text:
Is the QST2 method suitable to find the transition state of an enatiomeric R-S interconversion?
I am trying to calculate the energy barrier between the two possible enantiomeric components of a polymer with Gaussian. I wonder if I am able to locate a transition state between two of those structures with the keyword opt=QST2?

What kind of inversion would this be?

The polymer has two crossed arms. With a shape similar to a cross. Depending on which arm is in front or behind, there will be a R-enantiomer and another one S-enantiomer.

Maybe, but more likely no. Depending on the complexity of your molecule the guess from QST2 is not good enough. Additionally you might run into some unforeseen coordinate system troubles.
As a small demonstration I'll use $\ce{CHFClBr}$ and the DF-M06L/def2-SVP level of theory and Gaussian 09 Rev. D.01.
I'll optimise one conformation and then just exchange the $y$ with the $z$ column.

%chk=m06ldef2svp.chk
#p M06l/def2SVP/W06
gfinput gfoldprint iop(6/7=3)
symmetry(none)
opt

title: optimisation

0 1
C        0.000000000      0.000000000      0.000000000
H       -0.530433945      0.045629756     -0.953288460
F       -0.905627118      0.105968719      1.035862034
Cl       1.161565217      1.332282245      0.093221601
Br       0.942449903     -1.690284863      0.135370837

That gives the following file for the transition state search. It is important, that the ordering in the two input blocks is identical, otherwise the program will fail horribly.

%chk=m06ldef2svp.chk
#p M06l/def2SVP/W06
gfinput gfoldprint iop(6/7=3)
symmetry(none)
opt(QST2, maxcycle=50)

title: optimised conformation

0 1
 C     0.004249     0.005882     0.022885
 H    -0.510890     0.037500    -0.945427
 F    -0.880186     0.102687     1.016533
Cl     1.139127     1.361087     0.088671
Br     0.915653    -1.713561     0.128503

title: yz exchanged

0 1
 C     0.004249     0.022885     0.005882
 H    -0.510890    -0.945427     0.037500
 F    -0.880186     1.016533     0.102687
Cl     1.139127     0.088671     1.361087
Br     0.915653     0.128503    -1.713561

In this case the calculation will produce the following error:

 GradGradGradGradGradGradGradGradGradGradGradGradGradGradGradGradGradGrad
 Berny optimization.
 Bend failed for angle     2 -     1 -     5
 Tors failed for dihedral     2 -     1 -     5 -     3
 Tors failed for dihedral     2 -     1 -     5 -     4
 FormBX had a problem.
 Error termination via Lnk1e in /path/to/g09/l103.exe at {a date}.

The problem is that in any case of switching stereo centres, you most likely have have an $\approx180^\circ$ angle at one point and then the automatic generation of redundant internal coordinates will fail.
You can (maybe) prevent this at a much higher computational cost by optimising in Cartesian coordinates. Alternatively you specify the redundant coordinates by yourself, but even then it is probably best to use QST3.
In some rare cases, such calculations can actually give you a starting structure for a real TS search.
In any case, a little chemical intuition is necessary and often a single step mechanism is much, much higher in energy than a step-wise path. Sometimes it is better to use your imagination and guess a transition state (which you can then optimise with very low level tools like force fields or pm6), and refine later on.

The example I chose is certainly an extreme one. In this case isomerisation will likely happen via $$\ce{^{$S$}CHFClBr <--> [CHFCl]+ + Br- <--> ^{$R$}CHFBrCl}$$ or similar, and a transition state won't be easy to find.

Thank you Martin - マーチン♦ . The transition state that I am trying to optimize is the one that involves the conversion between these enantiomers: R and S-2,2′-bis(2,2′-bithiophene-5-yl)-3,3′-bithianaphthene.

https://www.google.es/imgres?imgurl=http%3A%2F%2Fars.els-cdn.com%2Fcontent%2Fimage%2F1-s2.0-S0013468610006274-gr2.jpg&imgrefurl=http%3A%2F%2Fwww.sciencedirect.com%2Fscience%2Farticle%2Fpii%2FS0013468610006274&docid=uh1jH_NBTGtnKM&tbnid=syZp48MlBN0cPM%3A&vet=10ahUKEwjz2eev8YvVAhUFthoKHe7kBqcQMwh4KAAwAA..i&w=376&h=489&bih=662&biw=1366&q=F%C3%B3rmula%20desarrollada&ved=0ahUKEwjz2eev8YvVAhUFthoKHe7kBqcQMwh4KAAwAA&iact=mrc&uact=8

You may have luck with these complexes, but I'd rather recommend trying a series of relaxed/rigid scans and at least qst3 if not a straight TS search. PM6 does wonders for such things

Thank you Martin - マーチン♦. I will check if my error is solved with PM6, because I can not optimize the TS by direct assumption due to the enormous steric impediment


text:
Relationship between freezing point depression and hydrogen bonds
Freezing point of a solvent is decreased due to the added solute particles in it. e.g. ions. Therefore electrolytes contribute to lowering freezing point (say for physiological conditions).
If a particular solution has a non-electrolyte solute, which is held together with hydrogen bonds (forming dimers), I want to know how do H-bonds affect the freezing point?
Pure water itself forms hydrogen bonds. Yet at 0'C it freezes theoretically. How to approximate effect of the hydrogen bonds in an acidic solution to its freezing point depression theoretical value? e.g. do H-bonds in CH3COOH halve the freezing point depression value?

The addition of one molecule (or salt) of solute produces $i$ effective solute particles. This defines the van't Hoff factor $i$, which encodes the strength of the solute with regards to colligative properties.
As simple (idealized) examples in aqueous solution, the van't Hoff factor of $\ce{NaCl}$ is 2, as $\ce{NaCl}$ dissociates into the two solute particles $\ce{Na+}$ and $\ce{Cl-}$ in solution, whereas the van't Hoff factor of glucose is 1, as glucose doesn't dissociate in solution.
These examples are idealized because they do not account for intermolecular or interionic interactions, and are therefore most valid in dilute solutions. In concentrated solutions, intermolecular and interionic effects are much more important: the presence of hydrogen bonding, for example, or ion pairs in solution. Attractive interactions "clump" the solute particles together and reduce the van't Hoff factor.
Returning to your example, if we assume that acetic acid does not dissociate in solution, and we're under conditions in which acetic acid dimer predominates, then indeed we will have a van't Hoff factor $i = 0.5$. It's important to note, however, that the hydrogen-bonding interaction in acetic acid is particularly strong, and in general intermolecular and interionic interactions will not change the van't Hoff factor so drastically.
It remains to consider how to correct for these interactions. Intuitively, it seems to me that the activity coefficient should be the desired correction factor, considering that the activity coefficient is a measure of how ideal (interaction-less) a solution is, but I do not have a more quantitative argument along these lines.

Appreciate this answer. It's not that I haven't looked into it. I take time to really sort things out and understand - because there are new theories I want to do some calculations. Once I do, I will reply to you again.

source: Stack Exchange

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/

sort: votes


text:
When precessing nuclei absorb electromagnetic radiations, am I supposed to consider the wave nature or particle nature of light?
I decided to peek into the NMR section of my spectroscopy book, and I found out lots of interesting stuff. And along with the interesting stuff, I did come along some stuff that I didn't understand.
My book tells me this (not copied word-for-word):

Under the presence of a magnetic field, the two spin states of the nuclei split up, and if a correct wavelength of light is incident upon the sample, the nuclei get excited to a higher state.

In the above scenario, I believe light behaves as particles, wherein a photon of light collides with a nuclei, and push it to higher spin states.
There's another thing my book says:

Under the presence of a magnetic field, the nuclei begin to precess (wobble like  a spinning top). If an incoming electromagnetic radiation has the same frequency as the nuclear precession, resonance occurs and the light is absorbed.

In this case, light behaves as a wave. It's the oscillating electromagnetic fields that are resonating the nuclei, and causing them to absorb the energy.
Out of the two cases that I mentioned, which is the one that actually happens? Is it resonance or spin excitation?

Both... Excitation depends on resonance, a wave-like property. But the photon is absorbed all at once, particle-like behavior. Such is the nature of quantum phenomena. "We are the watching of our lives."

The nuclear spin states in an atom are excited by the radio frequency radiation and one spin state moves to the other when a photon is absorbed.
The precession part is not so much about individual spins but the magnetisation of the sample which is the vectorial addition of all the nuclear magnetic dipoles; precession is motion about the permanent magnetic field. (Think of the magnetisation rather like a vertical rotating pendulum if you want.) As circular polarised rf is used the spins are excited coherently and all add up 'in phase', hence a large magnetisation is produced that then undergoes precession. Loosing this phase in the magnetisation corresponds the the T2 decay time and loosing population excess above equilibrium to the T1 time. It happens that the precession frequency is the same as the transition frequency.
(An individual spin angular momentum is a complicated nutation type motion as it experiences a force from the rf field which is at right-angles to he permanent magnetic field.)

Thank you for your answer, but I have lots of questions. What are the T1 and T2 times?  What is nutation? And what is an rf field. I'm really sorry to bug you, but I'm just a beginner in spectroscopy.

Ok :)  These are things that are in the textbooks and too long to answer here except rf field which just means the radio frequency radiation used to stimulate the nmr signal.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
What is the relation between molarity and molality?
Is the following equation correct:
$$m = \frac{1000 \times M}{M \times M_b - 1000 \times d},$$
where $M$ - molarity, $m$ - molality, $d$ -density of solution, $M_b$ - molar mass of solute?

You need to mention the units you're using for density. Is it $\pu{g/ml}$, or $\pu{g/l}$ or $\pu{kg/m^3}$?

$$m = \frac{M}{\rho - (M \times wt. of solute)}$$
$m=$molality
$M=$Molarity
$\rho=$density
all quantities in S.I. units


text:
Solubility of Aluminium Hydroxide in Sodium Hydroxide Solution
Aluminium hydroxide is not soluble in water at all, however I was doing some reading and read that it does in fact dissolve in a solution of sodium hydroxide. This just seemed a bit strange to me, so I was wondering if anyone could explain why this is the case.

That's because $\ce{Al(OH)3}$ is amphoteric in nature, it acts as both acid and base. When you drop a chunk of $\ce{Al(OH)3}$ into a solution of $\ce{NaOH}$, you would get a soluble salt, which is sodium aluminate, and some extra water.
$$\ce{Al(OH)3 + NaOH -> Na[Al(OH)4]}$$

Out of curiosity, is there any way to tell if a substance is amphoteric?

@Etched Yes, but you need to analyze the structure carefully. In the case of $\ce{Al(OH)3}$, it acts as a base since the $\ce{Al-O}$ bonds are broken, releasing $\ce{OH-}$. But at the same time, it can also accept $\ce{OH-}$ due to the presence of empty electron orbitals that can grasp onto electrons. This is just a little description, if you need more info, please ask a new question.

Another major point is that saying"Aluminium hydroxide is not soluble" isn't entirely correct. When a chemist says "not soluble" what he is really saying is that the solubility is very low. // I'd write the reaction as $$\ce{Al(OH)3 + NaOH -> Na^+[Al(OH)4]^- }$$


text:
Methyl vs halogen : order of precedence

How would you name this compound?
4-bromo-2-methylpentan-3-one
or
2-bromo-4-methylpentan-3-one

https://chemistry.stackexchange.com/questions/68624

It should be named as 2-bromo-4-methylpentan-3-one
The alphabatical order should be followed while naming this compound. As b in bromo comes before m in methyl, it should be named as above.
See here and here for more information.

Do halogen and alkyl group have the same priority? I was confused since my teacher said alkane comes before halide in the order of precedence.

It depends on the type of group present. For e.g. Between ethyl and iodo, ethyl will have more priority according to alphabatical order. See the links for more info.

@mettledmike Also see this answer.


text:
What are the effects of Ag/AgCl electrode liquid interface contamination?
I have been using a $\ce{Ag/AgCl}$ electrode with a porous teflon tip to measure open circuit potential between two electrodes separated by a membrane. I am worried that reagents such as iron chloride and ferri/ferrocyanide have contaminated the teflon tip because we have not been using a salt bridge.
However, when I place two reference electrodes in the same solution, the difference in potential between them is very low, ~2 mV.
What effect could having reagents in the teflon tip have on my measurements?


text:
Chemistry Behind TMOS Aerogel
I have found out how to make silica aerogel.
Currently, the solution that I need to create to make the gel requires tetramethyl orthosilicate or TMOS. Furthermore, the procedure requires me to make two solutions, one being a catalyst and mix the two.
What is the chemistry that goes on to form the ultra-light gel? How does the TMOS, methanol, and ammonium hydroxide interact with one another? What are possible substitutes for TMOS, that are less expensive?


text:
Rate-determining step of Claisen condensation
If we use a strong base such as LDA or NaH, will the rate-determining step  change from enolate ion formation to nucleophillic acyl substitution step?

If you use a strong base, you're likely generating the enolate in full before adding the electrophile, so I believe this question is moot.

@Zhe If the reaction were to be carried out in protic solvent, won't it also aid carbonyl bond polarisation.

But you choose when to add the electrophile. The rate effects are totally irrelevant.

Let me get one thing out of the way: the hydride ion is not a strong base. However, once a hydride is protonated to form hydrogen the acid-base equilibrium breaks down which makes it seem like a much stronger base than it is.

Your actual question — whether the rate determining step shifts from enolisation to nucleophilic attack — is harder to answer as it depends primarily on the reaction procedure. It depends on how you perform the Claisen condensation and what you want to condense.
If, for example, you want to perform a hetero Claisen condensation (i.e. couple two different esters), then you would typically make sure to deprotonate quickly and kinetically first before adding the second ester as Zhe implied in a comment. For example, you might use a very strong base such as $\ce{BuLi}$ or $\ce{LDA}$ for complete deprotonation followed by a waiting period before adding another ester. In this case, obviously you are already dealing with a fully deprotonated product and the question becomes moot.
However, you might be interested in a homocoupling product. For that, you might decide to add $\pu{0.5eq}$ of a strong base to a solution of the ester. Here, it depends on whether you add the strong base rapidly, (meaning the acid-base equilibrium is practically immediately established, in turn meaning that now the attack becomes rate determining) or whether you add it slowly and dropwise, in which case you, the experimentor, are rate-limiting deprotonation and causing it to remain the rate-determining step.

Finally, in response to a comment you gave under your question: You cannot use a protic solvent if you intend to use a strong base. Protic solvents are characterised by a relatively high acidity, meaning they will be deprotonated long before the ester ($\mathrm{p}K_\mathrm{a} \approx 25$). This will completely inhibit the reaction.

If a strong base is used, it will result in complete formation of the enolate ion, while in weaker bases, the RDS is enolate ion formation as it is a very weak acid.


text:
Book for understanding synthetic techniques and apparatus(with pictures)
Is there a standard textbook to understand numerous laboratory apparatus and the assembly techniques that can be used for carrying out different processes (with pictures)?
I have been cleaning my lab and came across many glassware, from as long as 30 years back which I don't have much idea about regarding their usage. Is there a book which could help me in understanding and classifying the resources that i have with me?
P.S. I am in a crystal engineering lab.

The best is rather subjective I'm afraid. Here at SE (sorry I forgot to welcome you, anyway, welcome), we deal with questions that can be answered with facts, and that have a definite answer. However, your question demands opinions, rather than facts, so we can't give you a precise and definite answer. Voting to close this question as Primarily opinion based. Since you're looking for resources (something asked commonly), we have a list of books. Do check it out: Resources for learning Chemistry

And I am sorry , though I know I should not be asking questions with multiple answers or subjective interpretations , but I had exhausted all my resources of getting good advice on this question. Sorry again. Cheers mates and thanks for help, please proceed as per policies. Thank you

Come to think of it, we don't have a section for practical/experimental chemistry within the Resources post. This would be a decent addition, since there are, of course, plenty of books on the topic.

I agree. This should have been closed if and only if there already is a pertinent subsection on the topic of resources. The OP knows that book opinions will differ. The word 'best' is hence not an issue (in my opinion); it could have been edited without changing the meaning of the question.


text:
Is there a flame which doesn't produce smoke?
I read a book where it states the following:

There are many types of flame and there are even flames which don't produce smoke.

After reading it, it just got stuck in my mind. Is there a flame which doesn't produce smoke? If so, then what is it?

I would say burning $\ce{H2}$ wouldn't produce any smoke, only water vapour ($\ce{H2 + \frac{1}{2} O2 -> H2O}$).

@FelipeSchneider You told while burning H2 there won't be smoke but look at the Hindenburg Disaster. https://en.wikipedia.org/wiki/Hindenburg_disaster

You also get smoke if you have an incomplete oxidation due to lack of oxygen. The smoke may then consists of all kinds of ugly compounds.

Methane, used in UK, for domestic cooking, doesn't produce smoke

@KarthikSrivijay Hm, it's not really that good a comment anyway. Basically, smoke is a mixture of unburnt stuff. I think by your question you meant "visible smoke", right? Since what is normally meant by smoke is mostly ash and stuff like that, anything that burns without visible gaseous or floating solid products can be considered "smokeless"

Alcohol, acetone and diethyl ether also burn with no visible smoke. A hydrogen flame is nearly invisible, as well as completely smokeless.

Yes there are flames that don't produce smoke, but the reasons are complicated and depend on the conditions of the flame as well as the key fuel creating the flame.
The obvious example of a smokeless flame is hydrogen. Pure hydrogen burns in oxygen to generate water and there is no way the normal things that make up smoke can be created (the smoke in the Hindenburg disaster was the gas-bag covers and the content of the zeppelin so isn't a counter example).
Smoke is, technically, a suspension of particulate matter in the atmosphere. Some things look like smoke but are not (like the steam from a kettle or the steam from a power station cooler tower: these consist of liquid droplets that will evaporate).
The main reason smoke is created is because the reaction driving the flame is incomplete and can produce small particles of something. Flames involving hydrocarbons, for example, have side reactions that can generate small particles of solid carbon or a variety of long-chain or aromatic hydrocarbons. The amount of these side-products (and therefore smoke) depends not just on the fuel but also on the control of the combustion reaction.
Flames where the fuel-air mixture is well controlled may generate very few side reactions and no visible smoke (most gas cookers are set up carefully and don't generate visible smoke, but a badly controlled Bunsen burner using the same fuel may be very smoky.) Candles emit smoke by design as this is what generates most of the light in the flame (small particles of carbon become incandescent in the flame emitting yellowish light just because they are hot). Candles are, essentially, optimised (by the design of the fuel and wick) to burn this way. But small burners using wicks and fuels like ethanol or acetone burn with very clean flames with little smoke (they are optimised for heat not light).
Even fairly well optimised burning can produce some smoke. Modern vehicle engines, for example, need to be as efficient as possible but both petrol (gasoline) and Diesel engines still emit some small particulates (though there are often not very visible unlike old Diesel engines which were notoriously smoky). A badly controlled flame from burning hydrocarbon can be very smoky, but a well controlled flame will emit little visible smoke.
Some things are hard to burn efficiently and smoke a lot. Raw coal is full of a variety of things other than carbon (lots of S and N containing compounds, for example) when burning these tend to promote smoky emissions and also release nasty sulphur and nitrogen oxides into the atmosphere which may react further creating more smoky nasties. "Smokeless" coal is heated before use to drive off the worst offenders and results in something like a halfway house on the way to charcoal, which is almost pure carbon. But the burning of solids is always harder to control than gases or liquids and even charcoal will emit small amounts of smoke though it may not be visible.
In short, smoke is usually a product of the control of a flame. Badly controlled flames smoke because side reactions are common; well controlled flames mostly don't as burning goes to the final products (CO2 and H2O which are gases). Well controlled flames don't smoke visibly (but might emit small amounts that are hard to see for some fuels). Pure hydrogen can burn with no smoke at all as there are no possible side reactions.

The chemical process in which a substance reacts with oxygen to give
off heat is called combustion. Substances that undergo combustion are
called combustible substances. It may be a solid, liquid or gas. Along
with heat, some produce flame or glow. The substances which vaporize
during burning give flames. For example combustion of kerosene oil
produces flame. On the other hand certain substances like coal do not
vaporize and hence doesn’t produce a flame.
A side effect of the chemical reactions (that takes place while a
substance is burning) is a lot of heat. The chemical reactions in a
fire generate a lot of new heat that sustains the fire. Many fuels
like gasoline burn in one step. Heat vaporizes gasoline and it all
burns as a volatile gas. There is no char. A candle  on the other hand
slowly vaporizes. As they heat up, the rising carbon atoms emit light.
This "heat produces light" effect is called incandescence. This is
what causes the visible flame. Flame color varies depending on what is
burnt and how hot it is.
Imagine burning a piece of wood. Some of the decomposed material is
released as volatile gases. These gases form  smoke. Smoke is compound
of hydrogen, carbon and oxygen. The rest of the material forms
charcoal, which is nearly pure carbon, and ash, the unburnable
minerals in the wood (calcium, potassium, and so on). Charcoal is wood
that has been heated to remove nearly all of the volatile gases and
leave behind the carbon. That is why a charcoal fire burns with no
smoke.

Source: Attempt N Win

Not, though 100% reliable. Coal clearly burns with a flame and even charcoal is not entirely flameless. And gasoline will produce smoke depending on the conditions of burning. So I think the explanation is missing at least some of the explanations for smoke.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
Laser cutting polyurethane
I have a laser cutting machine and have always read that PVC (or types of vinyl in general) are very dangerous to cut because it releases chlorine gas. Chlorine gas = toxic (plus the $\ce{HCl}$ thing), makes perfect sense. This is well-known information, and it appears all over informational sites about laser cutting materials.
But I now have someone telling me that cutting polyurethane foam is way more dangerous since it produces $\ce{HCN}$ (hydrogen cyanide). Now this information is in a few limited places and mentioned kinda as a footnote, but it is not nearly as widespread as the Chlorine gas risk that is talked about elsewhere. (I know that the chlorine gas presence during cutting of PVC effectively makes $\ce{HCl}$ gas which clearly does damage to the laser system itself which might explain the prevalence of the information about it)
I know that polyurethane is made with isocyanates, which I'm wondering if that is leading to some people seeing that word and getting confused and assuming that there is cyanide in the foam.
What is confusing about this is that polyurethane is generally considered a very "laser friendly" material to cut (stated all over the place on laser hobbyist and professional websites under acceptable materials). Producing the same gasses that were used in gas chambers seems pretty darn dangerous and like there should be red flags up everywhere.
Can anyone here speak to the chemistry of this, firstly how much if any HCN would be produced by using a 10000nm laser (meaning intense heat based, but nonflaming cutting). And if properly ventilated with an industrial air handling system designed to move the full volume of air from the cutter outside many times per minute what are the risks involved? Does HCN bond to surfaces and stick around? Or is it fully ventilated as a gas?
Clearly, I'm not a chemistry expert, so looking for someone who knows more to help me understand what is real and what is just scary sounding jargon.

Pyrolysis of PVC produces a whole slew of toxic chemicals, some worse than chlorine. See http://fire.nist.gov/bfrlpubs/fire87/PDF/f87015.pdf  Carnegie-Mellon recommends a good exhaust system for any laser cutting: https://www.cmu.edu/ehs/fact-sheets/laser-cutter-safety.pdf

Thank you @DrMoishePippik, do you have any other information about the cutting of polyurethanes similar in scope to that NIST article you shared? That was extremely informative about PVC, but as I was using PVC as my null hypothesis case of what NOT to do, why not to do it wasn't my specific concern!

There are a number of sites about laser cutter exhaust systems or vent hoods, e.g. https://www.engraversjournal.com/article.php/2780/index.html , http://www.instructables.com/id/Build-a-laser-cutter-fume-extractor/ and commercial products. Ideally, the exhaust should go through some sort of filter, e.g. a water bath, to remove toxic chemicals (e.g. HCl)  that would have been vented outside.

From SDS:

Special hazards arising from the substance or mixture: carbon oxides ($\ce{CO}$, $\ce{CO2}$) nitrogen oxides ($\ce{NO}$, $\ce{NO2}$ etc.) hydrocarbons, isocyanate vapours and hydrogen cyanide can be released in case of fire.
The polyurethane polymer formulation greatly changes so the fumes that can form at high temperatures (above the melting temperature). A suction and exhaust system is required. To measure the exact composition of the fumes very specific analysis are required.

Can you share the link to the SDS that this came from? Clearly a suction and exhaust system is present, but what is the likely hood of these elements being produced with no flame present?

I cannot share SDS from my suppliers, you can read this document. https://goo.gl/A452Cu

So based on what I'm reading here, I would say that cutting polyurethane foam may generate some hydrogen cyanide, but not really much more than even burning wood would. So it can be GRAS? (with the proper ventilation system in place). Is that the conclusion that you are driving at?

Yes, it can be safe. Think to TPU melting between 160-200°C. As long as the temperature is below 250° C you may assume that no combustion occurs (DSC test).

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
Can hydrogen alone be used as a fuel?
I have always thought that fuel such as petrol, diesel, etc.. are getting burnt to move the pistons in the car (I am not bringing physics). Why not hydrogen be used as a fuel but I know the incident about the Hindenburg disaster but the same thing could happen if the Hindenburg was filled with petrol. So both are combustible and that's what we want. Petrol and hydrogen do the same job what does he want?. When hydrogen is burnt there won't be any smoke so it saves the environment. PS: This is just to make this question a bit understandable I have mentioned the word "Alone" in the question.

Hydrogen-powered cars exist. The energy density of petrol is higher than hydrogen and hydrogen is more difficult to store safely which why we use petrol and diesel.

Hydrogen-powered cars exist?? I never knew about that @Waylander.

But @Waylander Hydrogen is abandoned around the world!

Hydrogen alone can't be used as a fuel, you need an oxidizer like Oxygen, or Nitrous Oxide. But, of course, you might've assumed it, and not mentioned in the question.

I will of course assumed it but anyways thanks @PrittBalagopal

Hydrogen isn't abandoned, it's still used a little, but it's very hard to store and carry around. Sometimes, the very fact that it's a ridiculously great fuel makes it hard to use, quite ironic.

But @PrittBalagopal why can't we store Hydrogen as CNG?

By the way, do check out: Hydrogen as a fuel really worth it? The answers claim that hydrogen is hard to store in vessels as it's hard to leak.

Another advantage over petrol is that with hydrogen, the world probably won't have to prop up oil states anymore.

Hydrogen fits the definition of a fuel perfectly. Of course it can be used alone. Whether or not it's practical or better is the issue.

@PrittBalagopal Your comment with "alone" only confused OP; fuel and oxidiser are different things, and are treated separately.

@Mithoron Probably, thanks for setting things straight now.

Hydrogen fuel cells which produce electricity from hydrogen and oxygen are over a hundred years old
https://www.thoughto.com/hydrogen-fuel-cells1991799
The recently introduced Toyota Mirai runs off a hydrogen fuel cell.
www.toyota.co.uk/mirai  as does the Hyundai ix35 and Honda Clarity
There aren't very many hydrogen filling stations in the UK which rather limits its appeal

You should try to improve your answers: make links, elaborate, maybe add pictures and stuff. They seem rather comment-like.


text:
Which unit should I use for m/z?
I've written m/z 300 - 1100 Thomson in my paper, but the reviewer said that the Th is already deprecated. So in this case, should I write m/z 300 - 1100 Da? or something else?

Generally I see it written as a dimensionless number, and occaisonally as dalton. Not dalton per anything, so calling it dalton is somewhat incorrect, but I have seen a few published papers on mass spectroscopy written as sch.

TL;DR: According to current IUPAC recommendations, $m/z$ is an abbreviation for a dimensionless quantity.
Use of thompson unit is indeed currently discouraged.

There is an overview provided in Definitions of terms relating to mass spectrometry (IUPAC Recommendations 2013) [1, p. 1516] (reference numbers were updated):

Labeling of mass spectra
The labeling of the $x$-axis of a mass spectrum engendered the most discussion during the creation of this document; however, in spite of a general desire for a better way to label the $x$-axis of mass spectra, there was no broad consensus for any of the proposed changes.
Therefore, this document continues the use of the definitions of the Gold Book [2] and the similar definitions in the Orange Book [3].
The Gold Book recommendation is for the use of $m/z$ as an abbreviation for mass-to-charge ratio, a dimensionless quantity obtained by dividing the mass number of an ion by its charge number [2].
The thomson unit, defined as the quotient of mass in units of u and the number of charges $(z),$ was proposed nearly two decades ago [4], but has not been widely adopted and is therefore not recommended.
Labeling the $x$-axis of a mass spectrum with any unit of mass such as dalton (Da), atomic mass unit (amu), or unified atomic mass unit (u) is strongly discouraged due to the confusion that would result when reporting spectra of multiply charged ions.
The quantity plotted on the $x$-axis of a mass spectrum is a function of both the mass and charge of the ion.
Furthermore, the use of amu in place of u is strongly discouraged in all cases; it has been used to denote atomic masses measured relative to the mass of a single atom of $\ce{^{16}O},$ or to the isotope-averaged mass of an oxygen atom, or to the mass of a single atom of $\ce{^{12}C}.$

Gold Book entry:

mass-to-charge ratio, $m/z$
The abbreviation $m/z$ is used to denote the dimensionless quantity formed by dividing the mass number of an ion by its charge number. It has long been called the mass-to-charge ratio although $m$ is not the ionic mass nor is $z$ a multiple or the elementary (electronic) charge, $e$. The abbreviation $m/e$ is, therefore, not recommended. Thus, for example, for the ion $\ce{C7H7^{2+}}$, $m/z$ equals $45.5$.

For historical purposes the aforementioned letter to the editor regarding the thompson unit [4]:

The ‘Thompson’. A Suggested Unit for Mass Spectroscopists
Recently, there has been a rapid increase in experiments in which multiply charged ions are generated in mass spectrometers. […]
This welcome development has permitted the measurement of molecular weights of compounds having masses in the tens and even hundreds of kilodalton.
These advances exacerbate the problems caused by a common imprecision,
namely, the terms mass measurement, mass range, etc are often used imprecisely when mass-to-charge ratio measurement, mass-to-charge range, etc are intended.
The implicit assumption that the charge state of ions is unity is no longer valid when very-high-charge states are so easily produced.
With this letter come two suggestions.
First, that the longer but more exact term be used whenever it is intended.
Second, that a unit of mass-to-charge ratio be adopted.
After all, this is the quantity which all mass spectrometers measure; mass is a derived quantity requiring an independent measurement or knowledge of charge.
Such a unit would be defined as the quotient of mass, in units of u* and the number of charges, $z.$
The number of charges could be positive or negative, depending on the sign of the charge.
The name Thomson suggests itself in view of J. J. Thomson’s contributions to measurement of this quantity and his preeminent role in the evolution of mass
spectrometry.
Using standard rules for abbreviation, we have $\pu{1 Th} ≡ \pu{1 u/\text{
atomic charge}}.$
For example, the molecular weight of the peptide myoglobin (isotopic average molecular weight $\pu{16950 u})$ can be deduced from measurement […] of a peak at $\pu{998.0 Th},$ provided it is known that the ion bears $17$ charges.
If this suggestion is accepted, other simplifications ensue.
For example, the benzoate anion, mass $\pu{121 u}$ and charge $-1$ atomic units, is $-121$ Thomson not $m/z~121.$
The latter is actually the mass-to-charge ratio of the corresponding (unstable) cation!

* $\pu{1 u} = m(\ce{^{12}C})/12 = \pu{1.660540E-27 kg}$
The unified atomic mass unit, $\pu{u},$ is also known by the name Dalton and
the symbol $\pu{Da}.$

References

Murray, K. K.; Boyd, R. K.; Eberlin, M. N.; Langley, G. J.; Li, L.; Naito, Y. Definitions of Terms Relating to Mass Spectrometry (IUPAC Recommendations 2013). Pure and Applied Chemistry 2013, 85 (7), 1515–1609. DOI: 10.1351/PAC-REC-06-04-06. (Free Access)
IUPAC. Compendium of Chemical Terminology (the “Gold Book”). Online Version (2019-) Created by S. J. Chalk, 2nd ed.; McNaught, A. D., Wilkinson, A., Eds.; IUPAC Recommendations; Blackwell Scientific Publications: Oxford, 1997.
IUPAC. Compendium of Analytical Nomenclature (the “Orange Book”), 3rd ed.; J. Inczédy, T. Lengyel, A. M. Ure, Eds.; IUPAC Recommendations; Blackwell Science: Oxford, 1998. https://media.iupac.org/publications/analytical_compendium/
Cooks, R. G.; Rockwood, A. L. The Thomson-A Suggested Unit for Mass Spectroscopists. Rapid Communications in Mass Spectrometry 1991, 5 (2), 93–93. DOI: 10.1002/rcm.1290050210.

Of course, the IUPAC books are the description of chemistry. No book should explain anything more clearly than those books.

Oh, but @Loong is the IUPAC here :D He even informed them about mistake in one of "Books" :)

This answer is wrong.  m/z is not dimensionless and does not involve mass "number".

According to IUPAC, $m/z$ is a ratio of "mass number" to "charge number".  Also according to IUPAC, "mass number" is the number of nucleons (protons + neutrons) in a nucleus.  According to IUPAC, saying that the $\ce{H3O+}$ ion has an $m/z$ of 19.01786 is wrong, because 19.01786 is the ratio of the mass, not the mass number of the ion in Da to the charge in elementary units.  Unfortunately for IUPAC but fortunately for chemistry, few working analytical chemists use their silly rules.

The m/z ratio is not dimensionless.  Mass to charge ratio should have a unit, since mass has an IUPAC unit (kg) and charge has an IUPAC unit (coulomb).  To me, Thomson is the convenient unit for biochemists, as we often use Dalton for molecular weight instead of kg.  Thus, I felt thankful that it was proposed.  If we don't like or don't adopt the Thomson unit for any reason, we'll have to propose a new unit or spell it out as Dalton/charge#.
Until the mass spec community proposes a different unit, I will continue to use the Thomson unit because it is the only available unit out there that is convenient for biochemists.  The change in mass (delta mass) would lead to a change in m/z.  Without a unit, how do we refer to the delta m/z?  For a charge state of 4, the delta m/z between isotopes would be 0.25 Th.  Without the unit, how do you communicate it effectively?  Now that charge detection is evolving, we'll have m/z in the tens of thousands.  Will we say 80,000 m/z? Or would we say 80 "k-m/z"?  I would say "80 kTh" would be a much more effective communication.  Similar to charge detection, MALDI also gives signals in the thousands.  When we have mass spectra with tens of thousands in the m/z axis, it would be cumbersome to label the axis with all the zeros spelled out (such as 130000, 140000, 150000, etc...) if we simply label the axis as m/z.  The better way to communicate would be using the kilo pre-fix just like any other unit, label our axis with 130, 140, 150,... and call the unit as kTh.  If my reviewer doesn't let me use Thomson, I can imagine myself calling the axix "m/z (x10e3) to work around the issue.  But kTh would be scientifically correct as m/z does have units, and it would be a more effective communications.
I think the Thomson unit is not widely adopted because many professors don't teach it in schools.  If you work in the academia, please make sure to teach your students about the unit of m/z.  Unlike absorbance or pH, m/z is NOT unitless.  When we teach Thomson in school, it'll be adopted similar to ampere (A), Celsius (°C), hertz (Hz), joule (J), newton (N), volt (V), watt (W). I think it's not widely "taught" because m/z is probably not taught until grad school, unlike other physical properties.  If you are a reviewer, please have an open mind to see the author's perspective if they write the unit Thomson.  If you don't like Thomson for any reason, then please propose another unit and influence the mass spec community in that direction.  But we can't say m/z is unitless, nor it is Dalton, or amu, or any other neutral mass unit.  Many thanks!
To answer your question specifically, if you can't convince your reviewer about the unit Thomson, you probably have to spell it out such as Da/charge#, kDa/charge#, or use "m/z" as if it is the unit, such as 12345 m/z, or 15x10e3 m/z.  It doesn't make sense to me, but I do understand that we are obligated to address our reviewers' requests.  Good luck!

The answer is not easily readable, the use of the unit $\pu{1 Th} = \pu{1 Da}/\pu{e}$, based on Wikipedia's article apparently did not find wide adoption: «The thomson is not an SI unit, nor has it been defined by IUPAC.  Since 2013, the thomson is deprecated by IUPAC (Definitions of Terms Relating to Mass Spectrometry). Since 2014, Rapid Communications in Mass Spectrometry regards the thomson as a "term that should be avoided in mass spectrometry publications."»


text:
How can I identify corrosion products of steel using simple process?
As I am doing a research regarding corrosion, I immersed a steel in a salt solution. Different colors of corrosion products were produced as time passed. There are blackish, the usual red-brownish, and others. But, I need to identify if it is really corrosion products or the iron oxide to confirm that they are not just contaminants. What is the simplest way possible?


text:
How would the "-COOCl" functional group be named?
I had this question that has been bouncing around for over 2 years.
Basically, I imagined this molecule:

This molecule, as far as my former (and current) knowledge of nomenclature goes, couldn't be named by me. At first, when I was in about 11th grade, I used to call it ethanoyl chloride, but that's $\ce{CH3COCl}$, not $\ce{CH3COOCl}$. I then thought of chlorine ethanoate, but that's ridiculous too, since chlorine can accomodate the negative charge way better than what ethanoate can.
I looked up a while, and I found out that the $\ce{-OCl}$ group is called a hypochlorite group, that gave rise to my third guess ethanoyl hypochlorite. However, I am not very sure about that either, since ethanoic acid isn't called ethanoyl hydroxide, and methylmethanoate isn't called methanoyl methoxide.
So, what would the "-COOCl" group be called?

Chemspider says it's 1-(1-chlorooxy)-oxoethane

One could be tempted to name this compound as acid halide of a peroxycarboxylic acid (here: ethaneperoxoic acid). However, the compound could also be named as anhydride of acetic acid and hypochlorous acid. Note that anhydrides have a higher priority in the seniority order for classes than acid halides.
And indeed, the current version of Nomenclature of Organic Chemistry – IUPAC Recommendations and Preferred Names 2013 (Blue Book), mentions that hypochlorous acid forms anhydrides such as benzoic hypochlorous anhydride. The rule for the formation of such names can be found in Subsection P-67.1.3.3:

P-67.1.3.3 Anhydrides of mononuclear noncarbon oxoacids
Neutral anhydrides formed between acids named by suffixes and mononuclear noncarbon oxoacids described in P-67.1.1 are named in the same way as described for anhydrides derived from carboxylic and the sulfur acids named by suffixes (see P-65.7). The names of the acids are cited in alphabetical order followed by the class name ‘anhydride’ prefixed by a numerical term indicating the number of anhydride linkages (the numerical prefix mono is not used); such names lead to preferred IUPAC names. (…) Halogen oxoacids form anhydrides with carboxylic and the sulfur acids expressed by suffixes.
(…)

Therefore, the preferred name for the compound that is given in the question is acetic hypochlorous anhydride.

Acetyl hypochlorite  CAS 758-11-2

I suppose there isn't really much to be explained here, but some sort of elaboration would (in general) be helpful.


text:
Does chlorine in tap water usually contain traces of cornstarch (or its derivatives)?
My online research has been challenging to separate out the corn starch or corn Derivatives that are obviously in chlorine weather in tap water or pools. There is medical support backing it but I would also like to hear from the wisdom from this sight. Still learning how to ask questions in the correct format. Thank you for your time.

"Trace of corn in chlorine?" I'm sorry, could you be a bit clearer on what you mean my this? Furthermore, why bring up polycaprolactone?

Just trying to find out if there is any corn product of any kind in chlorinated tap water and in chlorinated pools. Corn can be used as a declumping agent from what I understand

Containing corn is kind of a weird way to put it. You'd need to be exposed to some macromolecule in corn (or smaller I guess if you have the haptens). In tap water, molecular chlorine is likely used, so it is unlikely that any corn allergen survived...

@Zhe Apparently pool chlorine is supplied in tablet form, e.g. this. I suppose there could be corn-based excipients used in the pressing of such tablets? Not sure how to determine this w/o looking at the ingredients of each product, though. I agree municipal water treatment is likely to use molecular Cl2, though.

Zhe is right. Saying that chlorine contains corn isn't quite right, since chlorine (or chlorides) in pool water just consist of simple single molecules, but corn is a vegetable (or cereal?). Corn is made up of millions of molecules in an orderly fasion, they are biological organs. However, there may be a certain chemical (polycaprolactone) in starch that's causing your son's allergy.

So would it be more appropriate to ask if there is corn starch in chlorine? That is what was originally used before Polycaprolactone was created and cross linked I believe with DMS I think it's called.

---@hBy2Py --- Brian, cornstarch decomposes to (pyro)dextrins on heating. Not sure about chlorine tablets, but dextrins are used as a binder in fireworks. Then again, dextrins aren't unique to corn, so blaming it for "Corn allergy" sounds a bit off :P

Neither polycaprolactone nor DMS would be primary ingredients in the disinfectants used for municipal or pool disinfection.

Robbe, there's no conclusive answer here. Corn residues would only come from excipients used while pressing tablets of chlorine chemicals, and all of the products I've seen claim to be pure chemicals. Further, the concentration of corn residues would be less than that of the added chlorine, and the amount of chlorine itself in treated waters is quite small. It's much more likely that your son is reacting to the chlorine itself, which some people can be sensitive to, and not any trace corn ingredients.

Surely, you must mean cornine, the principal agent of corn starch?

Chlorine itself will not trigger a corn allergy.  That's simply because chlorine is a simple molecule and corn is a complex smörgåsbord of complex organic molecules.  However, pools do not typically use pure chlorine gas.  They use a compound such as troclosene sodium which is a powder that releases chlorine as it dissolves.  The manufacturing of such substances may include additives that could cause problems.  In particular, they may use cornstarch as an anti-caking agent.
Given that the material your son may be having issues with is not an active ingredient, tracking the problem down may be difficult.  Different manufacturers may use different mixtures.  It's entirely possible that one supplier might use cornstarch for anti-caking, while another uses sodium silicate.  Wikipedia's page on anti-caking additives doesn't even list cornstarch, so its reasonable to believe its rare in such products, but that doesn't mean your local pool's particular vendor doesn't use it.  You'd have to do some digging.
It is far less likely that tap water will have similar issues.  There's a simple reality that if drinking water caused harm to those with corn allergies, you wouldn't hear it from "hundreds of people," you would hear it from the doctor because it would be documented scientific fact.  I would recommend asking your doctor about the feasibility of such a source of corn derivatives.  That being said, do note that many bottled water sources are actually municipally sourced, meaning all they're giving you is tap water.
An interesting test (if you're willing to experiment!) would be to use an activated carbon filter such as those in Brita water pitchers instead of bottled water.  The activated carbon will remove the chlorine, but will most likely not interact with any corn derivatives in the water.  If your son has trouble with that brita water, but not bottled water, that's an indication that you may indeed be right about what's in the water.  If your son can drink the brita water just fine, that suggests it might actually be the chlorine that your son is responding to, independent of any corn allergy.
(Obviously use common sense: as a parent, your child isn't a guinea pig.  Learn what you can from experiments, but don't subject your child to anything life threatening in the name of science.  That'd just be silly!)

Thank you for the great information. Also I can only use certain bottled water since a lot of them are tap water and some of the plastic bottles have corn derivatives in the plastic.

Note to editors: please don't remove disclaimers.  People tend to put them there for a reason.


text:
Is it possible to make Spider-Man like web fluid?
Is it possible to make a substance which has similar properties to that of the web fluid e.g. High tensile strength, sticky (can attach to most of the surfaces), evaporates within few hours etc. If so then how?

What are the properties of the said "web fluid"? 8D

I suspect it is the same fluid used in GITS SAC S01E02 "Testation" in order to stop the tank when major Kusanagi failed to open the tank's hatch. I think this is a decent question.

I'm not sure if this question is broad. The OP isn't asking how to do it, he is asking if it's possible to do it.

There was similar question on Quora

https://www.wired.com/2014/04/the-physics-of-spider-mans-webs/

@PrittBalagopal Yes, OP is asking how, in the last sentence.


text:
Atoms or molecules with spin 1 in the ground state?
Is there any atom or molecule that has spin 1 in its ground state?
Do Hund's rules keep this from happening for an atom?
The reason I'm curious is that it would be nice to have a spin-1 example for use in pedagogical discussions of the Stern-Gerlach experiment.
[EDIT] Clarification: when I say "spin," I mean the total angular momentum, not just the sum of the spin-1/2's. (The total angular momentum is what you are seeing in the Stern-Gerlach experiment.) I deleted the part of the question about ions, because, as pointed out by Orthocresol, they won't be usable in a normal Stern-Gerlach spectrometer.

Atoms
Atomic carbon with its $\mathrm{1s^2 2s^2 2p^2}$ configuration has a triplet ground state ($S = 1$), precisely because of Hund's first rule.
However, in the context of the Stern–Gerlach experiment, you might run into a problem with orbital angular momentum, as carbon's ground state also has nonzero orbital angular momentum ($^3\mathrm{P}$ ground state, $L = 1$). The behaviour in a magnetic field will be rather more complex and you probably need to take into account spin-orbit coupling.
At the moment I can't think of any atoms with a ground state term symbol of $\mathrm{^3S}$. I actually suspect that it's impossible, but I'm not really up to proving it right now.

Molecules
Triplet dioxygen (as Zhe mentioned) has no orbital angular momentum ($^3\Sigma_\mathrm{g}^-$ ground state, $\Lambda = 0$), but I'm not sure if the inhomogeneity of the electron density would have any impact. (as in, I'm genuinely not sure.)

Ions
The $\ce{^2H}$ nucleus (which us chemists usually refer to as $\ce{D+}$) is lightest stable nucleus that has a spin of 1.
The only issue is that you then have a moving charge in a magnetic field. If you could ignore it, or take it out of the equation, somehow...

The behaviour in a magnetic field will be rather more complex and you probably need to take into account spin-orbit coupling. I don't think the behavior of a neutral particle can be any more complicated than what it normally is in a Stern-Gerlach spectrometer. The Hamiltonian is $z\mu_z=zgJ_z$, and spin versus orbital angular momentum would just have an effect on $g$. I'm not sure if the inhomogeneity of the electron density would have any impact. For similar reasons, I don't think this matters.

In an S state (L=0) you only have room for 2 electrons, one with spin up, one down.

@BenCrowell It is slightly complicated (imo, at least!). The coupling of the angular momenta $S$ and $L$ gives rise to the overall angular momentum $J$, which takes values $0, 1, 2$ (Clebsch-Gordan series). Of the three resulting terms, Hund's third rule dictates that $\mathrm{^3P_0}$ is the ground state (which unfortunately isn't magnetic). $\mathrm{^3P_1}$ (the one we're probably interested in) and $\mathrm{^3P_2}$ are then $16$ and $43~\mathrm{cm^{-1}}$ above the ground state.

@orthocresol: You're talking about carbon, right? All I'm saying is that a Stern-Gerlach experiment only measures the component of the total magnetic moment along the field axis. There is no other, independent parameter that is available to measure. The angular momentum isn't independent of the magnetic moment. Since the first two excited states are so close in energy to the ground state, they would presumably be equally populated in an atomic beam. Therefore you would probably see a superposition of results corresponding to the three energy states, each with its own quantized magnetic moment.

@BenCrowell Yes, I agree entirely. By "complicated", what I meant was that if you performed a S–G experiment on a beam of carbon atoms, the result wouldn't be a clean separation into three different beams. If you consider carbon to be a good enough example, though, then I guess we're done here? I was hoping to find an example that doesn't have a mixture of multiple states, but I don't think it's forthcoming.

Triplet oxygen has two unpaired electrons with the same spin, and a total spin value of 1.
In fact, by Hund's rule, the triplet states are preferred over the singlet states which have two electrons with opposite spins.
Further reading:

https://en.wikipedia.org/wiki/Triplet_oxygen
https://en.wikipedia.org/wiki/Singlet_oxygen

Rubidium 87 is one candidate, if you take into account hyperfine splitting, the ground state (which is part of hyperfine manifold of 5$S_{1/2}$) has total angular momentum $\vec{F}=\vec{J}+\vec{I}$ momentum of $F=1$.


text:
Can rays be considered a type of "element"?
First of all, I hardly know any chemistry, so let's get that out of the way.
I have been wondering lately, from a "common sense" point of view (owing to my lack of knowledge), that since air cannot be seen and yet is considered as being made up of elements, then what precludes us from considering things like x-rays and radio waves to be elements? What precludes the electromagnetic spectrum from being considered an element? What about light and gravity, or the four fundamental forces?
If someone could kindly elucidate the differences between these categories of "thing", I would highly appreciate it and it would remove much confusion that I have been experiencing lately.

They don't make anything what was considered a substance ages ago, so they aren't elements.

Element is a wrong word, but you could useparticle? That in fact, is a quite known fact: de Broglie wave-particle duality.

I'd argue that to be a proper chemical element, a "thing" must have rest mass, which would exclude photons.

Photons are NOT considered matter because they have no mass, since they don't interact with the Higgs field in a way the gives rise to mass. Since they don't have mass, they can't combine to give ordinary matter.

The very word "elements" is ambiguous, come to think of it.

@Zhe That is not true. The have relativistic mass-energy and can convert into for example electrons. Every particle is "matter" and I can't say I like this question here, it's rather for Physics.SE.

@Mithoron You're talking about pair creation, but the photon has zero rest mass. And it does not interact with the Higgs field in a mass-producing way.

Elements just refer to classification of matter based on atoms. Anything else is not an element. Otherwise, as you can tell from comments, you can really go off the rails in a non-helpful way... :)

@Zhe Yes, but neither do gluons, but without them protons wouldn't exist.

@Mithoron Everything you say is true. I'm trying to come up with a sensible dichotomy here, but I think I'm forced into the declaration by fiat. There are too many corner cases to account for otherwise...

I like your approach.  "Here's the way I see the categories of 'elements' and 'not elements.'  Why don't they draw the line this way?"  While the line you want to draw to include "rays" alongside the elements that appear in the air isn't the one science draws, it does point out something really important:  We draw the lines between categories because they are useful.
As it turns out, the differences in behaviors between "things" like hydrogen atoms and helium atoms, and "rays" like radio and X-rays is substantial as you dig further into science.  The "elements," for instance, are "massive," meaning they have mass.  Quantum scientists would say they "interact with the Higgs field."  However, you choose to phrase it, these atoms behave very differently than electromagnetic waves (which is the proper term for "rays").  Electromagnetic waves have no mass (other than some secondary effects that you'll learn when you study relativity).  Their propagation through space is well defined by Maxwell's Equations.  They don't form bonds (like most elements do).  They don't take up space (electromagnetic waves don't push each other away, but atoms can only get so close before the repulsive forces drive them apart).
Eventually, you will learn that the lines blur.  Relativity will start to explain why electromagnetic waves indeed do have mass.  Quantum Mechanics will point out that particles have wave-like properties.  However, as you start out, the differences between "massful things" and "electromagnetic waves" is pretty iron clad.  As you continue your studies, you'll become more comfortable with the distinctions drawn, and then we can slowly introduce all those strange line-blurring facets of our reality that makes science really interesting.


text:
Is 70% ethanol hygroscopic?
I'm currently doing some DNA extractions and I realized that in some protocols, they are saying that ethanol is hygroscopic.
In this source, they are saying:

85% ethanol made with nuclease free water (Note: 85% ethanol is hygroscopic. Fresh 85% ethanol should be prepared for optimal results)

I know that this is even more pronounced for 100% ethanol. But I'm wondering if

70% is still "very" hygroscopic so that the concentration might change over time. Say differently, how stable is the ethanol concentration over time?
How long would a 70% ethanol solution last? That may depend on the volume made, but I want to know if there could be a "best before" date so that it's no longer considered as a 70% ethanol solution. A year? A decade? 2 minutes?

I think real problem is that it's volatile

So still it would change the concentration... but how fast?

Sorry to state the obvious, but isn't it sufficient to keep it in a tightly closed bottle? Hygroscopic just means it will absorb water from the air, but if you don't let it in contact with moist air...; and it won't evaporate that much either. I don't see the problem, unless you plan to keep if for years and the concentration must be incredibly exact (like "70.00%"  rather than "70%", and that would be pretty hard to achieve in the first place, because, as you pointed out, pure ethanol is very hygroscopic).

It is in a tight closed bottle. I don't usually put ethanol in the sink to pipet-it! :D But if you're telling me that it doesn't really matter, then I'll just prepare a larger amount, and I'll reuse this for a longer period instead of making 70 % ethanol each time I need it! I was just wondering how freaked out I should be by the "hygroscopic" phenomenon for 70% ethanol and especially if it could affect my results.

@user6376297, If you have a source, I'd like to see it!

A source? Just physics and logic... If you mix two pure substances that don't react with each other and then you store the mixture in such a way that it is not exchanging matter with any external system, what do you suppose would make the composition change? I guess your source means that opening and closing the bottle several times (that's when matter, and in particular moist air, is exchanged), may alter the composition by enriching it in water. How 'freaked out' you should be depends on how important having an exact concentration is for your application, and only you can tell that.

Estimating the amount of water that is absorbed each time you open the bottle depends on many factors: how big the bottle is compared to the volume of liquid it contains, the size of the neck, how long it stays open, the humidity in the lab, how the liquid is extracted from it... incredibly complicated for very little gain. In practice people will probably use a small bottle with a small neck, so that there won't be many open/close cycles, and not too much 'air' will enter each time. As a rough estimation, if you use about 100 mL at a time, I would make bottles of 500 mL.

For the sake of completeness, you can have a look at this comprehensive guide to handling air-sensitive material (which includes liquids that must not come into contact with moisture from the air). But frankly, for a solution that has got already 30% water in it, it would be ridiculous to go through such hassle...

Thanks a lot for that. That's what I roughly thought. But since I'm not a chemist, I just wanted to make sure I don't forget anything for basic chemistry.


text:
Name for reaction producing carbon from CO2
Some years ago I heard of a reaction where hydrogen could be used to reduce $\ce{CO2}$ to carbon and water, similar to the Sabatier reaction. I believe it also used iron as a catalyst at high temperature. Can anyone remind me of the name of that reaction or correct me if I am mis-remembering the details.

Hello and welcome to Chemistry.SE. Feel free to take the short [tour] or visit the [help] if you have any questions about how the site works. Best of luck with your question!

related https://chemistry.stackexchange.com/questions/915/how-to-convert-carbon-dioxide-into-carbon-and-oxygen

I since found the answer to my question, it is the Bosch reaction: https://en.wikipedia.org/wiki/Bosch_reaction


text:
Multiple "/" in catalysts
What does it mean if a catalyst has multiple "/" symbols, such as $\ce{Mn/Fe/Al2O3}$? Does it mean either $\ce{Mn}$ or $\ce{Fe}$, or does it mean both are supported on $\ce{Al2O3}$?

Both metal are present, but otherwise it seems imprecise to me.

There should be only one "/" symbol. When more are used, it should be unambiguous from the context what is the carrier, e.g. in your case it would be more appropriate to use a dash:
$$\ce{Mn-Fe/Al2O3}$$
From section 1.3 Composition, structure and texture of catalysts of 1975 IUPAC recommendations for heterogeneous catalysis [1, p. 79] (emphasis mine):

Support. In multiphase catalysts, the active catalytic material is often present as the minor component dispersed upon a support sometimes called a carrier. The support
  may be catalytically inert but it may contribute to the overall catalytic activity. Certain bifunctional catalysts (§1.2.8) constitute an extreme example of this. In naming
  such a catalyst, the active component should be listed first, the support second and the two words or phrases should be separated by a solidus, for example, platinum/silica or
  platinum/silica-alumina. The solidus is sometimes replaced by the word "on", for example, platinum on alumina.
Promoter. In some cases, a relatively small quantity of one or more substances, the promoter or promoters, when added to a catalyst improves the activity, the selectivity, or the useful lifetime of the catalyst. In general, a promoter may either augment a desired reaction or suppress an undesired one. There is no formal system of nomenclature
  for designating promoted catalysts. One may, however, for example, employ the phrase "iron promoted with alumina and potassium oxide".

References

Burwell, R. L. Manual of Symbols and Terminology for Physicochemical Quantities and Units. Appendix II. Heterogeneous Catalysis; Pure Appl. Chem. 46, 71, 1976. https://doi.org/10.1351/pac197646010071 (Open Access)


text:
How to read Robin-Day Classification?
I am currently preparing for an exam and am stuck now with the Robin-Day classification. We had these three diagrams and also the explanations how the systems differ but we never talked about what the diagrams are supposed to show or how we have to read them.
The paper suggests that λ here is not (as one might suggest) the wavelenght required to excite an electron but the so called Marcus-reorganization energy and h(ab) is here the electronic coupling.
I understand that you have to treat the system a bit more complicated because you have a system that exchanges electrons without the formation or cleavage of bonds or any major geometric change. And there is also the concept of conductor and non conductive if we see class III compounds as being fully delocalized.
But why does the class I non conducting intervalence compound have these two energy curves, the class II a double well. I suppose because there is no λ in class three that, as there is no localization of the corresponding electrons, this reorganization is not important there. Still I'd be interested in how to actually understand the diagrams and what I can take from them.

Not sure if you've read the Wikipedia entry yet: https://en.wikipedia.org/wiki/Inner_sphere_electron_transfer#Mixed_valence_compounds

Theses are potential energy diagrams, representing the 'stable' forms of both the reactant and the product of IVCT (and their immediate relationship with ligands and surrounding solvent). This is most easily seen in the class I case, where the curves are not merged at all, and instead represent the species as distinct (and the reaction unable to proceed). The reason the curves appear to merge is because the class II and class III cases are where electron transfer is able to occur, and thus the reactant has a reaction path with which to proceed to the product.
There are in fact two reaction paths that a class II mixed-valence species can proceed to the product, one being optical (which is the IVCT transition, caused by absorption of light), and the other being thermal. The optical pathway occurs upon absorption of light at the bottom of the leftmost potential well, where it is excited to the top left of the upmost curve. This corresponds to lambda, which confusingly represents the energy of this optical transition (E=hv=\lambda). The position that the ion is now at on the potential energy curve is representative of a state where the electron has been exchanged, but the molecule is yet to rearrange bond lengths, and the surrounding solvent has not rearranged to accomodate the new distribution of charge. The molecule then relaxes down the adiabatic potential energy curve (not shown here, some diagrams have the same structure as class I in dotted lines, these lines being the adiabatic energy curves), to the rightmost potential energy well, where the molecule has completely rearranged the surrounding solvent and changed bond lengths to accomodate the new charge distribution. The thermal process is less relevant, except to note that the hump in the class II diagram is the thermal activation barrier, where an activation energy equivalent to or greater than this barrier is required for the reaction to proceed via this pathway.
A class III ion is one where there is full mixing of the two potential energy curves, such that they become the structure you see above. Clearly here there is no thermal activation barrier, as there is no hump, and the optical transition energy is actually equivalent to or smaller than 2Hab (twice the coupling constant). The result of the now missing thermal activation barrier is that the ion can freely transition between both states, equilibrating between them, resulting in partially charged ions, and a very delocalised system. Note that optical transitions still occur in class III ions, and are typically more intense than transitions in class II ions. There is just no good way of displaying where the transition occurs as it is in the same position as the 2Hab gap.
I realise this thread was created long ago, so this response likely won't be useful to the original poster, but hopefully anyone else who is interested will have a greater appreciation of how these diagrams work.
I strongly recommend reading this paper https://onlinelibrary.wiley.com/doi/10.1002/9780470166314.ch1 if you can get access to it. It is by far the most understandable of all the papers on the subject, and very comprehensive.


text:
Trioxane fuel tablet recrystalization
I had a bunch of old military trioxane fuel tablets.  I decided to open all of the packets, maybe a dozen, and place the contents in an empty tin coffee can with a plastic lid.  I put the can in a dark closet.  I let it sit undisturbed for several months.  Upon opening the can, I saw crystal structures growing off of the sides of the tin can.  They appear to be of a lightweight somewhat flexible plastic composition and are flammable in the same way that the rest of the fuel is.  If I disturb them by damaging them or removing them, more will grow over time, if I let the container sit.  What is this reaction?  Has anyone witnessed this before?

Trioxane is a trimer of formaldehyde. I wonder if you're under enough of an equilibrium to form paraformaldehyde.

It's subliming (aided in this case by decomposition) and re-depositing.
Substances that turn directly from solid to vapor (e.g. naphthalene, water ice) are said to sublime, rather than evaporate. The inverse, changing from vapor to solid, is termed deposition.
When water vapor deposits on very cold surfaces, such as "dry ice", it grows into amazing dendritic crystals:

The substance sublimating is 1,3,5-trioxane.  www.dtic.mil/dtic/tr/fulltext/u2/732431.pdf


text:
Which is a better nucleophile: hydroxide anion or amide anion?
I know that nucleophilicity order depends on what type of solvent we take, polar protic or polar aprotic and also on the basicity of nucleophile.

In DMF, which is a better nucleophile $\ce{NH2-}$ or $\ce{OH^-?}$

Since the solvent is aprotic polar, it doesn't have any donor hydrogen to form  hydrogen bonds with nucleophile. So, the nucleophilicity should depend on which among them is more basic. Now, since $\ce{N}$ is less electronegative than $\ce{O}$, it's lone pair is more readily available than that of $\ce{OH-}$.
So, according to me, $\ce{NH2-}$ should be more nucleophilic than $\ce{OH-}$, but the answer given is the opposite, that is, $\ce{OH-}$ is more nucleophilic than $\ce{NH2-}$. How is my reasoning incorrect?
And also what are the factors we should keep in mind while deciding nucleophilicity?

Problem with considering this is that OH- and NH2- react with DMF which makes it a poor solvent for these reactions.

@Waylander  I have also got a similar question in a book. But in that book, it was written , Which is a better nucleophile : (i) OH- (ii)NH2- in DMF . So I guess, it means that only NH2- is used in DMF and not OH-

@Waylander Also, Can you give some links for reactions of NH2- and OH- with DMF

@Jdeep  https://www.researchgate.net/post/Feasibility_of_N_N-dimethylformamide_solution_of_KOH_at_reflux     https://pubs.rsc.org/en/content/articlelanding/2019/qo/c9qo00842j#!divAbstract

I was not able to find data on this nucleophilicity of $\ce {NH2^-}$ and $\ce {OH^-}$ in dimethylformamide, $\ce {Me2NCHO}$. However, I have a plausible explanation as to why the answer may not be what you expected.
Firstly, it is important to remind ourselves that nucleophilicity is always relative to the electrophile that we are concerned with. However, it is also possible to discuss nucleophilicity generally, i.e. not with respect to any particular electrophile. This is the case now.
The fact that the solvent is polar and aprotic suggest that hydrogen bonds cannot be made with the $\ce {N}$ and the $\ce {O}$ of the nucleophiles. However, it does not restrict us to form hydrogen bonds between the lone pair on the carbonyl oxygen with the partially positively-charged $\ce {H}$ atoms on the ions. This is not at all unlikely considering that the strong delocalisation of the lone pair on the $\ce {N}$ in amides, gives a significant resonance structure with this carbonyl oxygen bearing a formal negative charge.
$\ce {NH2^-}$ has two such $\ce {H}$s and $\ce {OH^-}$ has only one such hydrogen. Thus, perhaps, we may still expect greater extent of solvation for the former. Relative to $\ce {OH^-}$, the greater amount of solvation of $\ce {NH2^-}$ increases the activation energy required to initiate the subsitution reaction since solvation can be said to bring down the energy of the reactant in the reaction energy profile. Thus, the rate of substitution with $\ce {OH^-}$ would be faster than $\ce {NH2^-}$ in this polar aprotic solvent.
Alternatively, the answer provided could very well be wrong. That is a possibility that cannot be discounted...

I think that We can not consider the strength of nucleophile based on the electronegativity in this case, but rather than the Hydrogen bonded to Oxygen and Nitrogen.
Positive charge of Hydrogen diminishes the nucleophile strength of Nitrogen or Oxygen via sigma bonding. The more hydrogen bonded with Oxygen or Nitrogen, the more nucleophile decreases.

This is clearly wrong.


text:
What does 2 vol (w/v) mean?
I came across a protocol saying:

Homogenize 50 g powdered plant material (accurately weighed and recorded) in 2
  vol (w/v) acidified methanol.

What does 2 vol (w/v) mean?

Weight per volume percent. It's basically the density.
$$ w/v = \frac{mass_{solute}}{volume_{solution}} * 100 $$
Usually it includes units too, but you didn't specify. I'm guessing it means grams per liter?
For more, see http://www.ausetute.com.au/wtvol.html

Yes, that is what "w/v" means. But on it's own it is not a satisfactory answer, as that doesn't address the "2 vol" bit, or give any indication of how the OP should interpret the quoted protocol.

It is actually a poor protocol telling you that the mass of your solute (or disperded material) should go in a double volumetric amount of liquid. Strictly speaking you do not have the knowledge of what you are supposed to do, as units are missing. I am experienced in lab and personally I wouldn't be able to interpret the protocol without guessing, and I'll use 50 times 2 = 100 millilitres of solvent.
As additional comment, the w/v concentration should be used with caution as it is susceptible of being practically meaningless (volume contraction / dilation upon solution preparation) without further knowledge. Also, if the attained solution has to be used as a source of solute, its density must be known (unless it is aqueous and very diluted).


text:
Is there a cheap, safe liquid that evaporates EXOthermically around room temperature?
The heating and AC system in the building where I occasionally work, works like this (basically a thermostat):
The administrators set some temperature that is maintained automatically. Say, in the winter, the outside temperature is 0 °C. The admins will set the desired inside temperature to 20 °C. There is some thermometer behind a little steel panel. It measures it to be cold in the room; therefore the heat comes on. I can trick the heat into coming on even more, by rubbing some alcohol or acetone on the steel panel. The acetone evaporates endothermically, making the steel panel REALLY cold; then the heater thinks that it's REALLY cold in the room and pumps the heat in.
The same system is in place in the summer, but in reverse. However, I can't trick the panel in the same way. If I put acetone on the panel, it'll cool off, and then the A/C unit thinks the room is just fine, because the panel is cool. I'm wondering if there is some liquid that will evaporate exothermically, and heat up the panel, in order to make the system think I need more A/C?
I don't believe that exothermic evaporate exists; here is a list of heat of vaporization for various substances; all positive (endothermic). http://www.engineeringtoolbox.com/fluids-evaporation-latent-heat-d_147.html
But maybe I'll be surprised and it does exist!
I have been holding my laptop's hot backside up to the panel instead, but that's a bit too manual for my taste.
Edit: To be a bit clearer, let me specify that this is just a thought experiment. I'm not actually expecting to find something which doesn't make sense like an exothermically evaporating liquid.

Why can't you just use the heatpack for summer? https://en.wikipedia.org/wiki/Heating_pad#Chemical
It should be safe enough.

Remember those USB coffee cup heaters? I would've never guessed I will ever see use case for these, but apparently here's one.

Last I checked, your body puts out a lot of heat.  Just lean on it and check your cell phone.

You could dissolve something that has an exothermic enthalpy of solution, like potassium hydroxide. Warning: this may eat through the thermostat, your hands and the wall.

You have a textbook XY problem here... What you want is to turn the air conditioning on, but instead you are asking about exothermically evaporating liquids. (In fact this is more classic than most classic XY problems...)

If your room is too warm in the summer then the first thing you should try is turn the thermostat down, or since it's locked, ask whoever has the key to turn the thermostat down.

@immibis This would be an XY problem if OP was asking for a solution to problem Y when they needed a solution for problem X. But since it seems the question was asked out of theoretical curiosity, I don't see how it is an XY problem.

@nitro2k01 You are right, the question is not an XY problem. It is fairly clearly asking about problem Y (exothermic evaporation), especially with the most recent edit. However, unfortunately, half the answers are answering problem X (how to heat up the thermostat).

, Problem X, and most of these answers addressing it, would be a great fit for LifeHacks.SE.

@immibis Sometimes, the solution isn't so straightforward, because politics. The employer has to keep the office in a certain temperature range, they don't have to please all employees. That said, iammax, there are various ways to heat that plate without any hazard. Maybe you can you tape your notebook's PSU to the steel plate, or how about a pocket warmer?

I'd suggest a different approach: A peltier element - one side heats up as the other cools down, this could be used to control the thermostat as desired.

FOOF. Neither evaporating nor safe, but exothermic like hell and fun to watch... from a very far distance. ;-)

If such a liquid existed, it wouldn't boil when you raised the temperature. Instead, it would boil when you lowered the temperature. No such substance exists.

No such liquid, safe or otherwise, can exist. Evaporation is a strictly endothermic process in all cases.
The change in state from liquid to gas is marked by the individual particles gaining enough translational kinetic energy to overcome the mutual attractions present in the liquid phase to "fly free" in the gas phase. It is logically inconsistent for a substance to increase its internal energy and release energy to the surroundings as heat in the same process.
In order to achieve both evaporation and a release of energy, one would have to find a liquid that reacts to (a) release heat and (b) form gaseous products.  The energy required to move from the liquid to the gas phase is substantial; more than likely the only reactions exothermic enough to provide a net release of heat are combustion reactions.
I somehow doubt dumping a flammable liquid into your thermostat, inserting a wick, and lighting it on fire is a satisfactory solution for you.

I somehow doubt dumping a flammable liquid into your thermostat, inserting a wick, and lighting it on fire is a satisfactory solution for you. That would be a great solution, although the OP wanted safe.

Of course, you could do the opposite - let some liquid condense on the thermostat. Like, spraying it with super-heated steam. Don't call me when the building burns down or something :P But it certainly isn't as convenient as evaporating a liquid.

One could also argue from thermodynamics: The transition from liquid to gas increases the internal entropy of the substance. If the reaction was exothermic, it would increase the entropy of its surroundings as well (or leave the surroundings alone and increase internal temperature and entropy even more), and it would therefore be spontaneous at all temperatures. It is precicely because it is endothermic, and thus lowers the entropy of the suroundings, that the reaction becomes spontaneous only above a certain temperature.

Good point, @Arthur!

There's always the option of using some alcohol-based hand sanitizers. I've seen many that are viscous enough to put on a vertical panel and they burn pretty low (small flame, not jumpy). Not really safe but safer than using gasoline.

Silly question, but why can't you get the thermostat adjusted to the temperature you want? You've got a nice control mechanism built in to the room and you want to tinker with it... to make it hotter in winter and colder in summer.
But if there is some reason you can't get the thermostat adjusted, instead of using an exothermic evaporating liquid (as hBy2Py says, because evaporation is increasing the kinetic energy in each molecule compared to the bulk liquid it's got to be endothermic) why not get a peltier cooler such as at http://tetech.com/peltier-thermoelectric-cooler-modules/?gclid=Cj0KEQjwkZfLBRCzg-69tJy84N8BEiQAffAwqlE6p-prIsm3udii0_qoEVpGT2gSRqXm4AgVWUNjos8aAolJ8P8HAQ and magnet it onto the panel in the right place. Feed it the appropriate (low) voltage one way it heats up the panel, reverse the voltage it cools it down.
That's far safer than evaporating acetone or ether!

To be honest, I was more interested in seeing people's answers/ideas about heating/cooling the panel thing than actually changing the temperature. But with regards to adjusting the thermostat, the thermometer inside the panel is not very good because it's right where the vent is. Even when I'm not tricking it, when the A/C is on the panel thinks it's colder than it really is because it's near where the cold air blows out, and that makes the A/C stop too early. Same problem when I want heat. So adjusting the thermostat doesn't really help much.

Ahh, a really well designed HVAC install then! A peltier cooler controlled by a remote thermostat might be the way forward.

I'm pretty sure they did it on purpose to save money. It's at a big city university so comfort is not a priority...

Or, wrap the thermostat with insulation, maybe?

@iammax : I've seen students install thin cardboard baffles to redirect the outflow away from the thermostat.  This sounds like a job for duct tape.  (... or Duck tape, if you get condensation.)

@iammax it's the same this side of the pond, and baffles work to some extent in our lab.  Can you rest the laptop charger under or even on the thermostat?

Can't you use something like a hair drier? It's a bit noisy but you don't need much time to trigger it.
Else, you can use the coffee machine warm  water, you just need it to touch the panel, not pour it onto it...
I second the heating pad idea.
If I would follow your principle I would find a way to have some evaporated liquid condensate at thermostat plate level, in a reverse way to what you have (inside a bottle, glass is a good thermic conductor). You can evaporate a few water inside a bottle then let it cool by transfering its energy to its environment. Problem being that it will also actually warm the room.

A heating blanket would be even better.

Forgive me for saying, but this feels like a problem statement in which the setup of the question inadvertently obscures the actual solution space. You state a number of constraining requirements, that other answers have dutifully adhered to by telling you there is no solution, but the actual problem does not seem to have those constraints.
For example is the phase of matter truly a requirement? Must it be a liquid? Must the mechanism of action be limited to evaporation? Are those really requirements?
It seems to me there is nothing about the explanation that requires those constraints. It seems to me that the constraints are actually: something that is clever, not unsightly or likely to lead to damage, and of course effective at making the thermostat turn on the air conditioning. In other words something as clever as your acetone solution, which is in fact quite clever.
However, as stated the question reads like "I'm hot, how do I build a fire that's cold?" And so, of course, hBy2Py is absolutely correct to tell you that there is no such fire.
Could the answer not be in the form of a solid? Maybe a mixture of particulate solids? Perhaps fine particulates that could even be aerosolized?
Because if that's the case I'd draw your attention to rust, iron oxide. You know the kind of thing you might find in disposable hand warmers. A fine particulate mixture of iron, activated charcoal, salt, vermiculite, and water.  Cheap, safe, exothermic, and clever.

Not at all sure about the practicality, but rather than trying to come up with a liquid that exothermically goes to a gas, could you take e.g. gallium (which is supposed to liquify in your hand), put that on and when it "freezes", that should give off heat (although I have no idea what the specific heat of fusion of gallium is)

To avoid having to stand there or wire up the panel, I would aim some low power laser to the thermostat cover. This can be started and stopped remotely. A dark sticker would enhance energy absorption.


text:
Role of nodes in stability of orbitals
What role does the node play in determining broadly the relative stability of various types of atomic orbitals with the same principal quantum number?

Nodes don't play roles. You can't have an orbital with nodes and the same orbital without nodes and compare them like "see, this is more stable".

As a simple answer one might say the more nodes the higher the orbitals energy, i.e. the less stable it is. But you can have orbitals with the same number of nodes and different energies.
In general the is a node rule in quantum mechanics, which states that the ground state of a system has no nodes and higher excited states have more nodes.
Two common examples are the particle in a box and the harmonic oscillator. Both links contain the equations for the eigenfunctions and their energy as well as their plots.
A simplified example dealing with orbitals would be Hückel MO Theory.
Now for atomic orbitals the story is a bit more complicated. First of all, orbitals are not the electronic states of an atom or molecule, but something you can approximately decompose the electronic state into. Furthermore we have not 1 but 3 quantum numbers (5 if you include spin) defining an orbital: $n$, $l$ and $m$. The node rule for atomic orbitals is:

$n-l-1$ radial nodes
$l$ angular nodes:

$|m|$ nodal planes shaped like a cone around $z$-axis
$l-|m|$ planar nodal planes perpendicular to $xy$-plane

So we have $n-1$ nodes in total. The energy of an atomic orbital mainly depends on the quantum number $n$:
\begin{equation}
  E_n \propto -\frac{Z}{n^2}
\end{equation}
where $Z$ is the atomic charge. So higher $n$ means more nodes and higher (less negative) energy, which is in agreement with the general node rule for $n$. This also agrees with the quantum numbers $l$ and $m$, as they neither change the orbital energy (it only depends on $n$) nor the total number of nodes. (This is considering an atom with one electron only).
However, the orbital energies furthermore depend on the coulomb interaction with other electrons. This is what leads to $2p$ orbitals usually having slightly higher energy than $2s$ orbitals, but they still have the same number of total nodes, they are just differently distributed between radial and angular part of the orbital.
Also note, that you cannot directly compare orbitals of same quantum numbers between different elements, as the nuclear charge $Z$ goes into the orbital energy as well. For example the energy of the $1s$ orbital in the Hydrogen atom is $-0.5\ E_{\rm h}$. In a Carbon atom this would be something more like $-16\ E_{\rm h}$ for $1s$, while $2s$ with around $-1.2\ E_{\rm h}$ would be energetically much closer to the $1s$ Hydrogen orbital.

Concerning the general node rule, here you can find a little bit of background information. Maybe its of interest.