it is a misconception, take a look at the mechanism of the aromatic electrophilic substitution on benzene, toluene/phenol, and nitro-benzene. I think the same concepts apply here.


text:
Question to find increment in total energy of a sample of He gas?
If a closed rigid wall container filled with 20 gram of Helium gas moving at a constant speed of 10 m/sec is stopped abruptly to stand still, find the increment in temperature of this He sample.
Way I have solved
I used Average kinetic energy formula and $nfRT/2$(f is degree of freedom here).
My sir told me that  $nfRT/2$ = Translational kinetic energy + Rotational kinetic energy.
So I used the formula  $nfRT/2$= ${\frac{1mv^2}{2}}$ + Rotational kinetic energy.
In the question , Rotational kinetic energy is not given.Rotational kinetic energy will be 0 here since He is has 3 degrees of freedom.
I get is Kinetic energy = $1/2 * 20/1000 * 10^2 =1
1 = $n3T/2$
Should I take n =1?Because it not given.
Have I solved it correctly and is there any other way to solve it.

Try to use more specific titles, as there are many questions about thermodynamics here. By 20 gm you probably mean g as gram.Include your complete computation procedure, and preferably use MathJax  for formula formatting. Note that the goal of the site is not a task proof reading nor solving, but helping to understand principles. It also prefers elaborated question. Do you have troubles to understand or apply a particular principle ?

Always think  thoroughly about possible answers to your question and search for them
before asking, together with providing of your intermediate  results of the failure.

Yes.one is that whether n will =1? Second , what I have written , is it correct? Is there any other to approach this problem.

@Poutnik This is not there online.Made by sir itself.

@Poutnik It took me to some researchers site which had many things I didn’t even know or needed to study now.

Improvement of the question belongs to the question, not to comments. The principles to solve are on internet. How long have you been thinking about it and searching for ways to solve it ?

Almost an hour.

Let us continue this discussion in chat.

The total mechanical energy (other kinds of energy are ignored here ) of moving gas is the sum of potential energy and translational, rotational and vibrational kinetic energy :
$$E = E_\mathrm{pot} + E_\mathrm{trans} + E_\mathrm{vibr} + E_\mathrm{rot}$$
These energies can be separated as the energy of the bulk volume of a gas and the energy of gas molecules in the inertial frame where the centre of the gass mass is in the rest:
$$\small E = (E_\mathrm{pot,bulk} + E_\mathrm{pot,mol}) + (E_\mathrm{trans,bulk} + E_\mathrm{trans,mol}) \\ + (E_\mathrm{vibr,bulk} + E_\mathrm{vibr,mol}) + (E_\mathrm{rot,bulk}    + E_\mathrm{rot,mol})$$
We can for our case consider $E_\mathrm{pot,bulk} = 0$, $E_\mathrm{vibr,bulk} = 0$, $E_\mathrm{rot,bulk} = 0$, $E_\mathrm{pot,mol} = 0$, assuming zero bulk gas and molecular potential energy and no gas vibration nor rotation.
$$E = E_\mathrm{trans,bulk} + E_\mathrm{trans,mol} +   E_\mathrm{vibr,mol} +  E_\mathrm{rot,mol}$$
For the translational kinetic energy of the bulk gas is the notorious formula:
$$E_\mathrm{trans,bulk} = \frac 12 \cdot m \cdot v^2$$
We can with just a little error consider helium as an ideal gas. For 1 mole of an ideal gas, each degree of freedom of motion has energy $E = \frac 12RT$, where $R \approx \pu{8.314 JK^-1mol^-1}$ is the universal gas constant and $T$ is absolute temperature. Their kinetic energy is then:
$$E_\mathrm{kin,mol}=\frac{f_\mathrm{trans} + f_\mathrm{vibr} + f_\mathrm{rot}}2RT$$
Atoms of monoatomic gasses like helium have 3 degrees of freedom $f$ for translational motion, but do not rotate nor vibrate, so $f_\mathrm{vibr} = f_\mathrm{rot} = 0$
Therefore, the kinetic energy of helium  molecules of the total mass $m$ and molar mass $M$ , forming helium thermal energy is:
$$E_\mathrm{therm}=\frac {3mRT}{2M}$$
So we have finally the energy of the gaseous helium as:
$$E = \frac 12 \cdot m \cdot v^2 + \frac{3m}{2M}RT = \frac m2 \cdot ( v^2 + \frac{3RT}{M})$$
As the helium mass is constant and as we honour the law of energy conservation, the following equations must be true ( with indexes 1,2 as initial and final states:
$$v_1^2 + \frac{3RT_1}{M} =   v_2^2 + \frac{3RT_2}{M}$$
$$ \frac{3R}{M}(T_2 - T_1) =   v_1^2 - v_2^2$$
As the final speed $v_2 = 0$, we can write:
$$ \Delta T =   \frac {M \cdot v_1^2}{3R} $$
The helium molar mass $M \approx \pu{0.0040026 kg mol^-1} \approx \pu{0.004 kg mol^-1}$
$$ \Delta T =   \frac {M \cdot v_1^2}{3R} = \frac {(\pu{0.004 kg mol-1} ){(\pu{10 ms^-1})}^2}{\pu{3 \cdot 8.314 JK^-1mol^-1}} \approx \pu{+0.016 K}$$

For formatting of chem/math formulas/expressions, use can use MathJax, also look here
and here. See also MathJax on Math SE.


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Conversion process of lanolin to 7-Dehydrocholestrol
What is the Chemical process to convert lanolin to 7-Dehydrocholestrol?
I don't know about the chemcial reaction, if I get the process I can try in lab scale.

Lanolin is a very complex mixture. I think you main problem is likely to be isolating the precursor lanolin alcohol is a decent level of purity.

Hydrolysis of lanolin to lanolin alcohols  https://patents.google.com/patent/US2824143A/en

The conversion of lanolin to 7-dehydrocholestrol is an important procedure in the production of vitamin D.
Lanolin is converted to cholesterol by a method which comprises of following steps:

dissociating cholesterol from lanolin by transesterification
reaction;

performing short-path distillation to obtain cholesterol crude
product

purifying by silica gel column chromatography

crystallizing to obtain refined cholesterol with purity higher than
95% and yield higher than 70%.

The extracted cholesterol is converted to 7-dehydrocholesterol and then treated with UV light to produce vitamin D3. The method has such advantages as high product consumption, little pollution, simple process and easy operation, thus is suitable for mass production.
References

https://patents.google.com/patent/CN1958596A/en
https://patents.google.com/patent/CN1263769C/en
https://www.sigmaaldrich.com/technical-documents/articles/biofiles/nutrition-research.html


text:
are refractometers generic?
I need to check the presence (and also purity approx) of a substance in solution. I have a very good reference (10mg/1ml - the substance in water).
Can I use a refractometer to do this?
I am trying to separate the substance from several other ingredients using ether initially then water. At that point, I'd like to see what I have. Is my target in the water? If so can I ballpark the purity?
I was hoping that I could use a refractometer because I have such an excellent reference.
Can I use a standard handheld refractometer from say amazon with say a Brix scale? Or am I way off?
Obviously, there isn't a handheld refractometer I can buy that measures my specific product.
Are refractometers essentially generic? Can I get one with a standard scale and use that to measure the reference and then compare with my extract/filtrate?

Any solute in water will change the refractive index. If you have a single substance dissolved in water, you can use the refractive index to estimate the concentration. If it is a mixture of unknown ratio of solutes, you need a different method. Also, a handheld refractometer will be fine for high concentrations (sugar in wine, etc), but not for diluted solutions

You can derive at most one parameter from one measured variable, not two.

Short answer is NO. Many are sold for specific application - e.g. determining urine SG (medical diagnostics) or sucrose concentration in liquids (food & beverage industry). They have limited range, one or two targeted algorythms and displays result/values as e.g. SG or deg Brix. You probably want something more versatile, so read some good articles on how to choose a refractometer.

If you know the theory, have a good eye (and steady hand if it's hand-held) and can do the math, get an old-fashioned Abbe type (analog) and you can use it for pretty much anything.

Sounds like chromatography might be helpful. It can be simple (paper), using water or alcohol (not ether - too volatile) to elute), or more elegant: thin film, or even gas chromatography. Develop with iodine in a jar, or maybe you can see a spot with some other reagent.


text:
The Use of AI and Machine Learning in Organic Chemistry
I am trying to do a bit of research into the current use of machine learning in chemical industry. I've been told my original question was too general so I'll try to be more specific about what I'm interested in.
I've read several articles about the computational techniques used for retrosynthesis, where a desired molecule is set as an input and the output being a series of feasible chemical reactions beginning with easily sourced/purchasable materials.
I'd like to know if/where techniques like this are being used in industry with any frequency or is it something still in the early stages of development and sees little to no practical application.

It is an open ended question, please make it more specific. One application, which has interested me a lot is the ability of machine translation of scientific chemistry texts. Chemical structures, mathematical equations create a lot of problems. A lot of useful and historical information of organic chemistry is in German and French. Search the Journal of Chemical Information and Modelling for such topics.

I have heard that retrosynthesis was also developed for the computers to suggest synthetic routes.

@M.Farooq Learn German, or French? You need but a basic understanding of the languages before you can read chemical texts just like that. Poetry and politics are a lot harder. ;)

@Karl, I was talking about machine translation of scientific texts. Both Google Translate and DeepL do a great job provided you follow some good practices and perhaps some "rules. I have published two survey articles on that topic. Since you already speak German, you may not worry about it :-)

@M.Farooq I have found I can also read Spanish OC chemistry (learned French at school?), and Swedish (finished half an A1 course). It's just extremely simple. ;-) Compared to the newspaper, where I get what it's all about, but would fail miserably in a basic reading comprehension test.

The question is1)  too broad, 2) essentially asking for references (which is not the goal of this site). For this question an answer about using google for research is a correct answer. If you formulate the question better, you get better answers. One way is getting more specific about what have you found so far, and where you are missing there. Have you read any reviews? How old results are you interested in (see chemoinformatics as a field)? Also, please note that AI is often just a marketing term, used vaguely.

Well, this seems to be the question you are asking: What is the current status of machine learning applied to materials or molecular systems?

In addition to «it is an open end question», (retro)synthetical planning by a computer is a field with long tradition (e.g., LHASA around the Corey group, see for example Derek Lowe's pipeline, https://blogs.sciencemag.org/pipeline/archives/2018/03/06/retrosynthesis-here-it-comes). For more recent examples, see the 2020 RDKit UGM, esp. https://www.youtube.com/watch?v=UyHVwJUBDIk&list=PLugOo5eIVY3HW8kJYnoBNyJvg8P6qtl44&index=13, and https://www.youtube.com/watch?v=4ntPCu-rp_k&list=PLugOo5eIVY3HW8kJYnoBNyJvg8P6qtl44&index=21).

I’m voting to close this question because a) there is too little evidence the OP took a moment to browse briefly the pages of publishers (example ACS).  And b), in its current state, the question appears to attract answers which are self-promotion (twice an answer pointing to a book by James Bonnar hosted by an unknown source).


text:
Driving force of Galvanic cell
Work can be extracted from galvanic cell which has a negative Gibb's free energy. I was wondering what is the driving force of it: I would naturally think its the Gibb's free energy, but the gaps between two fermi levels of the two electrodes also seem to pay a role in the potential difference. Could someone correct me or maybe tell me the connection between these two concepts?


text:
If stirring negatively affects crystal purity, why is it so?
My Intellectual Expectations
Intuitively, I would expect strong stirring to improve crystal purity during the process of crystalization. My rationale is that strong stirring would increase contact between each crystal and the variety of ions present in solution at each given level of solubility, and thus minimize the gap between reality and the equilibrium that would be reached if the conditions at each instant (solvent volume and temperature) would be kept equal for a very long time.
Information I have Read
I've read that as a general rule, the size of the crystals that form correlates well with the purity inside the crystals (i.e. that crystal
size in general is a good indicator of purity).
My Conflicting Observations
My observations are that under strong stirring the crystals that form are much smaller than if the crystalization is carried out in still water (at equal rate of cooling / solvent evaporation). This indicates that either crystal size is not a great indicator of purity, or that my rationale is wrong and by counterintuitive mechanisms which I ignore, crystals of greater purity would be formed without stirring.
My Question
Does anyone know 1) Wether stirring the solution during the crystalization  process positively or negatively affects crystal purity? 2) What is the physical explanation of the answer?

Comments are not for extended discussion; this conversation has been moved to chat.

Perhaps the solution relates to the impact of the speed of stirring on a solution's cooling rate. Basis: see this article in Journal of Physical Chemistry Chemical Physics here https://pubs.rsc.org/en/content/articlelanding/2019/cp/c8cp06650g#!divAbstract .

"Strong stirring" is a rather imprecise term.  There is such a thing as too much speed.  Rule of thumb for APIs, if you're working with a magnetic stirrer in a round bottomed flask in a lab (rather than in some industrial vessel or pilot plant setup) 350 rpm is too fast (and no, I can't quote a reference on that, it's one of those things you get told and then confirm by experiece - usually by being a wise-arse and bucking the "established wisdom" and ending up with a product that doesn't make medical grade).
Agitation for different crystallisation processes are generally kept in certain narrow bands to avoid inclusions as wel as a number of other reasons.
An example is described in Chow et.al [1] where when crystallizing paracetamol from water in a 500 mL round-bottom flask with p-acetoxyacetanilide present, increasing the stirring speed from 200 to 320 rpm increased product purity, but increasing further to 400 rpm lead to decline in product purity.
According to Meedara et.al. [2] stirrer-particle and interparticle collisions can facilitate crystal attrition  in suspensions with high energy agitation, leading to attrition-induced inclusions.
Urwin et.al. [3] did a reasonably thorough job testing various methods and reasons for presence of impurities in crystals, and is worth a read.
Another good reference is The Handbook of Continuous Crystallization, start with chapter 1 [4]
That said, in crystallisation especially of APIs, mechanisms can be compex and are still not completely understood. All kinds of workarounds have been developed, such as e.g. ultrasonics to help prevent agglomeration when crystallizing paracetamol from certain solutions.
If you have no literature on the process you are attempting, finding the "sweet spot" where you get the greatest purity of product, is only going to happen with a lot of experimentation.
[1] Chow, A. H. L.; Chow, P. K. K.; Zhongshan, W.; Grant, D. J. W. Modification of Acetaminophen Crystals: Influence of Growth in Aqueous Solutions Containing P-Acetoxyacetanilide on Crystal Properties Int. J. Pharm. 1985, 24 (2–3), 239– 258,  DOI: 10.1016/0378-5173(85)90024-9
[2] Meadhra, R.; Kramer, H. J. M.; Van Rosmalen, G. M. Model for Secondary Nucleation in a Suspension Crystallizer AIChE J. 1996, 42 (4), 973– 982,  DOI: 10.1002/aic.690420410
[3] Urwin, SJ, Levilain, G, Marziano, I, Merritt, JM, Houson, I and Ter Horst, JP
A Structured Approach To Cope with Impurities during Industrial Crystallization Development Organic Process Research & Development 2020 24 (8), 1443-1456,  DOI: 10.1021/acs.oprd.0c00166
[4] McGinty, J, Yazdanpanah, N, Price, C, ter Horst, JH and Sefcik, J  Nucleation and Crystal Growth in Continuous Crystallization , in The Handbook of Continuous Crystallization, 2020, pp. 1-50 DOI: 10.1039/9781788013581-00001

Thanks a lot for this answer Gwyn! I think that the truth may be in that direction. I think that to properly answer the question we need to identify all the major factors that affect crystal purity, and that some factors that favour purity are increased with stirring (e.g. homogeneity of the solution), as are as well some factors that disfavour purity (e.g. crystal collision & inclusions, as you mention), such that crystal purity is maximized at the agitation level at which the net influence of all factors is maximized (the intermediate level which you mention is observed).

Stirring during crystallization is positive for the purity of the crystals.

The explanation is that with stirring, your liquid phase will be homogeneous.

Then, there are some other details.
Many times, especially in research labs, the target of crystallizing is to get a crystal for XR.
What you need then is a crystal big enough to put it into the instrument.
Stirring accelerates the crystallization and nucleation rate, so you get smaller crystals.
That means, if you want a big crystal, don't stir and wait.
But if you are producing some chemical and you have a constraint on purity, and on batch time, stir as efficiently as you can.
Then, if the crystal size is too small, the filtration time will be longer so, do some optimization.
If you don't stir, the crystals grow slower.
The liquid phase can be very supersaturated without nucleation.
But, at the same time, your liquid phase is not homogeneous, so the surface of the crystal is not in equilibrium with the bulk of the liquid.
At that point, the less soluble compound, the one that is crystallizing, might not be locally supersaturated and some other impurity can start crystallizing on the growing crystal, yielding a final product of bigger crystals with lower purity.

It would be good if you explained from where you are generalising here.

I would say that in general it goes the other way around. Unless one is trying to precipitate / crystallize by-products or traces, which is rather non sense.

Thanks for your answer Raoul! I think, as Gwyn identifies, that more stirring is not necessarily always better, but that in general some level of stirring is necessary to maximize purity (because, at least in part, as you write, it increases the homogeneity of the solution).


text:
what is meaning that "reductive trapping" in organic chemstry?
Reductive trapping of intermediates, producing alkylated pyrroles, can be synthetically useful.

can you give more context? I suspect it means reducing an imine to an amine, which is no longer susceptible to hydrolysis, but there isn't enough info to be sure. In that case, you would react pyrrole with an aldehyde and then use mild reducing conditions to reduce the resulting imine without reducing the pyrrole ring.

Condensations of pyrroles with aldehydes and ketones occur easily by acid catalysis, but the resulting pyrrolyl - carbinols cannot usually be isolated, for under the reaction conditions proton - catalysed loss of water produces 2 - alkylidene - pyrrolium cations that are themselves reactive electrophiles. Thus, in the case of pyrrole itself, reaction with aliphatic aldehydes in acid inevitably leads to resins, probably linear polymers. Reductive trapping of these cationic intermediates(2-alkylidene pyrrolium cations), producing alkylated pyrroles, can be synthetically
useful...

How to understand “reductive trapping” in the sentence above

Okay, it's the 2-alkylidene pyrrolium that is reduced to a 2-alkyl pyrrole rather than N-alkylidene to N-alkyl.

In the presence of acid, the 2 position of pyrrole is nucleophilic and can attack electrophilic carbonyls such as aldehydes and ketones. The resulting carbinol (structure 1 in the scheme below) readily dehydrates to the 2-alkylidene pyrrolinium (2), which is itself quite electrophilic. As a result, it can be attacked by other pyrrole molecules (as happens in the Rothemund reaction, for example). To prevent this, compound 2 is reduced as shown in the scheme to yield a much less reactive 2-alkyl pyrrole. The 2-substituted compound is thus "trapped" by the reduction to prevent its further reaction.


text:
How to differentiate hydrous and anhydrous compound?
For an experiment, my teacher gave me containers of hydrates and their corresponding anhydrous compounds, but I have a feeling that for one of the compounds she switched the hydrate and anhydrous label by accident. How can I check if this is true?

Anhydrate is a word that does not exist. Anhydrides are existing, but it is not your question.

In a way, anhydrate is a word that should have existed. Pity it doesn't.

Most anhydrous materials are fairly hygroscopic,  so put a bit of it on a sensitive scale. If it "gets heavier" sitting there it used to be anhydrous.

Which compounds do you suspect were swapped?

There will be a difference in their appearance.

There is no general answer to this question.
Heating is probably the best approach. It is not the only one. For some compounds, heating the hydrated variety produces an emission of water vapor, and nothing else, like $\ce{CuSO4·5H2O}$ or $\ce{Na2CO3·10H2O}$. For others compounds, it may produce a total decomposition like $\ce{AlCl3·6H2O}$, which is transformed into $\ce{Al(OH)3 + HCl}$ at high temperature. Considering the corresponding anhydrous compounds, $\ce{CuSO4}$ is decomposed into $\ce{CuO + SO2 + O2}$ at $650$°C. $\ce{Na2CO3}$ resists decomposition up to $1200$°C. $\ce{AlCl3}$ sublimes at $177$°C. You see that there is no general rule.
You may also check the behaviour of your hydrated compound with substances that react violently with water like thionyl chloride $\ce{SOCl2}$. if there is a reaction, it is probably a hydrate. But it may also be an anhydrous compound that react with $\ce{SOCl2}$, a metal for example, or an organic acid.
If you want a better answer, please give us the names of the particular compounds you are interested in.


text:
Why can’t lanthanum through lutetium and actinium through lawrencium all be in group 3?
In 2015, IUPAC established a task force to “deliver a recommendation in favor of the composition of group 3 of the periodic table.”  Not much about their decision-making process has been made known to the public.
It is noteworthy that the task force must select one of two compositions for group 3:

the elements Sc, Y, Lu and Lr, or
the elements Sc, Y, La and Ac.

This seems to call into question the conceptual validity of arrangements that depict La–Lu and Ac–Lr as falling under the umbrella of group 3 by leaving a gap, such as the one currently published on PubChem (as opposed to arrangements that insert the f-block between groups 3 and 4, such as the one currently on RSC.org).
If all the lanthanides, including lutetium, are so similar, and if all the actinides, including lawrencium, are so similar, why was a third option—“the elements Sc, Y, La through Lu, and Ac through Lr”—disqualified as a recommendation for the composition of group 3 by IUPAC?

Since I am neither clairvoyant nor able to travel back and forth in time, I can only guess what IUPAC was thinking.  But there is no guessing that the inner transition elements do not completely fit in with Group 3 chemically.
One major chemical difference is in the oxidation states the elements may achieve, and thus the stoichiometry of compounds with nonmetaks that have strong ionic character.  Whereas the Group 3 elements overwhelmingly form compounds in the +3 oxidation state, other oxidation states are more likely for at least some inner transition elements.  While in some other groups there is more variance linked oxidation states, this is little seen in Group 3 whereas, below, variance in oxidation states is more common with inner transition metals.  Especially note the higher oxidation states in the examples that follow, which would not be expected in a group generally defined by elements normally having only three valence electrons.
Among lanthanides, cerium achieves a +4 oxidation state commonly enough to be called "ceric" and be available as a reagent for redox reactions, as in ceric ammonium nitrate and cerium(IV) sulfate.  Cerium can also form +4 as well as +3 ion cores in the metal by either including or excluding its $4f$ electron in the metallic band structure, leading to a transition between two FCC phases [1].  At the opposite end, europium accesses the +2 oxidation state much more readily than a true Group 3 element, leaving half-filled valence shell, and thus overlaps the heavier alkaline earth metals in its chemical properties.  Wikipedia reports that

In anaerobic, and particularly geothermal conditions, the divalent form is sufficiently stable that it tends to be incorporated into minerals of calcium and the other alkaline earths.

With actinides, oxidation states departing from the +3 predominance of Group 3 are more common.  Well-known examples include thorium forming a highly refractory oxide in the +4 oxidation state and uranium achieving oxidation states as high as +6 when combined with fluorine.
Reference
1.
Johansson, B., Luo, W., Li, S. et al. Cerium; Crystal Structure and Position in The Periodic Table. Sci Rep 4, 6398 (2014). https://doi.org/10.1038/srep06398

If it just boils down to oxidation state, how is that reconciled with other groups, like group 11?

Within G3, however, +3 predominates.

But the open question globally is what is group 3, so that comment seems self-contradicting to me