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Is the Self Consistent Field an actual field?
I know this is really stupid. But bear with me.
In everything I have read so far, the Self Consistent Field (SCF) is always used to describe a method or a process, namely, that of iteratively solving the Schrodinger's Equation to generate new wavefunctions, using them for a new potential, and solving until convergence.
If SCF is not an actual field, why is it named as such? And then when we describe the SCF space what are we really talking about? Does the iterative process create a series of objects that spans a field?
For SCF: https://en.wikipedia.org/wiki/Hartree%E2%80%93Fock_method or if you want a textbook for reference, there's always Szabo and Ostlund's Modern Quantum Chemistry.

It might not hurt to add a link so those who don't recognize SCF or SE might have a chance to learn something also. Possibly https://en.wikipedia.org/wiki/Hartree%E2%80%93Fock_method or maybe http://www.tlchm.bris.ac.uk/webprojects2002/grant/webcomp/scf.html or even https://www3.nd.edu/~johnson/Class01F/chap3a.pdf but you may have something you like better.

Thanks @uhoh I added the wiki, but really liked the other two links you shared.

@Karl I understand what you were trying to say, but it could have been expressed a lot more precisely without the judgement and overtones.  Please be mindful of Be Nice.

I'm tempted to ask "what is an actual field", but I'm allergic to rabbit holes.

I'm sorry, I should have clarified. I meant a mathematical field - the kind where division is defined, like the reals or complex field.

The notion of a self-consistent field comes from the orbital approximation - that a multi-electron wavefunction can be approximated as multiple one-electron wavefunctions.
At that point, the question remains - how does each of those electrons interact with the remaining $N-1$ electrons?
Well, there's an electric field from the electron density, right? Maybe you want to call this an electrostatic potential -- but Hartree considered it a spherically-symmetric electric field.
The problem, of course, is that since you don't know the final charge density, you have to make a guess and iterate until it's "self consistent."
Thus, SCF has been associated first with HF theory, and other related methods, including Kohn-Sham DFT.

so there's a strictly physical basis for the name and concept, not a mathematical one?

I'd have to go check Hartree's papers, but AFAIK, the "field" part is a physical one -- referring to the electric field experienced by the electron in an orbital.

The "field" part can't really be separated from the mathematical construct of  a field, but the more physically common field - an assignment of a value to all points in a space - not the algebraic field I think you're thinking of.
Here the relevant physical field is the scalar field of charge density or its derivative, the vector field of electric field.

Ah, yes, @user213305 makes a great distinction -- the term has nothing to do with an algebraic field.


text:
Colour of polytetrafluorethylene
Why are teflon coated dishes black or dark grey in colour when teflon as such is white (and the underlying aluminum is not black either)?

Afaik what's black is the coating on the aluminium, that helps the teflon stick to it. You know, teflon is very bad at sticking, so it needs a lot of help. ;-)

I agree with Karl. To get Teflon to stick to an aluminum surface, there are a few different techniques including sand-blasting the aluminum, applying resins to the aluminum surface, chemically modifying one side of the Teflon to make it more adherent, and combinations of these. Unfortunately I can't find any info on the specific appearance or color change from these process so can't fully answer the question. But given the changes made to the 2 materials (the Teflon and the aluminum) there is no reason to expect the appearance of the final product to resemble that of either unmodified product.

As @Karl states, treatment of the Al-PTFE interface with reducing agent such as Na dissolved in NH3 to promote adhesion likely leaves some carbon particles. See https://books.google.com/books?id=8OX_tcKCH8IC&pg=PA14&lpg=PA14&dq=ptfe+in+ammonia+and+sodium&source=bl&ots=91OepZreA6&sig=l8V9ouCD4nXyQDYc1QchtCXZw50&hl=en&sa=X&ved=0ahUKEwjdrdG8sKrVAhXEOT4KHXf5BFEQ6AEISzAH#v=onepage&q=ptfe%20in%20ammonia%20and%20sodium&f=false

Note: I was employed at DuPont in the 1990's, and my understanding of Teflon(R) cookware manufacturing is based on proximity to product research and development and not from hands-on experience with PTFE formulations.
Teflon(R) polytetrafluoroethene coated materials are manufactured in several steps.  The PTFE coating is specially formulated as a waterborne dispersion containing surfactants and pigments.
Versions of Teflon(R) coatings sold in the early 1990's used a primer/topcoat system to improve adhesion of the polymer film to the substrate.  These formulated coatings are applied to the cookware and the pan is fired at high temperatures to form the PTFE film.
Coatings can be applied to both grit-blasted and smooth substrates.
The color(s) observed on PTFE coated cookware results from interactions of the pigments, polymers and surfactants during the high-temperature curing process.

I realize this is an old answer, but I recall talking to DuPont employees from back then who said the original test cookware was actually an off-white color that was judged to be unappealing to consumers because it looked dirty, so they added the black coloring in the final product.

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Trustworthiness of IC50 values
My two questions: Is replication of recording $\text{IC}_{50}$ values prior to (primary?) publication less common than performing a reaction several times to determine a more reliable record? Provided the very small quantities used in these tests (vide infra), should a synthetic chemist rather stay skeptic if a newly isolated material is described as highly potent?
In a literature seminar, the story/history of maoecrystal V, a highly congested polycyclic natural product was presented:

(source first part of an overview)
Initially, both the molecular architecture as well as the the $\text{IC}_{50}$ value ($\pu{0.02 ug/mL}$ at a molar mass of $\pu{330 g/mol}$, $\pu{60 nmol}$) in comparison to cis-platin ($\pu{0.99 ug/mL}$ at a similar molar mass of $\pu{300.01 g/mol}$) against cervical HeLa cancer cells reported by Li et al. in  Org. Lett. (doi 10.1021/ol0481535) in 2004 triggered the interest of several chemists to synthesize this material.
About a year ago, the Baran group published again efforts towards this compound (doi 10.1021/jacs.6b06623, ACS AuthorChoice / open access), displaying again tremendous efforts in the lab.  With larger quantities of the material in hand, however, the initially claimed activity against cancer cells (not only against HeLa) could not be confirmed.
The supporting information of the last paper sheds some light on the frugal scale the $\text{IC}_{50}$ values were recorded:

Synthetic (–)-maoecrystal V (1) was dissolved in 100% DMSO at a     concentration of 1 mM. [...]  A day after cell seeding, 10 μL of prediluted compound was added to the cells (1:16 dilution).

Which would refer to low umol quantities of the material deployed per test.  Still, the question arises why the initially reported activity differs so greatly from the one eventually determined.  (Speculation: perhaps fewer groups would have joined the challenge to prepare the material if interested merely in its architecture.)
Addition
For the interaction of Maoecrystal V and other, already known, agents for example facing HeLa cells, page S52 of the SI of the 2016 publication shows this plot:

Note that Actinomycin D is considerably heavier ($\pu{1255 g/mol}$) than Maoecyrstal V ($\pu{330 g/mol}$).

I wish I had time to answer this right now, but generally in natural products chemistry they're awfully unreliable- there just isn't enough material to do the job properly with multiple runs for verification.  All too often two groups isolate/make the same thing and report wildly different values, not to mention the huge variety of different assays that groups choose for various reasons which can't be compared anyway.  Industry/medchem values, on the other hand are ran many many times over many weeks and generally hold true.

While I have no experience in recording IC50's, and do not know what level of dexterity is necessary to handle  amounts safely, precisely, and accurately (perhaps partially assisted by a robot?) at this small scale, the difference between the curve representing the interaction of Maoecrystal V (e.g. above, in blue; just added) or other agents against HeLa cells is more than expected. However again, this is my perception, as  someone who is not in the field; and not aware (beside the note in the 2016 paper) of other instances that an initial determination of IC50 was rectified as here.

I have worked in natural product isolation a little. Although I have never witnessed tests with HeLa cells, I can provide some insight into the general screening procedure.
In the lab I worked in which dealt with isolation of myxobacterial natural products, a novel strain was first tested as a crude extract in a preliminary screening via serial dilution assay. Tests were performed against a set of standard screening organisms including S. aureus, Candida albicans, E. coli and a few others I forgot.
If an isolated showed enough inhibitory activity at this stage, a semipreparative HPLC was performed to locate the peak(s) which caused said activity. The HPLC fractions were collected in a 96-well plate, the solvent removed by nitrogen flow and then the well plate was infected with the organism of interest. Hopefully, one or two wells would remain with out bacterial or fungal growth and hopefully these would not be in the fatty acid realm at the end of the 96 wells.
After that, a slightly larger separation was performed with the goal of gaining a few milligrams of the active compound (after confirming via HR-MS and dereplication that it was not already known) to perform another serial dilution assay to identify the minimal inhibitory concentration (MIC) — the isolationists’ equivalent to an $\mathrm{LD_{50}}$. This was typically performed around three times.
After that, the paper was drafted and submitted to peer review.
(Okay, there are a few steps I left out, but they contain other, unrelated stuff like making sure the myxobacterium was properly classified, elucidating the structure, hoping for crystals and maybe even performing some low-scale fermentation of the relevant myxobacteria to gain enough sample for proper structure elucidation. Ths last step could also preceed the MIC value determination.)

Interesting! I never realised your group worked in isolation as well as synthesis (jealous...).    I would caution your mixing of MIC and LD50 though- both are commonly measured during isolation with reference to different kinds of assays (MIC for things like inhibition of fungal growth, LD/IC50 with reference to the actual killing of cells such as those involved in cancer.  Basically inhibition of replication vs cytotoxicity).

@NotEvans. Yeah, afaik $\mathrm{LD_{50}}$ values were never measured only ever MIC’s (also not $\mathrm{IC_{50}}$, just MIC’s). And it wasn’t exactly my group although we did previously have two people working on isolation. It was the neighbouring group in the same research institute and because of the graduate school basically everyone went there at least once. They also never touched human cells afaik, only ever bacteria and fungi for testing.

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What can be a metal cleanser for aluminum?
I want to remove corrosion products in aluminum, but I don't have supply of nitric acid, as suggested by an ASTM method. Which can be a good substitute?

Really, ASTM procedures must be followed as written for certified testing. However, if this is for your edification or for a school project, you might use a commercial cleaner, such as those below, or just one ingredient such as phosphoric acid.
There are a number of commercial aluminum corrosion cleaners, such as Loctite Duro Aluminum Jelly and CRC MaryKate Aluminex Clean and Shine.
If you want to make your own:
Ingredients listed in the Aluminum Jelly Material Safety Data Sheet include phosphoric acid, diethyleneglycol monoethyl ether, nonionic surfactant, isopropyl alcohol , polysaccharide and sulfuric acid. [Similar formulaions may have contained hydrofluoric acid to break the highly-adherent $\ce{Al2O3}$ surface film and corrosion, but due to the hazards of $\ce{HF}$, I believe it is no longer sold in most places, for good reason.]
Aluminex Clean and Shine's MSDS includes ethoxylated tallow alkyl amines, sodium decylglucosides, hydroxypropyl phosphate and potassium iodide.

Can't I use hydrochloric acid? Most of these chemicals are not available and hardwares in my place don't sell this kind of products.

You can use anything you want -- but that is not ASTM testing.


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How to reconcile conflict between assumed oxidation state and the most stable multiplicity from electronic structure calculations?
Consider either of the small, neutral molecules containing two Zn atoms below. If I were to determine the oxidation state of Zn from this molecule, I would formally assume each Zn to be Zn(I) in order to balance the overall charge (if I give 2- for each O, 3+ for the Al, and 1+ for the H). I also assume that each Zn(I) has 1 unpaired electron as a result of this oxidation state. With this, I would assume that the molecule may have a triplet ground-state (putting aside the possibility of any broken-symmetry behavior for now).
The left and right images are the result of density functional theory geometry optimizations where I set the multiplicity to be 1 and 3, respectively. I find the (closed-shell) singlet structure is lower in energy than the triplet by 28.7 kcal/mol, in contrast with the prior discussion.
How can I reconcile these two conflicting notions? If I were to study reactions involving this molecule, do I assume the structure is a triplet because that's what the 1+ oxidation state on the Zn atoms would likely yield? Alternatively, do I assume that the structure is a singlet because that is what I calculated, even if that does not seem to agree with the oxidation state assumption?
If the appropriate approach is the former, then perhaps the oxidation states of each atom is not what one would formally assign. If the appropriate approach is the latter, then perhaps since Zn does not tend to be in the 1+ oxidation state, there is some unusual change in the electron configuration to stabilize this structure.
Computational details: this is computed using the M06-L functional, I have checked the stability of the wavefunctions, there is minimal spin contamination, both structures are local minima, and the trend holds true for other model chemistries.
Singlet state:   Triplet state:

You don't see that both molecules have exactly same oxidation states?

@Mithoron I assume you mean both transition metals, not both molecules? I manually set both molecules to have 0 charge (of course, molecules don't have oxidation states). With regards to the Zn atoms, the oxidation state isn't something you can directly obtain from DFT. I don't necessarily know what you're asking. Typically one would likely associate di-Zn(I) with the triplet state and di-Zn(II) with the singlet state, but this is admittedly a simplistic approach.

I'm really not getting it. If there is a bond between Zn atoms or not, they have both +1 (if it's neutral molecule) so what problem with ox. state you have?

The oxidation state is not an input parameter for electronic structure programs. The overall charge and spin multiplicity are. I have set the charge to be neutral and the multiplicity to be 1 and 3. I am implicitly assuming that each Zn is Zn(I) in order for the net charge to be zero. Typically I would associate a triplet with a di-Zn(I) complex since each Zn(I) would formally be $[Ar] 4s^{1}3d^{10}$. It is clear from my calculations that, instead, a (closed-shell) singlet has lower energy, and it is not clear to me how to reconcile the  Zn(I) oxidation state with a singlet spin state.

Well, that's the molecule with Zn-Zn bond - singlet, both Zn have +1 state, everythings OK!

There seems to be some miscommunication, perhaps because you are used to working the other direction if you're not a computational chemist. I already know the singlet molecule with the Zn-Zn bond is most stable (and I assume that each Zn is Zn(I) since I've set the charge to 0). I am trying to rationalize how this could be and why it's more energetically favorable to have a singlet when I have two Zn(I) atoms that would normally give a triplet. Just because I've found the singlet to be lower in energy does not resolve the contradiction.

As an aside, could you explain what you mean by "putting aside the possibility of any broken-symmetry behavior.." OR basically, what does broken-symmetry behavior mean, within electronic structure theory? Any references etc. will help too!

@seavoyage If you consider an isolated Zn atom in the 1+ oxidation state, its electron configuration is [Ar] $3d^{10} 4s^{1}$. In my structure, I have two Zn(I) atoms. I would anticipate then that there is 1 unpaired electron on each Zn(I) atom. The spin-multiplicity could then be a triplet. However, it could also be a broken-symmetry (BS) singlet. The "broken-symmetry" part means that the unpaired electron on each Zn(I) atom has an opposite spin (i.e. alpha on one and beta on the other). This then leads to a molecule with $s=0$ (all paired). This is a BS singlet.

@seavoyage As for references, here are ways to implement it in Gaussian and in ORCA, with some description for why you would consider it. The literature abounds with examples of its use as well if you want a more formal description. Essentially, you are "breaking" the spin-symmetry by introducing some antiferromagnetic coupling such that the unpaired electron(s) on one atom are antiparallel with that on another.

First reflex: As long as the calculations are otherwise reliable, I don't think you can ignore the possibility of a broken-symmetry/multiconfigurational solution. If such a solution ends up even lower in energy than your closed-shell singlet, I think you have your answer right there.
Alternatively, if the closed-shell singlet ends up exhibiting a lower energy than a BS solution, perhaps this is some sort of cationic Zn analogue of a peroxo species? If this were the case, then the $+1$ oxidation state wouldn't be nearly so surprising.  As you note, the non-$\ce{Zn}$ portion of the system would be $\ce{[AlO3H]^{2-}}$, and thus the species as a whole might be better thought of as $\ce{[Zn2][AlO3H]}$. I have no feel for how realistic this hypothesis is, though Wikipedia notes that organozinc(I) compounds have been prepared, with decamethyldizincocene being the first such compound reported (public domain image; click to enlarge):

Thanks for your input. I will try running the BS singlet and see. I have occasionally seen this "odd" behavior when I consider other transition metals aside from Zn. For instance, I'd expect a di-Co(I) structure to be a quintet, but I find the septet is lower. For di-Fe(I), I'd expect a septet but I get a nonet.  For all three of these examples, I have not yet considered a BS solution (but intend to). I will try it for the di-Zn(I) case now and will report back here for others to reference. Thank you for your point about the "peroxo"-like behavior. I'll look into that as well.

@Argon Beware that DFT is notoriously unreliable for metal spin states, especially with transition metals.  For rigorous work, AFAIK one really can't avoid running multireference calculations (best augmented with perturbation theory or MRCI) to confirm the DFT conclusions. For a main group system this small (I would think Zn would behave as main-group here), I'd think such calculations should be feasible even on modest hardware.

Yeah, I've kept that in mind as well. It's part of the reason why I wondered if I should go with the anticipated multiplicity even if it is not the lowest energy structure obtained from DFT in case DFT is not correctly capturing the energetics. Many computational papers I've seen on complex bumetallic complexes simply state they model the assumed high-spin multiplicity associated with the $d^n$ count without clear justification that other spin states are higher in energy. I don't feel extremely comfortable with that though. I may need to consider something beyond DFT I suppose.

Also, for what it's worth, I have not yet been able to obtain a BS singlet solution using Gaussian. I read in the orbitals of the triplet state and mix the orbitals for my initial guess. This creates a BS singlet, but as the geometry optimization progresses (and it progresses toward the closed-shell singlet structure), it inevitably switches off the BS singlet potential energy surface and onto the closed-shell singlet surface. I haven't had luck with defining "fragments" to obtain the BS singlet either. We'll see if I can get it another way to compare the energies.

UPDATE: @hBy2Py's second answer was it, at least according to a natural bond orbital analysis. Excellent suggestion!

@Argon Neat! Glad to not have been completely off-base. :-)  And yeah, in my (limited) experience with BS calculations, if there's a BS state that's unambiguously lower in energy than the closed-shell singlet, the convergence algorithms usually solve to it quite quickly.


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How to rationalise the increasing bond length order in the carbonate ion, carbon monoxide, and carbon dioxide?
I am unable to rationalise the order of increasing bond length in $\ce{CO3^2-}$, $\ce{CO}$ and $\ce{CO2}$.
Having gone through the factors affecting bond length in two different books, my approach to the problem was:

$\ce{CO}$ has bond order 3 while carbonate and $\ce{CO2}$ have bond order 4 so
$\ce{CO}$ should have longest bond length.
$\ce{CO2}$ is sp hybridized while $\ce{CO3^2-}$ is sp2 hybridized.
Since carbonate has lesser s character and therefore it should have a greater
bond length than $\ce{CO2}$

Thus,
  $$\ce{CO2  <  CO3^2- < CO}$$
However, I am unsure about my method. What would be the best approach to determine the relative bond length order?

Actually, in regard to bond length, you should be looking at the bond order of a single bond within the molecule. The bond order is 2, not 4, for $\ce{CO2}$, and the bond order for the carbonate ion is somewhere between 1 and 2 due to resonance. Thus, the carbonate ion has the longest bond length, followed by carbon dioxide, and finally carbon monoxide.


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Color difference among oxovanadium species
Why dioxovanadium $\ce{[VO2]+}$ is pale yellow, whereas orthovanadate $\ce{[VO4]^3-}$ is colorless?
Both of them should be colorless if we ignore charge transfer for a moment, because in both cases vanadium is at +5 oxidation state.
Now considering the charge transfer is the reason for the pale yellow color of dioxovanadium species, why orthovanadate does not show charge transfer band?

The colour will depend on several factors. You've identified the oxidation state (+5 in both cases) as one.
Other factors are:

Ligands
Complex shape

With $\ce{[VO2]}^{+}$, the actual complex ion is $\ce{[VO2(H2O)4]}^{+}$ (Taken from ChemGuide). So our ligand is water, and the shape is tetrahedral (or square-planar).
The $\ce{[VO4]^{3-}}$ ion usually only exists at high pH in solution but polymerises at lower pHs. These polymeric ions can have colour (e.g. $\ce{[V10O28]^{6-}}$ is orange).(Link for colour / polymerisation)
However, I could find no details as to the ligands (I would assume that they would be hydroxide ions at high pH) nor shape.
If we are talking about the solid, then the same applies... does the solid have any water of crystallisation. (Think copper sulfate... blue when "wet" and white when "dry").


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How to interpret the shape in this representation of an electride?
This excellent answer describes the appearance of solvated electrons in ammonia as first a blue, and then metallic color. It is produced by dissolving sodium in ammonia at low temperature. According to Wikipedia this is called an electride:

An electride is an ionic compound in which an electron is the anion. Solutions of alkali metals in ammonia are electride salts. In the case of sodium, these blue solutions consist of $\ce{[Na(NH3)6]+}$ and solvated electrons:
$$\ce{Na + 6 NH3 → [Na(NH3)6]+e−}$$
The cation $\ce{[Na(NH3)6]+}$ is an octahedral coordination complex.

Question: How to interpret the shape in this representation of an electride?
Included in the Wikipedia article is the image shown below. Is it fairly self-evident to chemists what this image represents? If so could someone help me understand it? I see little things which might represent the location of solvated electrons, and a larger continuous/contiguous? surface which might represent all of the ions and molecules and the fact that any moment most of them are in some degree of bonding with each other, but that's just my guess. What defines the surface, some sort of threshold on electron density?
In this case the original source might be hard to track down, so an "educated guess" by someone familliar with this kind of representation would be welcome.
I'd like to read more about this kind of 3D representation of an ionic solution, so an introductory source would be appreciated.

below: "Cavities and channels in an electride", from here.

Yes, it is fairly self-evident: this is a 3D isosurface of God knows what, maybe electron density. Little things look like artifacts.

@IvanNeretin since this particular solution can have such a high solvated electron density that it actually behaves optically as a metal, I'm thinking that in this case the solvated electrons would have to show up as isolated, localized regions of high electron density themselves. But they do look a bit too scrappy for that.

If the solvated electrons were isolated, then the whole thing would behave like an insulator and not like a metal in any way.

@IvanNeretin I only said ".. show up as isolated.." in an electron density plot. It's a matter of the arbitrarily chosen density threshold chosen for a given plot. I'm not sure what isolated means in your statement, but I'm just talking about the appearance of the surfaces shown in the plot, nothing more profound than that. I now have more papers to read about solvated electrons, so I will be giving it more thought and study to understand what it really means, but I believe these are truly if transiently unbound electrons, free to respond to external electric fields at optical frequency, no?

True, it is  a matter of arbitrary threshold, and it can be that the electrons are not really isolated.

@IvanNeretin et al. I've just asked the follow-up question How to think of solvated electrons?

@IvanNeretin coming back to this after having spent time drawing surfaces in other applications, I realize you're probably absolutely right. If you'd like to capture your comments into an answer I would be very happy to accept it. Thanks!

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text:
What is the reason for colour in 2,4-Dinitrophenylhydrazine derivatives?
2,4-Dinitrophenylhydrazine is an important laboratory reagent for the detection of Carbonyl (C=O) group. It reacts with Carbonyl group by the typical Nucleophilic Addition-Elimination reaction and if positive the test results in the formation of bright yellow orange coloured derivatives.

I was reading from a source and it says the reason for showing color is -"Extended conjugation". Okay that may be the reason but what exactly happens that they look such beautiful bright yellow, orange solids ?

It would be useful if you could explicitly say at which level you want an explanation. It can be quickly done as "it depends on the electronic energy levels of the molecule and the allowed transition between them" or it can require much more effort. I would assume that someone doing such a test can more or less answer himself, or not pose this question at all.

Do not forget the more important value of 2,4-dinitrophenyl hydrazones (or of the 3,5-dinitrobenzoates, [for alcohols]; the benzenesulfonamides and picrates [for amines], just to mention a few examples of preparing derivatives) is / was lesser about a little varying colour and determination of the absorption maximum (which may require a spectrometer). It is / was to obtain a crystalline compound of sharp melting point, which subsequently may be compared with reference tables. On the other hand, instead of preparing several derivatives of X, modern spectroscopic techniques  have own benefits.


text:
How to think of solvated electrons?
This excellent answer explains at length what's happening in this fascinating video entitled Liquid Electrons - Periodic Table of Videos. In the screenshot below, the metallic-looking solvated electron layer (middle) has a bronze color.
I am trying to understand what "solvated electron" even means. Is the way that these electrons move around in solution more like a solvated ion, or proton in an acidic aqueous solution, or more like the conduction electrons in a typical liquid metal we might see in a laboratory, such as mercury or gallium? Or are both of those completely inadequate analogies?
I chose proton as the lightest thing I could think of that could be roughly thought of as a "bare charge". It is, of course, the opposite sign, so maybe a lithium ion?

Related: https://chemistry.stackexchange.com/questions/73010/would-a-quarternary-ammonium-electride-conduct-electricity/73018#73018

@IvanNeretin Another solid electride question: Is it possible create crystalline solvate of electrons? and a comment linking to this interesting pdf. Here I'm just asking about the liquid. Do you think your answer applies? I have no problem making this as a duplicate if it is, but the problem is that that question asks about a "crystallised a quarternary ammonium electride".

@IvanNeretin Still I have a hunch a significant amount of your answer applies here as well.

Another related answer to a question about solvated electrons in liquid.

The analogy with a proton is actually a good one if you are careful to remember that an electron is nearly 2000 times lighter than a proton. What does that mean? It means that despite the fact that an electron is very "small", the electron is actually going to be very large because lighter particles will tend to spread out and have a much more diffuse wavefunction. For instance, if we just take a case where the environment sets up a more or less constant potential for the electron to sit in, then it becomes very clear that a proton will spend much more time towards the bottom of the well than the electron will. Of course the environment wouldn't set up the same potential for both of them because this potential is determined by interactions with the electron or proton itself.
Ok so that's a basic picture. Essentially, you have to think of the solvated electron as being a very quantum mechanical ion if you wish to think of it as an ion.
Before we can get to full solvation, however, it is useful to understand the behavior of an excess electron in simpler systems. One good example, because they're studied all the time and are usually fairly representative of what might be going in the liquid phase, is a water cluster. In Sommerfeld and Jordan's paper[1] studying the behavior of excess electrons on water clusters, they provide the following wonderful image:

This gives a great picture of what a solvated electron could look like. The point is that it can look like a lot of things. If you wish, you can think of these isosurfaces as being an "orbital" that the excess electron is free to move around in. The difference is that we draw orbitals as 90% probability densities, only these are drawn at a constant volume, and hence don't all contain the same "percentage of the electron". In fact many of these surfaces contain less than 50% of the charge density, which demonstrates these electrons are very diffuse!
Now, the arrangements where the electron is primarily inside the cluster is going to resemble a solvated electron the most. It is worth noting structures such as 20c and 20d. In these structures, the electron interacts electrostatically with the free $\ce{O-H}$ bonds. In structures such as 13a and 24b, the electron is actually bound by the dipole field of the cluster (these clusters all have large dipole moments). Because these electrons are bound by dipole interactions, they will be quite diffuse.
One way in which electrons like these are definitely not like a proton in solution is that correlation effects become very important for the behavior of these electrons. Indeed, some of the structures shown above only give a negative binding energy when correlation is included at the CI level. (See 1 for details on all this.)
Some more interesting papers on electrons bound to water clusters are 2 and [3].
So, this sets us up for thinking about fully solvated electrons (sticking with water because it's where the most work is done since water is very important). As it happens, our orital analogy applies quite nicely in liquid water. Below is the little cover art from a paper hot off the presses (2017) by Ambrosio et al.. [4]

They actually show that the solvated electron in water is bound strongly enough to have multiple electronic states (they match with experiment quite well), and they can be seen to be s-like and p-like. That is, one of them is roughly spherical while the other has a node with a smaller lobe. Again, keeping our analogy with a proton in mind, these electrons are much larger than how we think of a proton in water. Both, however, induce a local structure in the water molecules to stabilize the excess charge.
The fact this aqueous electron has at least two electronic states tells us already that it is probably bound more strongly than the electrons in the sodium/liquid ammonia solution (at least for the concentrated solution). The difference being that excess electrons in water are going to be very dilute (there's not many of them), while the sodium/ammonia ones are quite concentrated when we see them.
These solvated electrons in water are actually quite mobile, as we might expect. See for instance[5], in which it is shown that a surface-bound electron actually has almost the same energy as a fully solvated electron. That doesn't really make sense to me besides that the stuff from the water clusters might be relevant. That is, for this to be true, you would expect that the dominant interactions at the surface would be different than in the bulk. I'd guess dipole-bound at the surface and primarily dispersion and other correlation effects in the bulk.
As one last way to beat the proton analogy to death, we can maybe understand the mobility of the solvated electron by noting that protons are also very mobile. That is, if I drop $\ce{DCl}$ into a solution of $\ce{H2O}$, I will eventually end up with a bunch of $\ce{HDO}$ molecules and some $\ce{D+}$ and mostly $\ce{H+}$. This is to say that the proton which I drop in and say "that's my proton" is going to get attached to a water molecule and some other hydrogen is going to become the proton, and so on.
The big difference for the solvated electron is that electrons are indistinguishable from each other, so I can't say that this is the solvated electron and those are the valence electrons of the surrounding solvent. Rather, we have to picture the solvated electron as just being part of the wavefunction describing the electrons of all nearby waters (well really the whole solution but let's not get carried away). Maybe this is where we start talking about solvated muons...
In any case, if you use google scholar there is a ton of reading on solvated electrons, so feel free to check it out, and hopefully this helps you have a picture for whatever people might be working on.

Sorry I could only speak about water. In general an aqueous electron might behave quite differently in other solvents, but it's definitely always going to be quite quantum mechanical. The fact it can be bound by purely correlation effects tells us what kind of beast we're dealing with.

[1]: Sommerfeld, T., & Jordan, K. D. (2006). Electron binding motifs of (H2O) n-clusters. Journal of the American Chemical Society, 128(17), 5828-5833.
2: Sommerfeld, T., DeFusco, A., & Jordan, K. D. (2008). Model potential approaches for describing the interaction of excess electrons with water clusters: Incorporation of long-range correlation effects. The Journal of Physical Chemistry A, 112(44), 11021-11035.
[3]: Jordan, K. D., & Wang, F. (2003). Theory of dipole-bound anions. Annual review of physical chemistry, 54(1), 367-396.
[4]: Ambrosio, F., Miceli, G., & Pasquarello, A. (2017). Electronic Levels of Excess Electrons in Liquid Water. The Journal of Physical Chemistry Letters, 8(9), 2055-2059.
[5]: Coons, M. P., You, Z. Q., & Herbert, J. M. (2016). The hydrated electron at the surface of neat liquid water appears to be indistinguishable from the bulk species. Journal of the American Chemical Society, 138(34), 10879-10886.

+n! This is exactly the kind of answer I was looking for, but even more thorough and candid than I expected. I appreciate the quantum mechanical insight. Water is a great place to start; I'm not asking for the final word on the subject, just how to think about it and this really provides a good reference point. Now, off to the library. Thank you!

A really nice explanation, but it is so long that I don't think the most important point is as distinctly stated as it should be. // A solvated electron won't be "freely" floating around but rather attached to a solvent molecule. Because the interaction between solvents molecules are labile the electron is extremely likely to pass from one solvent molecule to another as the solvent molecules form and breakup clusters.

@MaxW Just out in Nature Communications; Electron affinity of liquid water (discussed here).


text:
Bromine monochloride equilibrium
The value of $K_p$ for the reaction
$$\ce{Br2(l) + Cl2(g) <=> 2BrCl(g)}$$
is $\pu{1 atm}$. At equilibrium in a closed container partial pressure of $\ce{BrCl}$ gas is $\pu{0.1 atm}$ and at this temperature the vapour pressure of $\ce{Br2(l)}$ is also $\pu{0.1 atm}$. Then what minimum moles of $\ce{Br2(l)}$ to be added to $\pu{1 mol}$ of $\ce{Cl2}$ initially to get the equilibrium mixture from above?
In this question should I include the vapour pressure of $\ce{Br2}$ in calculating $K_p$ or not?

Yes. How else would you know how much bromine gas is actually in the gas phase.

@Martin-マーチン but why so as the thing that is taking part in the reaction is Br2 liquid and not Br2 vapour which is the cause of vapor pressure.

Yes
As the reaction is written above, you may incorrectly conclude the bromine is in a liquid state throughout and hence is omitted from the equilbrium constants.
However as the question reminds us bromine is highly volatile and significant amounts of gas will accumulate above the liquid, hence most of the reactions will be between gas molecules of bromine and chlorine rather than gas and liquid.
The vapour pressure tells us the partial pressure of these gas.  Liquid bromine will evaporate until the sealed vessel contains bromine vapour with a partial pressure of 0.1 atm.
This means it is actually the equilibrium constant for the all three gases:
$$K_p=\frac{[\ce{BrCl}]^2}{[\ce{Br2}][\ce{Cl2}]}$$
where $[\ce{A}]$ is the partial pressure of $\ce{A}$.  As the vapour pressure of $\ce{Br2}$ is known to be 0.1 atm, that is the partial pressure of bromine you use in your equilibrium constant.
Hence the minimum amount of bromine to add to establish any equilibrium with the liquid is the point where partial pressure of bromine is 0.1 atm and all the bromine is in the gas phase.  Any more bromine added will condense out back into liquid.

I may have read this question wrong as your equilbrium constant has units suggesting $Kp=\frac{[\ce{BrCl}]^2}{[\ce{Cl2}]}$, when the mine is unitless: (pressure^2 / pressure^2) = 1.
If so, I would think there is something wrong with the the chemistry of the question being asked about.


text:
Some materials emit more photoelectrons than others - why?
I've been experimenting with some materials by changing the wavelength of the incident light on the material and detecting photoelectrons, like in the diagram below:

I might get a response that looks like this

Both materials have the same HOMO (organics) / Work Function (metals). For example, I hardly detect any photoelectrons (emitted electrons) from stainless steel, but loads from copper (both roughly 4.5eV). However, I am more interested in organics.
Why might some substances (e.g certain blends of perovskites) emit more photoelectrons than others (even if I have exceeded the energy required to lift an electron from the HOMO to the vacuum level)?
I thought it might be related to Density of States in metals, but this effect is also found in non-lattice molecules.
Could this be related to the reversibility of the reaction? I heard mentioned by a scientist (name now unknown) that molecules that easily lose or win an electron by an outersphere process (?) (which are likely to have a clean reversible CV voltagramm) exhibit a strong response in spectroscopy where an electron is given.

Three words - density of states.

I suggest that you google "photocathodes" for a lot more information. Most public university libraries have some public access which will allow you to get articles from many pay sites for free.

Do you mean organic molecules blended with perovskite? You have one perovskite, and you change the organic composition?

Some material emits more photoelectrons because of there low value of work function(minimum amount of energy to be dissipated for photoelectrons to be emitted.

Is it a simple as the reflectivity of the substance under investigation? If a photon is reflected, it cannot remove an electron. You mention stainless steel, in my experience stainless steel is very reflective and the neutral colour suggests it is reflective over a wide range of frequencies. Copper clearly is less reflective at shorter wavelengths, given its colour.

In accordance with @MrBishopMiller comment, consider the state of the surface. In preparing the specimen, is the surface ordered? Is it a single crystal face? Is it a jumble of crystals? Is it smoothed, annealed, non-crystalline? Is the surface contaminated, e.g., by oxides? Surface preparation affects photon reflectivity and electron emissivity. See the images in https://hal.science/hal-00629948/document, for example.

Based on photoelectric effect, the amount of photons emitted is dependent on the intensity of the incident radiation. All factors that would emit for photoelectrons include; Energy of light and potential difference, the nature/ chemistry of material, intensity of light, and the frequency of light. Ive learned that increasing the light frequency increases the Kinetic Energy of photoelectrons and increasing of the light amplitude will increase the current.

When EM radiation encounters a barrier, it can be reflected/scattered, transmitted, or absorbed. Only absorbed photons can have their energy converted to photoelectrons. But even if the radiation is absorbed then its energy could be converted to many forms that don't involve the emission of a photoelectron.
The photon's energy could be converted to heat, increasing the kinetic energy of particles within the material without changing their mean position or the energy could alter the position of particles with no emission, altering the substance's microstructure. In some cases, chemical bonds can be altered or new photons emitted at a different wavelength. Finally, photons that penetrate through the outermost layers of the material might create photoelectrons that could interact with those layers before escaping.
Chemical properties and physical microstructure at and near the surface would have an impact on all those things.


text:
Which corrodes faster at 5% NaCl solution at room temperature: Steel or Aluminum?
Which corrodes faster at 5% NaCl solution at room temperature: Steel or Aluminum? Is it true that Al has high corrosion resistance? Or is it just a myth as stated in this article.

Come on. At this point it would be better to do the experiments and report. It seems that whatever you are doing you run its virtual version here. I more or less explained to you that Al is a sacrificial anode to iron. Al cannot have a stronger resistance than iron, nor obviously to stainless steel. The point is that aluminium oxide has pretty good mechanical features and does not go away as iron rust. As long the iron (steel) is covered no oxidation can proceed (aluminium is already oxidised and iron is cover protected). Actually it is what you should have found as basically you did the exp.

Actually, as I ask several questions, I am conducting a thorough experimentation. I am asking to verify some of the results, just to know if I am doing the right procedures, since I am very new in this field and my brain cannot comprehend all reasoning behind my project.

Besides, I didn't specified that the Al is coupled with steel in my question.

It is not really the number of question... I am nobody here to tell they were too much. But looks like you ignored the previous answer and explanations, as there should be no reason to reask basically the same, and pointing to a link the quality of which is indeed low.

Corrosion usually refers to objects. A bar of Al corrodes slower than a bar of iron. But because it gets self passivated. Al it easy to oxidise, tough. It is always the same argument..................

I've decided to answer only because the confusion about "what is readily/faster oxidised/corroded between..." is quite common.
Given a couple of elements/substances in a given state, than the one with lower oxidation potential is the one which is readily oxidised (this is a rather obvious statement).
If I take corrosion = oxidation, then Al gets easily corroded than Fe or steel.
This will be evident if we were to make a nice dispersion of very very fine particles of the metals and compare their behaviour, for instance weight gain by oxygen intake at given times.
However, although the chemistry underlying is obviously the same, corrosion is not a synonymous of oxidation, in the sense that it refers to the loosing of the integrity (shape, mechanical properties, even electrical properties) of an object.
If  the Al or steel of the question are, instead of very very fine particles, in form of bars, rods, etc., the oxidation process is still easy in case of aluminium. HOWEVER, while iron oxide may take longer to form (at the VERY beginning of the experiment), its mechanical and coating properties are poor. It can continuously detach from the surface it originates from, and so doing lets the underlying iron exposed to the corrosive environment. Things go like this, until the complete object is destroyed - or at least lost is original function.
Conversely, Aluminum oxide has a compact structure and relatively good coating properties. It does not go away. Therefore, as the outer layer of the Al gets oxidised but not detached, an inert passivating layer is formed, and oxidation/corrosion of the object stops.
This may lead someone to say that Al is corroded less than Fe, but now it should be clear that no one means that is indeed harder to oxidise Al as compared to iron.
As the intrinsic properties of Al and Al oxide stay the same, the same happens if we have aluminium coating an inner metal, for instance steel or iron as in the discussion above or in the previous question by the same user. As such, Al is an effective protection layer for steel.
Finally and related: if Al doesn't cover the iron object, but it is merely in contact with it, still it maintains a protective function. A galvanic cell is form, of which Al is the anode.( Look for sacrificial anode).
As still Al can get passivated, a more effective sacrificial anode will be Mg, both in terms of even lower oxidation potential and especially reactivity of the oxide. It gets literally consumed and must be replaced, but meantime you won't have to buy a new boat or engine :)
I hope this makes some clarity.
(Note that the very very fine particles are intended to reduce the mass/surface ratio, little particles can self-passivate, too)
Edited: this is a general picture. Technologically/Material science: I do expect lot of details, much of which shall be on the practical attainment of the passivating layer, its morphology, its resistance to diffusion. It seems well known that aluminium oxide undergoes pitting corrosion upon prolonged permanence in solutions containing chloride ions, such as that of the question.


text:
if you have 2.5 * 10^21 atoms of magnesium how many moles do you have?
If you have $\pu{2.5 * 10^21}$ atoms of magnesium, how many moles do you have?

What have you trief so far? How is a mole defined? This is a homework question, it needs to show some effort from you what you have tried to solve the problem.

divide however many atoms you have (2.5*10^21) by 6.022*10^23 - Avogadro's Constant

why was this voted down?!

Read the "Why don't you provide a complete answer to homework questions?" section of the homework policy

I don't really care about people answering homework questions. However if you do want to answer them, you need to do it properly (as with any other question on this website). Your answer doesn't explain anything, it just states the solution. It would be more helpful if you explained what a mole is, the significance of Avogadro's number, ...


text:
Why does sodium hydroxide react with aluminum oxide?
I have seen all the chemical formulas for the reaction between sodium hydroxide, water, and aluminium. I understand that the hydrogen gas comes from the aluminum reacting with the water and the sodium hydroxide just breaks down the aluminum oxide. But why does sodium hydroxide break it down? Also since aluminum oxide is amphoteric can you do this reaction with a strong acid instead of a strong base?

Hydrogen is in reaction with metal so which one you really mean?

Sodium hydroxide would react with aluminum oxide precisely for the reason you explained yourself: because the latter is amphoteric.

@IvanNeretin thanks Ivan, so I understand that sodium hydroxide does interact with aluminum oxide because it is amphoteric, but what is the reason behind it reacting and how does it react?

If you use a "strong acid" to break down the passive oxide you need to be careful which acid you choose.  Sulfuric or nitric acid is oxidizing and thus could maintain the oxide.  Hydrochloric acid would be best, among common mineral acids, for actually getting to the metal while being relatively easy to handle.

C2H6O2 is a chemical formula.
HOCH2-CH2OH is a chemical formula.
CH4 + 2O2 → CO2 + 2H2O is a chemical equation. You should learn the difference.
Reaction products are usually lower in Free Energy than the reactants. If it's helpful, then the answer to your question of why does NaOH react with Al2O3 is: because the products are lower in free energy than the reactants. In my more experienced view, this is a circular answer: of course the products have more negative delta G, but that doesn't answer "why?". Science is much better answering "how?" than "why?". But at your level of understanding, you should be aware that speaking about the oxide coating on aluminum and its reactions with acids and bases is (arguably) categorically different than speaking about Al° and its reactions. You've asked about the oxide. Al+3 exists in aqueous solution as [Al+3].6H2O with its counter ion (in HCl(aq) is would be the Cl- anion) and as [Al(OH)4]- with the counter ion (in your case it would be Na+ ). BOTH of these are stable, one in acid, one in base (it's obvious which is which, right?). So, the question is: is the dissolution of Al2O3 spontaneous in acid (in base)? The answer is:"in sufficiently low (sufficiently high) pH it is." Strong base attacks skin, so does strong acid. The reactions are different, they are NOT opposites. I don't see anything unusual in the fact that a material reacts with strong acids and strong bases.  My guess is that one reason you might see a dichotomy between acids and bases is because in water solutions you don't generally have both simultaneously. (at least, since the equilibrium constant of water is 1E-14, in most solutions of chemical interest (contrasted with biologically interesting situations), either acid or base is present in overwhelming concentrations...). While dealing with Bronsted acid/bases, it's sometimes convenient to consider acids as opposites to bases, that approach has limits beyond which other definitions of acidity or basicity are more useful. HTH


text:
Thermal stability of alkali metal hydrides and carbonates
Why is it that thermal stability of alkali metal hydrides decreases down the group, but for carbonates, it increases?
I used Fajans' rule to check for ionic character but somehow this is only applicable for carbonates.

Related: What is the explanation of the changes in stability going down a group for carbonates, bicarbonates, fluorides, and chlorides?

Down the group, atoms of the alkali metals increase in both atomic and ionic radii, due to the addition of electron shells. This results in the charge density of their corresponding cations decreasing down the group. Thus, as we go down the group, the cations become "softer".
Now, note that the hydride ion is "hard", having high negative charge density. While the carbonate ion is "soft", having a lower negative charge density.
Applying the "hard-soft acid base" (HSAB) principle, we would expect the "hard" ions to form more stable compounds with each other and the "soft" ions to form more stable compounds with each other.
Since the heavier alkali metal ions are softer, we would expect them to form more stable compounds with carbonate. As for the harder lighter metal ions, we would expect them to form more stable compounds with hydrides.

Can we also say something about lattice enthalpy

@KingTut The formula for lattice enthalpy does not explain this trend.

What about florides then?


text:
Does it undergo Chloroform reaction or Canizzaro reaction?
What are the products formed when Trichloroacetaldehyde is made to react with NaOH? I see 2 possible routes the reaction can take:

It can form chloroform and Sodium formate, since this is the final stage of the chloroform reaction
It can undergo Canizzaro reaction to give Trichloroethanol along with the sodium salt of Trichloroacetic acid.

Please tell me which reaction occurs and why.

Probably both...

https://chemistry.stackexchange.com/questions/50469/isnt-the-reaction-of-chloral-with-sodium-hydroxide-a-disproportionation-reactio

Well that gets interesting https://books.google.pl/books?id=YyoUNYgBYJoC&pg=PA239&lpg=PA239&dq=chloral+cannizzaro+reaction&source=bl&ots=OUBbAA7vAr&sig=MXtYb4RM_6Hz4d2x2iJD_oFlKBQ&hl=en&sa=X&ei=uJUFVe_cGMjsgwSExYDgDQ&redir_esc=y#v=onepage&q=chloral%20cannizzaro%20reaction&f=false says it undergoes both!


text:
Bleach turned towel bright pink before fading to white
We have some relatively new white towels that were getting dirty, so we decided to clean them with bleach (standard Clorox). When we sprayed the diluted bleach on the towels they immediately turned bright purplish pink, but the color faded back to white within seconds. We initially thought it was somehow transferring color from the pink nozzle on the spray bottle, but we noticed the same effect when the diluted bleach was poured directly onto the towels. No harm appears to be done, but what would cause the towel to turn pink, especially since it fades almost immediately?

Although Dr. Laundry says it happens to the incoming brighteners of a detergent and not to those fixed to the fabric, I think that your new towels should have enough brighteners - deposited into and/ or onto them - to react as well. Additional note : brighteners absorb light in the near UV and emit in the blu tail of vis, so making the fabric "extrawhite". https://www.clorox.com/dr-laundry/detergentbleach-reaction/

@Alchimista That could quite possibly be it, and would be worth posting as an answer.

I do it, just it is not evident to me which oxidation product (dye) is formed.

@Alchimista If it's helpful, this is the detergent we use.

You sprayed bleach directly on the fabric? you are not supposed to do that!   the rx is towel + bleach not detergent. something was chlorinated or oxidized. To Do: new towel check very small area with Cl2 bleach and a peroxide bleach Wash towels w/o bleach and retest. At First blush the towels are chemically treated probably to look white, might be a good idea to wash them before first use.

Although Dr. Laundry says it only happens to the incoming brighteners within a detergent and not to those fixed to the fabric, I think that your new towels should have enough brighteners - deposited into and/or onto them - to react as well.
Optical brightners and bleach reaction:  https://www.clorox.com/dr-laundry/detergentbleach-reaction/
Additional note : brighteners absorb light in the near UV and emit in the blu tail of the Vis spectrum, so making the fabric "extrawhite". Search for Optical brighteners in Google or Wikipedia.
Perahps someone here could know or propose the ongoing reaction and is transient behaviour. As the OP said in a comment, the towels were washed with a detergent containing a stilbenedisulfonic triazine derivative(s).

Possible sunscreen was on the towels? Sunscreen will do that when bleach is added.

Interesting. Can you explain why?


text:
Which poison was used in the failed attempt to assassinate Khaled Mashal of Hamas?
This question is both historical and chemistry-related, but I'm posting it here because I am more interested in the chemical part:
In 1997, Israel tried to assassinate Khaled Mashal, a high-rank official of Hamas. This was to be done using an extremely potent poison that was supposed to be sprayed on him topically and kill him within a few days without a trace (a "quiet" murder).
I searched for the name of this poison, but I was only able to find out that it was a modified version of the drug Fentanyl (here, page 16). It was probably an internal formulation of the Mossad, but I was wondering if the formula was ever publicly released or if anyone knows how Fentanyl's structure was modified in order to make it so deadly upon contact.

I don't know which analog was used for the assassination attempt. However, regarding your last question, there are fentanyl analogs 100x the potency of fentanyl itself, which is basically due to increased lipophilicity, allowing more efficient transfer to the CNS. All fentanyl analogs readily cross into the blood stream transdermally. Just some tidbits, sorry I don't have a full answer (it does seem surprisingly hard to find).

" I was wondering if the formula was ever publicly released or if anyone knows how Fentanyl's structure was modified in order to make it so deadly upon contact."
Well, formula is not publicly released but you can increase the lipophilicity substituting one or more C-H bond with C-F bonds or adding a -CF3 group.

According to several sources online the chemical weapon used in the failed assissination of Mashal was levofentanyl.
This Wikipedia article states:

On September 25, 1997, Mashal was injected in the ear with a toxin
  (thought to have been a derivative of the synthetic opiate Fentanyl
  called Levofentanyl).

Another source reads:

In September 1997, an Israeli hit squad was sent to Amman to
  assassinate Khalid Mishal, the Hamas leader. The chosen instrument was
  levofentanyl, a deadly poison that leaves no traces and produces
  effects like a heart attack. It was administered by a slight physical
  touch.

And another:

...The agents entered the city using Canadian passports and waited for
  Mashal at his office. As he was entering, one agent injected a
  chemically modified toxin called levofentanyl into his left ear.

Concerning the structure I don't think it was ever publicised considering the case was high profile (perhaps all traces were taken offline or can be found in deep when-no one knows)
I've been trying to figure out how fentanyl could have been modified, but have no clue either.

Levofentanyl sounds like it's simply one enantiomer of fentanyl (see e.g. levocetirizine) but just a guess on my part.

@orthocresol Pretty much like it, I think it was one enantiomer of fentanyl which was lethal

@orthocresol Fentanyl is achiral.

[OP] This was to be done using an extremely potent poison that was supposed to be sprayed on him topically and kill him within a few days without a trace (a "quiet" murder).

Poisons leave a trace. In the case of fentanyl derivatives, which are dangerous in very small doses, it is technically challenging to find the trace, though. The path of entry is described differently by different sources. Some speak of "injection" into the ear while others say the intent was to spray on the back of the neck, but Mashal supposedly turned his head to the side, and the spray went into one ear.

[OP] It was probably an internal formulation of the Mossad,

I did not find any evidence, or even other claims, for this.

[OP ...] but I was wondering if the formula was ever publicly released or if anyone knows how Fentanyl's structure was modified in order to make it so deadly upon contact.

In general, fentanyl and its derivatives are contact poisons (see airhuff's comment to the OP's question). As of 2022, I don't think Mossad disclosed which substance was involved. However, this peer-reviewed medical paper comments on reports claiming the use of the fentanyl derivative carfentanyl by Russian safety forces in an anti-terrorist case:

Carfentanyl has also been reported to have applications in anti-terrorist interventions as incapacitating agent, used by Russian safety forces in the hostage rescue operation in October, 2002, in the Dubrovka theatre, Moscow. Carfentanyl and remifentanyl were found on a shirt sample and carfentanyl metabolite norcarfentanyl was found in a urine sample. It is possible that an aerosol containing a mixture of carfentanyl and remifentanyl was used.

If the poison was indeed a fentanyl derivative (targeting the opioid receptors), a possible antidote would have been naloxone, approved in the U.S. since 1971 to treat opioid overdoses. It competes with opioids, so the stronger the opioid binds, the higher the necessary does of naloxone (for the example carfentanyl, 100 mg of naloxone are administered to treat a 1 mg exposure with carfentanyl).

[OP] I've been trying to figure out how fentanyl could have been modified, but have no clue either.

Fentanyl itself is a synthetic product. The derivatives, of which many are known and characterized, are not always obtained by using fentanyl as a starting material. Sometimes, they are made using a different series of synthetic steps. So derivative (or modification) often refers to similarity in structure, not necessarily making a claim about the path of synthesis.


text:
Number of stereoisomers and symmetry of pentane-2,3,4-triol

The answer is apparently 4, but the solution does not make sense.
I can only identify 2 chirality centers (the central carbon cannot be a chirality center) so I thought 2^2 = 4 stereoisomer possibilities, but because of the existence of a plane of symmetry, there is a meso compound: thus there should only be 3 stereoisomers.
However, the solution claims that initially, we can deduce 3 chirality centers and claim 8 stereoisomers, then eliminate some of them through the observation of plains of symmetry (there is only one as far as I can see either way). This solution does not make sense, but when I checked by looking at all of the structures, it seemed true. Can someone help me what I am missing?

Central C is chiral when the configuration around the external ones is not the same.

Why someone did downvoted my comment? It is obvious that a C atom between two fragments with different configurations is a chiral center. Obviously if the two remaining substituents differ from each other. I really do not see the reason for such complicated answers with such a simple structure. Especially considering that the question is based on assuming that the molecule has only two chirality centers when indeed, as I explained in my comment, there are three.

@Alchimista it's not possible to downvote comments, what are you talking about? The central carbon can also be a pseudoasymmetric centre - search on here or look for it in the IUPAC gold book

Not a real problem @Orthocresol.Does not matter much... :) I thought that is not possible but I was convinced that I read that and I was wondering why. I wanted to made clear to the self-teaching student why he got stuck with the max number of stereocenters and consequently the max number of stereisomers. It did not seemed to have problems with identifying symmetry elements, as he said that after seeing the structure he agreed with the exercise book.

Related: Number of diastereomer pairs of 1,3-dichloro-1,2,3-triphenylpropane

@Orthocresol. I know what is a pseudoasymmetric stereocenter. However, it is true that explicitly referring to its definition makes the answer more elegant. I am going to edit my answer for future readers.

Related: Mismatch of plane of symmetry vs Gold Book's definition for a pseudochiral carbon

It is easier to compare molecules is you rotate them all into a comparable position.  Say 1 with the central C-3 hydroxyl group out of the plane of the screen/page like in 1, 4, 5 and 7.
So when we rotate a molecule by 180 degrees along a vertical axis in the plane of the paper, something going into the plane of the page on one end off the molecule rotates to become something coming out of the plane of the page on the other end of the molecule.  Also central group flips from into to out of the page and vice-versa.

Hence we can tell that actually pairs (1,2), (3,4), (5,8) and (6,7) are the same molecule, just rotated by 180 degrees.
The absolute positions of the three centres doesn't matter, only the relative orientation of the end two to the central site.

Also, adding to ans of @user213305, molecule number (5,7) and (6,8) are enantiomeric. Hope it helps

It seems to me that your question is motivated by considering the central C atom as having two identical substituents. However, the fact that the two substituents have their own R or S makes them not identical, at least not always (only conformational differences do not reflect to the optical ones).
When the two sides of the molecule are not in the same configuration, it easy to see that the central atom is a stereocenter, too.
From here you proceed with max number of enantiomers = 2^n and so on, in which the previous answer might help you in how to identify symmetry elements - if you do not see them at a glance.
In this situation, the carbon is referred to as a pseudoasymmetric centre.

Stereoisomerism comes by the atoms making up the isomers, joined in the same order, but still manage to have a different spatial arrangement.
Here is the molecule projected in space with 3D axis and are you able to see now if there's a plane of symmetry or not? NOTE: this is not the mirror image. Rotation of its mirror image does not generate the original structure. To superimpose the mirror images, bonds must be broken and reformed.
And you can see if you do spatial arrangements for all other structures in your question, they do have mirror images that are superimposable.

EDIT
I was trying to get a mirror image of the molecule using the application, but it doesn't turn out well. So I leave just the molecule here.

The above drawn structures are 4 stereoisomers of pent 2,3,4,triol.
Note that structure 1 and 2 are meso compounds and therefore do not possess mirror images.
Structures 3 and 4 are an enentiomeric pair . Now you have 4 stereoisomers.
I am quoting from http://www.chem.ucalgary.ca/courses/351/Carey5th/Ch07/ch7-7.html.
"When relating one Fischer projection to another, it's important to realise that it may only be manipulated within the 2D plane in which it is drawn (that is, it may not be arbitrarily rotated within 3D space), and even then, it can only rotated in the plane it is drawn (2D) by 180 degrees ".
If structure 5 is rotated by 180 degrees in the plane of paper then we get structure 3.
Similarly if structure 6 is rotated by 180 degrees in the plane of paper then we get structure 4.
Therefore we get on;y 4 isomers.


text:
Uncertainty of Atomic Mass
How should the uncertainty of the atomic masses of elements be quoted?
E.g., Mg has an atomic mass of 24.31, what is that value's uncertainty?

In general, if you have a measured value of upto 1 decimal place, then the uncertainty in its measurement is +- 0.1 (plus minus 0.1). Similarly for two decimal places, its +- 0.01 and so on. This is easy to remember, however @Paul has given you a mathematical proof as well.

First of all it is important to realize that the number you quote is not the atomic mass of magnesium, but the avarage (molar) mass of all naturally occuring isotopes. If you visit the wikipedia page on the isotopes of magnesium you will see that the naturally occuring isotopes have the following masses and abundances
isotope       mass (amu)          abundance
Mg-24         23.985041700(14)    0.7899(4)
Mg-25         24.98583692(3)      0.1000(1)
Mg-26         25.982592929(30)    0.1101(3)

As you can see the last numbers of the mass and abundance are within parenthesis. This is a very common way to display (1$\sigma$) uncertainties and the numbers within the parenthesis represent the uncertainty in the last digit, e.g. 0.7899(4) can also be written as 0.7899$\pm$0.0004 and 23.985041700(14) as 23.985041700(14)$\pm$0.000000014. You'll probably see why people prefer the former notation as it is so much shorter than the latter.
To calculate the average mass ($\bar{m}$) you take the weighted sum over the different isotopic masses ($m_i$) using the abundances ($h_i$)
$$
\bar{m}=\sum_i h_im_i=h_{24}m_{24}+h_{25}m_{25}+h_{26}m_{26}=24.30505
$$
To calculate the uncertainty in the average mass you should apply error propagation to find
$$
\Delta \bar{m}=\sqrt{\left (\frac{\partial\bar{m}}{\partial h_{24}} \right )^2\Delta h_{24}^2 + \left (\frac{\partial\bar{m}}{\partial h_{25}} \right )^2\Delta h_{25}^2 + \left (\frac{\partial\bar{m}}{\partial h_{26}} \right )^2\Delta h_{26}^2 + \left (\frac{\partial\bar{m}}{\partial m_{24}} \right )^2\Delta m_{24}^2 + \left (\frac{\partial\bar{m}}{\partial m_{25}} \right )^2\Delta m_{25}^2 + \left (\frac{\partial\bar{m}}{\partial m_{26}} \right )^2\Delta m_{26}^2}\\
=\sqrt{m_{24}^2\Delta h_{24}^2 + m_{25}^2\Delta h_{25}^2 + m_{26}^2\Delta h_{26}^2 + h_{24}^2\Delta m_{24}^2 + h_{25}^2\Delta m_{25}^2 + h_{26}^2\Delta m_{26}^2}=0.013
$$
The average mass of magnesium is thus 24.305(13) amu. Note that the uncertainty in the final answer is dominated by the uncertainty in the abundances.


text:
How to calculate what mass of halothane is inspired in one minute given the partial pressure and temperature?

If the anesthetic mixture is inspired at the rate of $\pu{100 mL/min}$, what mass of halothane, $\ce{CHCIBrCF3}$, molecular weight $M = \pu{197.4 g/mol}$, is inspired in one minute if the partial pressure of halothane is $\pu{7.6 torr}$ and the temperature is $\pu{21 ^\circ C}$.

$\pu{0.08g}$
$\pu{0.80g}$
$\pu{1.80g}$
$\pu{3.36g}$

My working:
\begin{align}
P &= \pu{0.01 atm} &&(\pu{1 atm} = \pu{760 torr})\\
T &= \pu{294 K}    &&(273 + 21) \\
V &= \pu{1 L}      &&(\pu{1000 mL} = \pu{1 L})\\
R &= 0.08          &&(\text{L-atm})\\
\end{align}
Equation:
\begin{align}
  0.01 \times 1 &= n \times 0.08 \times 294\\
  0.01 &= n \times 23.52\\
  n &= 4.3
\end{align}
Which suggest $\pu{4.3 mol}$ of $\ce{CHCIBrCF3}$ which would give the mass $\approx\pu{849g}$.
That is way off any of the answers. Can someone see what I have done wrong?

It looks you have two errors. You used 1L instead of .1 and in your algebra you multiplied where you should have divided.

Volumetric Flow Rate (VFR) is given to you.
By applying ideal gas equation to the gas, in terms of volumetric flow rate,
$$\frac{PV}{t} = \frac{nRT}{t}$$
And in an ideal gas mixture (the assumption),
the partial pressure of a gas is what is exerts alone in same volume in same temperature as of the mixture.
$$\frac{P_\mathrm{gas}\cdot V_\mathrm{tot}}{t} =
\frac{n_\mathrm{gas}\cdot R\cdot T_\mathrm{total}}{t}$$
amount of substance = mass of the compound/molar mass (or molecular weight of the compound)
$$n = \frac{m}{M}$$
$$\frac{P_\mathrm{gas}\cdot V_\mathrm{total}}{t} =
 \frac{(\frac{m_\mathrm{gas}}{M})\cdot R\cdot T_\mathrm{total}}{t}$$
Apply the values, mind the units:
\begin{align}
V &=\pu{0.1 L}\\
T &= (273+21)\pu{ K}\\
P_\mathrm{gas} &= \pu{0.01 atm}\\
R &=\pu{0.082057 L atm mol^-1K^-1}\\
M_\mathrm{gas} &=\pu{197.4 g mol^-1}\\
t &= \pu{1 min}\\
\end{align}
$$\frac{\pu{0.01 atm}\cdot\pu{0.1 L}}{\pu{1 min}} =
\frac{
  \frac{m_\mathrm{gas}}{\pu{197.4 g mol^-1}}
  \cdot\pu{0.082057 L atm mol^-1 K^-1}\cdot\pu{294 K}}{\pu{1 min}}$$
For $m_\mathrm{gas}$ I can see the value in your multiple choice.


text:
How to wedge d/hash notation a molecule?
This is probably trivia for many of you (I already feel the pressure by even posting this question). Yet funny enough, most of the time the materials I have come across show the wedge-d/hash notation. Then due its cumbersome or what not, the material decides to propose Fisher project as a better notation. But frankly that doesn't give the proper picture of the stereochemistry or least understanding of how the atoms would arrange in space.
When revising Rectus-Sinister (R/S) system for stereochemistry, it has almost always indicated about assigning priority to the groups by descending order of the atomic number and that lowest priority substituent should always point away from the plane of the central atom. Speaking of a simple molecule like $\ce{CH4}$, Wedge-Dash is ideally suited to show structure of sp3 hybridised (tetrahedral atoms).
But what about more complex compounds/molecules like amino acids or things like drugs like morphine etc? or even an imaginary molecule given in exam? I dread at the thought of it.
I went through this question for few days now, before posting this question. Perhaps it has more advanced perspective compared to the simplicity of my question I think.
So are there direct guidelines to draw the wedge-h/dash notation for a given complex molecule? Am I really missing something from preliminary general chemistry here?

Obviously there are online applets that allow to get a SMILE of a molecule and generate the 3D structure like JSmol. I also know there are complex methodologies employed in higher level of physical science to determine the structure of molecules. But at this beginning level, is there anyway to determine the 3D structure of a molecule? At least to get a feeling of it, and to predict that what atom/side chain would pop out of plane and remain in the plane?
I am a beginner in Chemistry, so should I really bother about this at all?

If not using a Fischer projection, structural formulae with hashes and wedges allow to be more specific.
In the structure drawn in your current question, you provide information about the connectivity of atoms within the molecule, up to a point to indicate if the bond between two atoms is a single, double, or (not present here) a triple / quadruple.
Using wedges and dashes, equally provides this information; however additionally informs the reader about the sterochemical relationship of the atoms bound around an atom in a centre, addressing a specific (R)/(S) configuration.  Returning to your example, there are three sterogenic centres that I marked by an asterisk:

and for each centre you may find a pair of either configuration; for example for the hydroxyl group in $\alpha$ position to carbonyl alone

In total (for 3 stereogenic centres), there are $2^3 = 8$ distinct stereoisomers possible with different properties, which all fit into the first "line only" general representation.

Buttonwood, thanks for this! Perhaps you are not getting my question? If you give me this molecule, I wouldn't know how to draw wedges/hashes at this state. That's why I wanted to know if there's a checklist, guidelines that has reasoning and which can be followed. Also I have no issues with Fisher. What I mean was most books talk about Fisher as a substitute for wedge/dashes. But do not mention how to begin wedge/dashes. I personally prefer the latter, because it really gives me a good idea of the spatial arrangement/stereoisomerism, geometry etc. So guidelines please? :)


text:
What causes the stairstep structure in bismuth crystals?
I've been interested in bismuth crystallization since I found out they existed, but have a question on their formation.
According to Wikipedia, the stair-step structure of bismuth crystals is "the result of a higher growth rate around the outside edges than on the inside edges."
What would cause this higher growth rate around the edges? Cooling?

Hopper crystals, that is, the "staircase" form, develop when a crystal forms so rapidly that material is used up before the interior finishes.
Table salt, $\ce{NaCl}$, also forms nice hopper crystals. If you want to try this, use "kosher" salt or CP $\ce{NaCl}$, which doesn't have anti-caking and iodide additives that would spoil the crystal structure.


text:
Dubowski’s Formula vs Henry's law applied to alcohol sensor 'calibration'
I'm working with some hobby-level gas sensors and I've run across a "calibration" system I don't understand. A general description of this kind of gas sensor is described a bit more in this question and also here. They have some selectivity between gases, but anything that will react with atomic oxygen at elevated temperature in the presence of a catalyst ($\ce{CO}$, $\ce{H2}$, $\ce{CH4}$, etc.) will register.
Since I don't want to mess with explosive gases, the alcohol sensor is a convenient place to begin. Many hobbyists become voluntary test subjects, and heroically subject themselves to alcohol, then breath on the sensors. Since I'm not so brave, I thought I'd try the recommended calibration test setup below. I'm not interested in doing a careful calibration, but just get some kind of ballpark quantitative agreement.
Henry's law suggests a proportionality between the concentration of a dissolved gas and the partial pressure of the gas above the liquid, and the van 't Hoff equation provides for a temperature dependence within some limited range of variation. However it's of the form $exp(\frac{1}{T}-\frac{1}{T_0})$ and the equation shown below is only a straight exponential of temperature.
This is described more thoroughly in this excellent answer:

Henry's law works for small concentrations of ideal mixtures at equilibrium. Henry's Law constant varies with temperature according to the Van't Hoff equation:

$$
k(T)=k(T_0)exp\left[-C\left(\frac{1}{T}-\frac{1}{T_0}\right)\right]
$$

Here $C$ is a constant related to the enthalpy of solvation for each gas, and $T_0$ represents the standard state, T = 298 K and 1 atm.

Question: Is Dubowski’s formula just a further approximation, or does this method  contain something new and/or different that I should be paying attention to?

below: Screenshot from Adafruit's AN4 Using MiCS Sensors for Alcohol Detection.pdf

Dubowski's formula is a parameterization of the temperature dependence of the water-vapor partition constant (Henry's law constant). It is based on data accumulated over 63 years (see Ref 1). It is assumed that Henry's law holds, in other words, that the vapour pressure over the solution is proportional to the concentration in the solution as $\frac{p}{c} = K_{a/w}=$ constant.
It can be used as a proxy for the concentration of alcohol in the blood although it is not expected to be entirely accurate, as Dubowski comments (Ref 2):

We believe that the conversion of a breath quantity to a blood concentration of ethanol, for forensic purposes, should be abandoned and that the offense of driving while under the influence of alcohol should be statutorily defined in terms of the concentration of ethanol found in the breath in jurisdictions employing breath analysis.

Irrespectively of the details involving breath-alcohol analysis, Henry's law states that the vapor pressure is linearly proportional to the concentration in solution as $p=K_Hc$, and is applicable in practice when the alcohol concentration in the blood is very low (it is in general applicable as a limiting law).The following figure shows the partial pressure of ethanol in dilute aqueous solution at $25 ^\circ C$ as a function of the mole fraction of alcohol in solution, where the dashed line represents the pressure predicted by Henry's law:

For reference, a blood alcohol concentration (BAC) of 0.1%(w/w) translates into a mole fraction of $\pu{3.9E-4}$.
The Henry's law constant is obtained by extrapolating to zero concentration of the solute (here alcohol). In general, water-ethanol solutions are highly non-ideal due to hydrogen bonding and would not be expected to follow Henry's law except for very dilute concentrations of one component in the other. As soon as interactions between solute molecules become significant the law can be expected to break down.
The difference in the functional form of the temperature-dependence in Dubowski's formula compared to Van't Hoff's can be ascribed to differences in units: one uses T in Kelvin, the other in degrees Celsius. With proper application of Taylor series formulas they can be shown to be equivalent within narrow T ranges.
References

K.M.  Dubowski.  Breath-alcohol simulators: Scientific basis and actual performance. J. Anal ToxicoL 3" 177-82 (1979).

M F Mason, K M Dubowski. Breath-alcohol analysis: uses, methods, and some forensic problems--review and opinion. J. Forensic. Sci. 1976 Jan;21(1):9-41.

Beautiful, thorough and clear answer, thank you very much!


text:
What is the difference between an electron in ground state and an electron in excited state?
What is the differnce between a ground state electron and an excited state electron? Upon excitation what exactly changes about the electron?

What makes you think there's a difference?

Bohr was the man! Just a matter of orbital change on absorption and emmision.....

Could you explain what your understand of a ground state and an excited state is?

You can think of electron states as book shelves and electron as a book. Is there anything different about that book if it stands on lower or upper shell? Not really. Same is with electron in different states.
I guess what made you think otherwise is the fact that electron in lower electronic states has a lower potential energy. When electron occupies higher(excited) electronic states it has more energy, and by releasing it in a form of photon it may be relaxed to lower(ground) electronic state. So the difference is in amounts of energy electrons in different states have.
Also, since electrons in different electronic states are described by different wavefunctions, probability density will be different. And other observables, besides energy, will have different values.

thancu for u r elaborate explanation


text:
Do I use the Nernst equation when the concentrations of electrolyte in both half cells are equal?
I'm doing a chemistry report, and for my experiment I investigated the amount of voltage produced by different half cells in a voltaic cell.
However, I tested them under nonstandard conditions as the temperature at the time was $\pu{18^\circ C}$, and the concentration of the electrolytic solution in the half cells was $\pu{0.1 M}$.
I tried the Nernst equation, but since the concentration of both my electrolytes were $\pu{0.1 M}$, calculating the reaction quotient gave me an answer of $1$, and $\log 1 = 0$, therefore I'm left with the same cell potential I started with (cell potential at standard conditions) and I'm stuck there.
Here is an example for a magnesium-zinc voltaic cell at $\pu{291 K}$ ($\pu{18^\circ C}$):
$$E_\text{cell} = 1.593 - \frac{0.05778}{2} \log_{10} \frac{0.1}{0.1} = \pu{1.593 V}$$
Could you please explain the solution to me?

Yes, you still use the Nernst equation, but one of your implicit assumptions is not correct.
A point about the Nernst equation that often confuses people is that, at first glance, it doesn't predict any change in $E$ as temperature changes, as long as $Q$ remains equal to $1$, and $\ln Q = 0$.
$$E = E^\circ - \frac{RT}{\nu F}\ln Q \tag{1}$$
However, a slightly more careful analysis reveals that if you set $\ln Q = 0$, it only implies that $E = E^\circ$. The conclusion that $E$ doesn't change is based on an assumption that $E^\circ$ doesn't change, which is not true.
"Standard conditions" do not stipulate any temperature. So $E^\circ$ itself is a function of temperature. The standard cell potential $E^\circ$ at $\pu{298 K}$ is going to be different from the standard cell potential $E^\circ$ at $\pu{250 K}$, and hence $E$ will vary with temperature, even if $\ln Q = 0$.

So, how does one determine the temperature dependence of $E^\circ$? The best way is to measure it, but if that is not possible, then you have to somehow find an expression for it. The Nernst equation alone has no answer for this. Instead you would have to turn to thermodynamics. We know that
$$E^\circ = -\frac{\Delta_\mathrm rG^\circ}{\nu F} \tag{2}$$
Note that $\Delta_\mathrm rG^\circ$ is a function of temperature, and hence so is $E^\circ$. So, the problem becomes one of determining the variation of $\Delta_\mathrm rG^\circ$ with temperature. The most primitive way is probably to look up the data for $\Delta H_\mathrm{f}$ and $S_\mathrm{mol}$ of each of the compounds, calculate $\Delta_\mathrm{r} H^\circ$ and $\Delta_\mathrm{r} S^\circ$ for the reaction (using Hess's law), and then find
$$\Delta_\mathrm{r} G^\circ = \Delta_\mathrm{r} H^\circ - T\Delta_\mathrm{r} S^\circ \tag{3}$$
Of course, when you look up data, the chances are that the data you find are specified for $T = \pu{298 K}$. So, when you use these data to calculate $\Delta_\mathrm{r} H^\circ$ and $\Delta_\mathrm{r} S^\circ$, you are finding the values of these quantities at $T = \pu{298 K}$. If you plug these values into equation $(3)$, then you also assume that the values of $\Delta_\mathrm{r} H^\circ$ and $\Delta_\mathrm{r} S^\circ$ at your desired temperature are equal to their values at $\pu{298 K}$.
Depending on how much accuracy you need, this may or may not be tolerable, and there are certainly more thorough ways of calculating it. However, that discussion is best left for another question.


text:
Which definition of Acids and Bases conforms to the pH scale?
It's all in the title. Is it the Arrhenius or the Brønsted-Lowry definition? Or is it some other definition?
Also, our teacher told us that $pH$ stands for "Power of Hydrogen", but some sources say it is a number. So which it is?

A few things first...
Arrhenius Theory:

An "acid" is a proton donor; a "base" is a hydroxide donor.

So for example, $\ce{HCl}$ and $\ce{NaOH}$ (both in water) ionize to produce $\ce{H+}$ and $\ce{OH-}$ respectively.
$$\ce{HCl -> H^+ + Cl^-}$$
$$\ce{ NaOH -> Na^+ + OH^-}$$
Which gives us the all-too-familiar neutralization reaction:
$$\ce{HCl + NaOH -> NaCl + H2O}$$
Brønsted–Lowry Theory:

An "acid" is a proton donor; a "base" is a proton acceptor.

At first, the base being a "proton acceptor", may appear to be in contradiction with the Arrhenius Theory... but it isn't.

 The following's from Chemguide. It's been put so well, that I didn't even bother with paraphrasing it
The Brønsted–Lowry theory doesn't go against the Arrhenius theory in any way - it just adds to it.
Hydroxide ions are still bases because they accept hydrogen ions from acids and form water.
An acid produces hydrogen ions in solution because it reacts with the water molecules by giving a proton to them.

When an acid in solution reacts with a base, what is actually functioning as the acid is the hydronium ion. For example, a proton is transferred from a hydronium ion to a hydroxide ion to make water.
$$\ce{H3O^+ + OH^- -> 2H2O}$$

 I'm assuming you already know what the hydronium ion is...so I won't get into that

Or, meh, I'll re-phrase that last bit. Remember that example/equation we first  brought up?
$$\ce{HCl + NaOH -> NaCl + H2O}$$
Looking at it from the Brønsted–Lowry POV, your "acid" is really just the hydronium ion, because that's the real proton-donor here; the "base" is really the hydroxide ion, because that's the real proton-acceptor here.
The hydronium ion (acid) passes on a proton to the hydroxide ion (base).
$$\ce{H3O^+ + OH^- -> 2H2O}$$

Back to your question:

Which definition of Acids and Bases conforms to the pH scale?

Both the Arrhenius and Brønsted–Lowry definitions are "compatible" with $\mathrm{pH}$ . They both point in the same direction ;)

Our teacher told us that pH stands for power of Hydrogen, but some sources say it is a number. Which is it?

$\mathrm{pH}$ is, by definition, (approximately) the negative logarithm (base 10) of the molar concentration of $\ce{H+}$ ions in a solution.
Therefore, $\mathrm{pH}$, is indicative of the $\ce{H+}$ strength, in a solution.
I've been led to believe that the $\mathrm{p}$ in $\mathrm{pH}$, comes from the German "Potenz", or the French "Puissance", both which mean the English "Power". I'm under the impression it was originally "Potenz" since German was the language of science well up to the end of the 19th century (and then the whole World War thing ruined that...); besides, the man who came up with the $\mathrm{pH}$ notation was a Dane, and Danish is a Germanic language with little semblance to French. However, Wikipedia suggests it's more likely "Puissance", since the laboratory the man worked for was French-speaking ...that's still just a guess though.
Your teacher isn't wrong, the $\mathrm{p}$ does mean "power" (or even "potential").
However, $\mathrm{pH}$ is a number (it is dimensionless/unit-less...but not without meaning either), so your "sources" are correct too ;)

A better answer could not have been written... +1, however, while acids and bases are the topic of discussion, shouldn't the Lewis acid-base definition also be included?

@Abhi Thanks! Acids-bases are on-topic here...but the question implicitly requires a comparison between the Arrhenius and Brønsted–Lowry definitions of acids-bases. So I didn't bring up Lewis acids (it wouldn't be wrong to do so, either) O:)

@paracetamol Thank you very much, good sir! As Abhigyan said, a better answer could not have been written. Again, many thanks!

paracetamol's answer to your first question is superb. He stumbles in answering your second, I think. pH doesn't "stand" for anything - it is not an abbreviation nor an acronym, contraction, nor initialism. The Oxford Shorter English Dictionary (which is, in my view authoritative) claims it is derived from the German potenz. Salary is "derived from" the latin word for salt (sal), but no one would claim that salt has any relevance to its current meaning. It might be of historical interest to learn its derivation, but that knowledge is mostly irrelevant in the practice of chemistry. pH is generally accepted to be the negative logarithm of the H+ concentration expressed in moles per liter (molar concentration, [H+] with the understanding that H+ represents the sum of the cations of nH2O + H+ where n is a small positive integer ). Practically there are some serious problems with that definition. Pragmatically, pH is what a well-calibrated pH electrode measures. Anyway, pH "means" the negative of the value of the logarithm of the molar concentration of H+ ions in a given system. It does not "mean" the "power of H+". (The OED states that pH is a noun, i.e. a word). HTH.

Meh, still feel Potenz Hydrogen ( "Power of Hydrogen") is indicative of the "power" or concentration of hydrogen... though, as you mention, it isn't a measure of concentration per se. Very insightful answer, thanks for the constructive criticism ;)


text:
What is the definition of liquid phase?
Why is a supercritical fluid not a liquid? My book says that because in a container above the critical temperature and critical pressure, a fluid of density similar to liquid exists uniformly in the container and there is no surface or meniscus that separates the liquid from the gas. Does that mean that a liquid always has a meniscus which separates it from the liquid's vapour?
Does that mean that in free space without no gravity, a gas/vapour will try to expand and the liquid will try to contract and form spherical blobs due to surface tension? Can I draw the conclusion from this that a liquid is the phase when attractive forces are dominant and repulsive forces are dominant in gas/vapour phase?

Why is a supercritical fluid not a liquid?

Beyond the elementary school list of solid liquid and gas, there are many states of matter which exhibit peculiarly different properties. Wikipedia gives a list of the states of matter.

My book says that because in a container above the critical temperature and critical pressure, a fluid of density similar to liquid exists uniformly in the container and there is no surface or meniscus that separates the liquid from the gas.

That is true.

Does that mean that a liquid always has a meniscus which separates it from the liquid's vapour?

No, if the container is absolutely full of liquid then there is no gas phase.

Does that mean that in free space without no gravity, a gas/vapour will try to expand and the liquid will try to contract and form spherical blobs due to surface tension?

Yes, but for the liquid there will also be evaporation. The rate of evaporation depends on the temperature of the liquid. As evaporation occurs the liquid will also cool. If there is enough liquid in the blob, then the matter can get cold enough to form a solid which will then sublime.

Can I draw the conclusion from this that a liquid is the phase when attractive forces are dominant and repulsive forces are dominant in gas/vapour phase?

This is more or less true. I'd quibble about the nature of the "repulsive forces." I don't think that "repulsive forces" is the best way to model water in the form of steam. Rather the water molecules have too much kinetic energy to stick together.


text:
Why doesn't a temperature and pressure gradient exist in a container of real gas?
In the Kinetic theory of gases, it is assumed that there are no intermolecular forces between the atoms/molecules. But real gases, have intermolecular attractive and repulsive forces.
Suppose a container filled with a real gas.

The molecules well inside that container will feel no net force of attraction/repulsion on any side. But considering molecules near the boundaries of the container, a net inward force will be felt. I think this will make velocities of atoms less than that of the atoms well inside the container.
So, is the pressure at the centre of the container more than that of the pressure at the boundary(measured actual pressure)?
My book says that the slowing down does not mean that the gas is cooler close to the walls. The relation of $\sqrt{T} \propto v_{\mathrm{rms}}$ is valid only in the absence of intermolecular forces. Temperature is defined by the Zeroth Law. Will net energy/heat transfer take place between the atoms of the gas and the metal bar if I keep a metal of temperature $T$ inside the container near the boundary? Here, $T$ is the temperature at the centre of the container such the no heat transfer takes place if a metal bar is kept at temperature $T$ at the centre of the container.

Most gases under normal conditions are closer to ideal than van der waals states. The extra terms in the VDW equation account for the non-ideal behaviour when they are close to liquefaction and are insignificant under normal conditions.

Most "containers" are "small," so temperature and pressure variations are insignificant. But think of the earth's atmosphere. Temperature and pressure variations are significant in the atmosphere. The sun is essentially a ball of gas. Look at what happens there...


text:
Is there a drug that can encapsulate benzene in the liver?
I'm doing a project on the prevention of leukemia and upon researching on the metabolism of benzene in the liver (since high levels of benzene can lead to leukemia), I thought of a question that was puzzling to me:
Is there a drug that can encapsulate benzene in the liver until the benzene is released through urination or defecation(pooping)?
Thanks

Most drugs can't break apart benzene. It's incredibly stable.

Well, it is actually its epoxide which causes cancer. If benzene was to be so dramatically stable it will induce less harm.

@Alchimista I'm pretty sure you don't need a drug to convert benzene into an epoxide. There's enough biochemicals in the body that can do that.

@Pritt Balagopal. Yeah, that is why! :) I just said that benzene is archetypal for its special stability, not for being almost a noble compound (see comment by Melanie). By the way, the OP would like to isolate benzene from the body, not destroying it :)

The OP is seeking a sort of benzene sequestration, though wasn't that clear.

@Alchimista Yeah I was just making an extra point. But here in By the way, the OP would like to isolate benzene from the body, not destroying it I don't think so. He/She is just studying leukemia prevention and was looking for a chemical that can encapsulate (is that right?) benzene to prevent biochemical action on it. Besides, with all that petroleum around, trying to obtain benzene from human blood? Unlikely.

Yes. Extra point not needed as far I concerne, as I said that. The same is for that he would like to encapsulates benzene, to prevent exactly epoxydation or whatever causes cancer. I do not see see reasons for much discussion. I just corrected a strong unmotivated comment. Still

You took "isolate" like at the bench :) we were saying the same

I edited my question in case it was too confusing. Thanks