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Implications of the excitation spectra
The fluorescence excitation spectra show the change in fluorescence intensity as a function of the wavelength of the excitation light.
I'm interested in the certain physical implications of the above.
Does that mean, that at the wavelength of excitation maximum the most of the molecules get excited? Or do they just emit more photons per molecule per unit time?
Additionally, if the former is correct, is there any relationship between the percentage of excited molecules and the excitation spectra?

The fluorescence excitation spectra show the change in fluorescence intensity as a function of the wavelength of the excitation light. Does that mean, that at the wavelength of excitation maximum the most of the molecules get excited? Or do they just emit more photons per molecule per unit time?

You have to understand how the excitation spectrum is collected. You have to know (or at least have an estimate) of the emission maximum of a given molecule. For example, if we have quinine, I should know that it emits blue light and maximum wavelength of emission is ~ 450 nm.
Now we will fix the emission wavelength at 450 nm and scan the entire UV-VIS range for excitation. Say, we would excite quinine with 200 to 700 nm. One wavelength at a time, and count how many photons corresponding to 450 nm were emitted. This plot is called the excitation spectrum.
An excitation spectrum basically tells you which wavelength (from a given instrument) is going to show the highest intensity. I emphasized "a given instrument" because an excitation spectrum has to be corrected for the light source intensity. It will vary from instrument to instrument. Nobody corrects it because it is a long procedure as unfortunately, normalization cannot correct it.
Anyway, a corrected excitation spectrum should match in shape with the absorption spectrum provided that there is only one fluorophore in the molecule.
I wanted to connect the excitation spectrum with an absorption spectrum because we can now connect the molar absorptivity with the excitation. Consider molar absorptivity as a measure of probability of excitation. Higher the value of molar absorptivity, more molecules will absorb the light and hence more will emit. So your first interpretation is correct. It is a collective behavior of the all the molecules which are being illuminated in the cuvet.
Also remember that excitation emission is normally a one photon excitation and one photon emission in majority of cases until and unless you are using fancy lasers. So you cannot have one photon in, and multiple photons out in routine cases. In rarer cases, you can indeed have two photon in for excitation and one photon out. You need lasers again.

Good books have been written to explain all the implications of fluorescence spectroscopy. There is no universal answer because of the complications of the experiment.
In general there are atomic spectra and molecular spectra.

Atomic spectra tend to give line spectra.
For molecules however the binding energy of the electron depends not only on the electronic energy but also the vibrational state of the molecule. This coupling creates a microstate and thus molecules tend to give broad peaks.

There is also the Stokes Shift which indicates that the exciting photon must be of greater energy than the emitted photon. This is mostly true, but there is also resonance radiation, and anti-Stokes radiation.
Given:

The fluorescence excitation spectra show the change in fluorescence intensity as a function of the wavelength of the excitation light.

Does that mean, that at the wavelength of excitation maximum the most of the molecules get excited?

Maybe...
Normally when trying to measure excitation sensitivity you'd normalize the number of excitation photons. Think of it as how many emission photons would be given off per excitation photon versus the energy of the exciting photon. (This will normally be considered to be less than 1). However a real excitation spectra need not have the same intensity of photons at each energy. So a lot of excitation photons at a non-optimal energy might product more excitation photons than the optimal energy at a lower intensity.

Or do they just emit more photons per molecule per unit time?

In general one excitation photon creates one excited state which may decay by the emission of a photon.
There are of course lasers where the atoms or molecules are pumped into an excited state then one photon can stimulate the multiple emissions.

Additionally, if the former is correct, is there any relationship between the percentage of excited molecules and the excitation spectra?

The fluorescent quantum yield is the number of excitation photons emitted per excited state.


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Manufacture an artifact with compression
Room-temperature superconductivity has finally been demonstrated, as reported by Quanta (and others), with the caveat that it requires compression of the substrate at nearly 2 Mbar.  Obviously, this is an impractically high pressure, but if a different material were found that only required a much lower pressure, would it be feasible to manufacture wires under compression?  If so, what is the current state of the art?  I have not been able to find any examples of such a product.  Most apropos to chemistry, what is a practical upper-bound on tension/compression we can expect from currently known materials?
A trivial example would be to put a spring in a box, which only requires compressing the spring and sliding it into a box.  For the wire, one could perhaps imagine a strange "cylindrical press" (not sure if there is such a device) and a jacketing material with extremely high tensile strength into which the wire is thus threaded (or, more likely, the jacket is somehow rolled onto it).
A more sophisticated system would be a compressive jacket which is applied in a relaxed state, possibly at high temperature, and then cools/shrinks into a high-tension state which applies the necessary compression.

I doubt that any sort of high pressure superconductor has any practical value. However it would of immense interest to theoreticians to help understand the nature of superconductivity.

@MaxW suppose that further research reduces the pressure requirements but only gets down to, say, 10 bar, or even 2 bar. Are you saying even that is infeasible? I find that hard to believe.

It is not forbidden to dream. But the conductivity of practically all known substances and mixtures have already been studied so far, at ordinary pressure. It would be extremely improbable that a superconductor at room pressure and room temperature could be discovered. Why was this substance not yet studied ? The substance reported by Quanta is a mixture $\ce{CH4 + H2S}$. and this is a gas at room pressure and temperature. It becomes a superconductor under 1 Mbar.

@Maurice there's lots of new materials discovered every day, like twisted graphene. But the question is specifically about the feasibility of manufacturing a material under compression. I know that concrete shrinks somewhat during curing, but I'm not aware of any material designed to create permanent compression in the finished product, or what the maximum feasible compression would be.

@LawnmowerMan - your Pyrex glassware in the kitchen has undergone ion exchange to form a compressive layer on the surface. This reduces crack formation and growth, toughening the Pyrex. Shot peening does similar things on metal surfaces.

If it were only a few hundred or thousand bars, run the cable underwater. 10 m depth = ~1 bar. Challenger Deep ~1,000 bar.

@DrMoishePippik very creative! But I think there are more economical ways to obtain 1 Kbar of compression...

But for off-shore wind power, it might work... if pressure needed were many orders of magnitude lower. But I'm a  bit out of my depths, here.

Jon Custer helpfully noted (in a comment) two common production processes that result in compression of the final product.  I did some research to discover the typical pressures involved.
Tempered Glass
Glass may be tempered by cooling the outer layer quickly, while letting the inner layer cool slowly.  As the inner layer cools, it shrinks; but because the surface layer is already cool, it cannot continue shrinking, so it experiences compression instead.  One can think of glass tempering as producing an end result in which the core glass is literally pulling the surface inward.  Very cool!
According to J.H. Nielsen, tempered glass retains compressive stress on the order of 100 MPa, if I am reading Figure 12 correctly.  This is equal to 1 Kbar.  That's 3 orders of magnitude short of the Mbar pressures created by the diamond anvil, not to mention the difficulties entailed by embedding wires in molten glass, and yet, this is at least a plausible way forward.
Shot Peening
The surface of a cold metal can be further compressed simply by banging on it really hard.  Most commonly this is accomplished by firing small metal shot at the surface at high velocity and volume.  This induces plastic deformations which have a higher density than the underlying metal, and thus a compressive strain within the surface which helps strengthen various industrial products like springs, crankshafts, and jet engine blades.
According to this source, shot peening can induce compressive strain on the order of 800-1000 MPa, according to Figure 1. However, it should be noted that the compressive layer is only about 100 μm deep.  So, this gets us up to 10 Kbar or so which is even better than glass tempering, and doesn't require high temperatures.  Very interesting!

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Isolating additives in alcohol products
Given an alcohol-water solution (Liquor products) that may have additives and/or may have been aged in wood, I'd like to boil off the alcohol and the water to leave behind the various additives in a highly repeatable fashion.
I.e. a pan on the stove works OK, but I'd potentially like to throw together something a bit more precise.
Two lines of thinking so far.   Starting with the same amount of solution in both cases:

Use test tubes and a heater block.   Hold the temp at ~180ºF to evaporate the alcohol.  ~212ºF to evaporate the water.   That should concentrate the additives in the bottom of the tube.

Place a some sort of sample paper (?) that exactly covers the bottom of a container.  That should concentrate everything into the paper.

I'm really looking for:

Keywords to search for.   Been a long long long time since I spent time in a science lab of any sort and I don't know what I'm looking for or what is available.    I don't want to throw tons of money at this-- it is just a bit of a personal exploration thing-- but I don't mind spending some $$$ to create a repeatable, quantifiable, process for doing this sort of thing.

Any suggestions on process.  I could be entirely missing the obvious easy solution here because I don't know what I'm doing. :)

A process that potentially creates samples that can be sealed in something -- ziplock bags, perhaps? -- for later reference.   The sample paper process seems to be a better fit for this.

You don't want to keep any of the volatiles, that might significantly contribute to flavor?

Why do you want to do this?

Specifically, I want to take already made product and get rid of the alcohol / water -- the volatiles -- to see/taste/smell what is left behind.   Ideally, I'd like to do this in a repeatable fashion.

@DrMoishePippik Nope, I do not!  I want to specifically focus on the non-volatile additives that may be in play.   Really, I want to focus on everything but the alcohol and water.

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Can someone help with the nomenclature of this metal complex. MarvinSketch is giving me trouble
I’m trying to get a name for the structure above using MarvinSketch for my undergraduate senior thesis. This is the first time I’ve had to draw a metal complex in MarvinSketch, and I’m having trouble getting the program to name it as is.
Right now, using the “structure to name” feature, the name I’m getting is 8,14‐dimethyl‐2-λ-4,20-λ-4,21-λ-4,27-λ-4‐tetraaza‐1-cobaltaheptacyclo[12.6.6.11,9.02,7.015,20.021,26.013,27]-heptacosa2(7),3,5,9(27),10,12,15(20),16,18,21(26),22,24‐dodecaene‐1,1‐diol, and I’m pretty darn sure that’s wrong.
I think this issue is that I drew the water molecules as being attached to the carbon via coordinate bonds but the polypyridine ligand as being attached by normal single bonds. I did this because the program wouldn’t even give a name if all bonds were drawn as coordinate bonds, and if I try to show the water molecules being attached by normal bonds, the program always either gives the waters a formal charge or removes one of the hydrogens to make it an -OH bonded to the cobalt.
I was wondering if someone could help me figure out how to get the program to properly name this complex, and perhaps also help me get the program to stop adding a formal charge/removing one of the hydrogens from the waters if I change the bond type to not be coordinate.
This is not a very important part of my thesis so if there’s no relatively easy way to get the IUPAC name I’m not going to worry about it (though it would be nice). I also unfortunately don’t have access to books on nomenclature from my school with us being remote.
Thank you very much

You should just name the ligand and then it's a small step to whole complex that program won't make, but you should be able to do easily.

Hello, thanks for the response! So if I just name the ligand in MarvinSketch, I get the name: 2‐[1,1‐bis(pyridin‐2‐yl)ethyl]‐6‐[1‐(pyridin‐2‐yl)ethyl]pyridine. Now that I have that, how would I go about naming the metal complex? I've not taken inorganic yet* and I've never learned IUPAC nomenclature for metal complexes.
*The class is only offered in the spring and unfortunately, I've had to wait for senior year spring to take it.

As I said this is the easy part. You can check for example Wikipedia for details.

Thanks for the link! Would the name therefore be: diaqua(2‐[1,1‐bis(pyridin‐2‐yl)ethyl]‐6‐[1‐(pyridin‐2‐yl)ethyl]pyridine)cobalt(III)?

Well, there's probably need for curly bracket, but more importantly, is this Co(II) or (III) 'cause (II) is more probable.

In my lab notebook, it is Co(III). We used hydrogen peroxide to oxidize the Co(II) to Co(III) because we wanted it to be NMR active. By curly brackets, do you mean it would be diaqua{2‐[1,1‐bis(pyridin‐2‐yl)ethyl]‐6‐[1‐(pyridin‐2‐yl)ethyl]pyridine}cobalt(III)? Is this the full name of the complex? Thanks!

Let us continue this discussion in chat.


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Placed in equivalent freezers, would a liter of water or a liter of lava turn from liquid to solid first? (question from a 6 year old)

Placed in equivalent freezers, would a liter of water or a liter of lava turn from liquid to solid first? (question from a 6 year old)

Based on this page in a “Blaze” book, my six year old asked “which would win?” between water and lava. On further investigation, we refined the question to: which would turn solid first in similar conditions, a liter of room temperature water or a liter of volcanic lava?

Starting temperature for each would be good to know. Good book though.

Paging  Randall Monroe....

I would think a truck named "Blaze" would prefer the lava to water...

Do you really mean "based on"?  I think "inspired by" might be a better description of asking this other question, separate from what's happening on that page.  (Perhaps that phrasing is why you got an answer that analyzes the situation on the page, not your question.  The heat energy per mass needed to boil water to steam is even higher than the energy needed to melt it (or conversely, energy it needs to release when freezing).)

The fridge would melt first

You've got the right idea — you want to simplify the problem — but I don't think you're using quite the right simplification.  Take a look at the book's set-up of the situation, and ask yourself how the water is stopping the lava. You'll see the idea is that we're using liquid water, not ice, to solidify the lava.
So your question should really be:  How much heat do we need to absorb from a liter of lava to turn it into a solid, and how much heat can a liter of water at room temperature absorb before it turns to steam?  If the latter is larger than the former, then a liter of water can cool a liter of lava to the point where it solidifies before the water all changes into steam.
[I'm using "heat" when I should really be using "thermal energy", but this is for a $6$ y.o., so I'm keeping it simple.]
First, let's do the calculation for water.  Here (since it's for a $6$ y.o.), I'm not going to show all the steps in the calculations:
Energy to heat $\pu{1 L}$ liquid water at room temp ($25 \,\pu{^{\circ}C}$ ) to $100 \,\pu{^{\circ}C}$ = $\pu{75 kcal}$
Energy to turn $\pu{1 L}$ liquid water to steam at $100 \,\pu{^{\circ}C}$ = $\pu{533 kcal}$
Total = $\pu{608 kcal}$
According to https://en.wikipedia.org/wiki/Lava, lava is typically $700 \,\pu{^{\circ}C}$ to $1200 \,\pu{^{\circ}C}$, so let's call it $1000 \,\pu{^{\circ}C}$.
And, using this source (https://ocw.mit.edu/courses/earth-atmospheric-and-planetary-sciences/12-109-petrology-fall-2005/lecture-notes/Nov3notes.pdf), let's assume it melts at $900 \,\pu{^{\circ}C}$ (there's actually a wide range of lava types, and thus a wide range of lava temps and melting points).
Density of lava = $\pu{3.1 \frac{g}{cm^3}}$, so $\pu{1 L}$ lava weighs $\pu{3100 g}$
Energy released when $\pu{1 L}$ lava cools from $1000 \,\pu{^{\circ}C}$ to $900 \,\pu{^{\circ}C}$ = $\pu{93 kcal}$
Energy released when $\pu{1 L}$ lava solidifies at $900 \,\pu{^{\circ}C}$ = $\pu{310 kcal}$
Total = $\pu{403 kcal}$
So, based on the above, $\pu{1 L}$ of water is enough to solidify:
$$\pu{1 L} \times \frac{\pu{608 kcal}}{\pu{403 kcal}} \approx \pu{1.5 L of lava}.$$
Finally, why does the water win?  The main reason is that the water is going between a liquid and a gas, while the lava is going between a solid and a liquid.  And, in general, it takes much more energy to change liquids into gases than solids into liquids, because in the former case you are pulling the molecules completely apart from each other.
But I said it takes "much more energy" to change a liquid to a gas than a solid to a liquid, yet the difference here is only a factor of $1.5$. The discrepancy is because  in this case you're not comparing masses of water and lava, you're comparing volumes (and for the same volume, the mass of lava is $3.1$ x greater). Thus, if you do it on a mass basis, $\pu{1 kg}$ of water is enough to solidify $\pu{6 kg}$ of lava (because, while $\pu{1 L}$ of water weighs $\pu{1 kg}$, $\pu{1.5 L}$ of lava weighs $\pu{6 kg}$).
[For these quantities, mass is typically a more fundamental basis of comparison than volume, which is why heat capacities, heats of fusion, and heats of vaporization are usually quoted on a mass basis. When you see the term "specific", as in "specific heat capacity, that means "per unit mass".  Of course, one can also use a number basis (moles).]

Oh dear, why isn’t the calorie dead yet? D=

@Jan Using calories here surprisingly makes sense, because it directly involves heating water, which the calorie was based around. Not my favorite unit, though.

This was a really interesting answer! I was I expected the ratio of water to lava to be closer to 1:1. It didn’t answer my exact question, but I’ll share your answer with my son nonetheless.

@T.D.Smith  Right, but in your question you explained that your son asked "which would win" based on the picture in th Blaze book.  And that picture shows liquid water vs. liquid lava.  So, as I explained in my post, your refinement of the question, which is about ice vs lava, isn't the right refinement to answer your son's question, because it doesn't address the specific set-up your son asked about. So while I didn't answer your question, I answered your son's, which was what I thought was your question's purpose.

@theorist understood, I appreciate your answer. Thank you

@T.D.Smith Happy to help.  Just noticed by calculation for the energy given off by the lava was off by a factor of two, so I've corrected my answer.  Thus 1 L of water is enough to solidify 1.5 L of lava (as opposed to 3 L of lava).  But, if you do it on a mass basis, then 1 kg of water is enough to solidify 6 kg of lava (because, while 1 L of water weighs 1 kg,  1.5 L of lava weighs 6 kg).

@theorist I shared your answer with my son. He said it made sense (and explained some of it back to me) and then theorized what the rest of the lava coming down the mountain would do (his guess was form a wall when it hits the hardened lava).

@T.D.Smith -- when your son's a bit older, you should absolutely have him read John McPhee's "The Control of Nature". It was a series of New Yorker essays that got published as a book, but one of the main sections focuses on Iceland's efforts to control lava flows by solidifying it into walls, using water: https://en.wikipedia.org/wiki/The_Control_of_Nature

@BillThePlatypus Just because $\pu{1atm}$ makes sense in the context of air pressure at sea level or $1'$ makes sense in the context of measuring feet doesn’t mean these units have any reason to still exist. Every unit was based on something and probably is well suited to measure that, but in science we should propagate use of the standard SI units for better comparison and understanding. I don’t like $\pu{1eV}$ either, for the record.

Did the question change, why are we considering steam at all? How is water absorbing heat from a freezer (-40C is considered industry standard for Ice Cream)?

@Jan It's the Joule I don't like; eV is a little better, though it loses the elegance of a Joule-like definition. Natural units ftw.

@mckenzm in the question as posted I attempted to distill a simple comparison between the phase change of lava and of water. My question was inspired by, but does not exactly mirror, the book I referenced. This answer is an exploration of the scenario depicted in the book.

Good question
You need to specify the conditions a bit more completely:

At 1 °C, the water would never freeze, while the molten lava might in an hour.

At -50 °C, not uncommon near Mt. Erebus, an uninformed guess on my part is that water would freeze first... depending:

What is the shape of the sample
Since water might freeze first on top (since below 4 °C, it is less dens as it gets colder), would that block off some loss of heat?
On the other hand, the lava might also freeze first on top, since it's viscous and might not sink.
What type of lava are you discussing? Aa or pahoehoe? One is much more viscous than the other.
What is the thermal conductivity of water and of rock?

However, the two of you have asked an interesting question that should be interesting to explore!

What is the environment of Mt. Erebus?
What effect does the shape of the container have on freezing? ? You and your offspring might experiment with a cubic container (1 liter = 10 cm/edge) and with a wide, flat pan, into which you pour a liter of water.
What is the initial temperature? 23 °C? Explore the Mpemba effect, in which, counterintuitively, hot water can freeze before cold.
What is the melting point of common rocks and minerals? You might try to melt some, such as salt (rock salt is a mineral) or quartz, with a propane torch or alcohol lamp, taking safety precautions

Hot rock can "pop" and shatter, sending pieces across the room.
Wear a face mask, and perform the experiment outdoors, with plenty of water handy should one get burned!

There is a table showing the temperature at which some minerals melt or convert to others, Bowen's Reaction Series, and the link is nicely illustrated, perhaps not too advanced to pique the interest of a six-year old.

No credible studies have confirmed the Mpemba effect though

I am curious to try your rock salt experiment.

@RomanOdaisky, that would be a good subject for controlled experimentation then... something a six year old child could do, with parental guidance. It's not the answer, but the process, that is important here.

The thermodynamics involved might overwhelm the child, but the question is a good one.  It shows the child has taken the information in the book and re-applied it to a new problem (the stated question was about freezing and has nothing to do with steam, contrary to a previous answer).  He seemingly already understands that the cold water has cooled and frozen (solidified) the lava.  He has moved ahead and designed a new experiment to test the differences between the rates of change of two different materials and how much each one would need to change state to a solid.
This is a golden opportunity to explain "heat", rates of change, and state-change in general terms, rather than needing a hard answer.
Disclaimer: I am an armchair scientist/engineer, but I love this stuff (until the greek symbols show up).
The real consideration is how much heat does it take to change the state of each material and how fast it can move.  Well, "it depends on a few things", is the answer.  But that is not a bummer if you can follow up with some basic and important concepts, which is the real gist of the question, and this leads to a great discussion: thermal energy transfer, states of matter and state change based on temperature.  You may or may not want to omit pressure from that discussion, depending on this child's understanding of science.  If omitting, then:
Discuss that water has a 100C range between frozen and gas.
Magma has (I don't know offhand, but it is going to be a LOT more -- how hot does it need to be to vaporize?).
So it depends on where the material started:
Each material will gain or lose heat at a certain rate.  the temperature in the freezer can be considered constant (if the kid is a genius, let them know that the magma will heat up the freezer a bit, and the water less so, but it will).
This will open the door to the critical effect here: heat transfer.
From there, you can discuss the idea that if the magma starts at 1000C, but gets to its freezing point faster than water that started at some temperature gets to its own freezing point, then the magma will freeze first.  But at a starting point higher than that, it will not freeze first.  And so forth.
You can use the analogy of two buckets with different sized holes in them.  Even if one bucket is bigger to start out, if you make a hole in the bottom that is the size of the whole bottom, it will clearly empty first.  If the holes are about the same size, but there is more water in one bucket than the other, then... you get the idea.  So there are two factors: how much water (heat) is left to lose before empty (freezing), and how big is the hole in the bucket.  Now, instead of water, the thing that is emptying out of each bucket is heat.
Again, the two takeaways here are:

The concepts of heat transfer, and the rates at which each material will transfer heat.  Heat always "flows" from warmer to cooler areas.  You might also include that air doesn't accept heat very well, but metal is very good at heat transfer.  You could demonstrate this by using an insulator and a conductor, maybe some wood, and some metal.  Place your hand on both, see which one heats up faster.  The metal has a noticeably better heat transfer rate.  Important to pause here to note to your child here that the heat left your hand and entered the material (not exactly, but the child is six and this will suffice).  Don't keep your hand on the materials too long or the wood will eventually reach the same temperature as the metal (although that is another good experiment that will show that neither material will ever get HOTTER than your hand).

The concept of state change.  Some materials change states in a smaller range than others -- the "bucket" is smaller.  Liquid is the only bucket, for now.  Materials can be super-heated and super-cooled.  The intelligent child can be informed about absolute zero, so technically there are three buckets (states of matter which have boundaries to another state or hard boundaries where temperature cannot be changed further), but again, that's a good follow-up for later.  It might very well be the next question they ask... "what happens when you keep heating/cooling?"  Once you get this far, explaining what heat is is the logical next topic and from there will make absolute zero very easy to explain, and why there is nothing colder than that. Absolute zero  AZ can inaccurately be simply described as the place where atomic vibration (the "definition" of heat) stops. Plasma is probably a bit more difficult for a child to understand.  I am taking some liberties here, generalizing and glossing over some important scientific details, but give me a break; the child is six years old.

In my experience, inquiring minds like this one don't crave hard, exact answers nearly as much as the reasoning and concepts behind them.  There's time for the math later, but understanding heat transfer and state change is the real opportunity.  Remember kiddo, you aren't "getting colder" by going outside in the winter, you are simply losing heat.  Since your head loses heat faster than your body, that's why you gotta wear this hat.
PS - one liter of magma or water in a thin sheet will cool off much faster than if it is a perfect spherical volume.  Again, "it depends on a lot of things".  The bucket analogy there, to explain surface area, is that there are more holes in the bucket, so there is more exposure to the colder air (that air contains less heat) at the same time, so more heat will transfer at once.


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Are burns from Lye or Potash that bad?
I read that the chemicals potassium hydroxide and sodium hydroxide are extremely corrosive and will cause severe burns but I also see videos of people saying that if you get lye on you, then just wash it off and you’ll be fine. Is that actually ok. The fact that the stuff is sold in hardware stores makes it seem even less dangerous than the warnings make it out to be. Which is it?

Knifes are sold in hardware stores as well, yet they may cause harm. Indeed, potassium hydroxide and sodium hydroxide are extremely corrosive and will cause severe burns if you won't wash them off right away. If you will, they won't. That's pretty much the size of it.

The key point about hydroxide contact on your skin is to WASH IT OFF IMMEDIATELY. If you get some on your skin and wash it off it will feel soapy; this is your skin dissolving!

I have had a student who drank a solution of NaOH by mistake. Although he immediately reacted by drinking bicarbonate, he has now an artificial oesophagus.

There are developed and deployed artificial burying procedures for dead human bodies being dissolved in pressurized hot sodium hydroxide solution. It takes reportedly few hours. I am not sure I would want to end this way, though.

It is like with a flame or hot water. You can safely push your fingers there, if you pull them immediately. If you do not, you are out of luck. Like being poisinous is a question of amount, being harmful is a question of time. With hot water and drawn tea bags, I perform "a kingfisher catching fish" trick by 2 fingers.

Lye/Potash are used in saponification, and the skin contains mostly fat. Put two and two together and it's easy to see why it's extremely corrosive to human skin.

Skin irritation is bad. Worse is eye contact: the cornea becomes opacified so quickly that a drop of caustic (NaOH or KOH) in the eye cannot be neutralized quickly enough to prevent permanent blindness. (Perhaps cornea replacement is possible.) Similar to Maurice's student' esophagus.

It also depends on the concentration (if in liquid form). Extremely concentrated sodium hydroxide or potassium hydroxide will corrode faster than a dilute solution.

As noted in the comments, lye burns are very damaging to skin and especially mucous membranes. This damage can be drastically reduced if the lye is washed off as quickly as possible.
One reason that these solutions are considered particularly dangerous is that there is no discomfort immediately upon contact (in contrast to acids, which tend to produce burning sensations immediately). Instead, the affected surface simply feels a bit soapy.  As a result, the presence of lye on skin may not be noticed until it is too late to prevent damage by washing.
On a side note, the "it's sold in hardware stores" argument is not a good test for chemical toxicity. There are numerous highly toxic chemicals that can be purchased readily.

Lye easily penetrates into the skin, and keeps doing damage for roughly as long as you had it on your fingers after you wash it off.

This is a purely anecdotal example: I once set up a target at a shooting range which involved mixing lye. My hands were damp because I had just wiped off some snow, which provided a tacky surface for some lye to stick against.
About 2-3min later, it itched. About 10min later, it felt like an open wound, because it was; the skin had turned into a "soup" about the size of a penny. I rinsed with soap and water, added a bandage; I have a scar now.
TLDR it's definitely bad for skin.


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Dispersal of pyrethroid insecticide by vaporization - role of petroleum distillate in distribution in air
I live in Indonesia where there are many mosquitoes and products sold for their control.
One such product is this one:

The product contains 22ml of liquid, the contents of which likely vary slightly by jursidiction
https://www.whatsinsidescjohnson.com/ph/en/brands/Baygon/baygon__liquid_starter

Prallethrin - 1.316% (Wikipedia indicates 1.6% in typical products in India https://en.wikipedia.org/wiki/Prallethrin)
Petroleum Distillate (carrier) - Enables the delivery of a product to a surface.
Butylated hydroxytoluene (BHT) - preservative
Fragrances

They sell a liquid for use in a sprayer like this:

Unlike the plug-in which releases very small amounts of insecticide continuously, a sprayer would be sprayed manually perhaps once or twice a day. The product is sold in quantities of around 300-700ml.
Active ingredients vary by jurisdiction: in the Philippines
https://www.whatsinsidescjohnson.com/ph/en/brands/Baygon/baygon__multi_insect_spray_waterbased_refill

Water
Transfluthrin / Cyfluthrin
Isopropanol "wetting agent" - Helps a formula spread across a surface.
Benzyl Alcohol - anti microbial preservative
Sodium Lauryl Sulfate - detergent
Acetyl Tributyl Citrate - Helps ensure even distribution of a product's ingredients.

The Indonesian version is:
https://www.whatsinsidescjohnson.com/id/id/brands/baygon/baygon__liquid_spray_floral

Water
Cybermethrin, imiprothrin, prallethrin
Propylene Glycol Butyl Ether and PEG-60 Hydrogenated Castor Oil instead of the Acetyl Tributyl Citrate
Methylisothiazolinone & Benzisothiazolinone instead of the benzyl alcohol
Ethoxylated Alcohol in place of the SLS

Another company sells a similar liquid product to the Baygon liquid

This is sold in two formulations, the regular and the water-based which notes

'tanpa minyak tanah' (literally 'without ground oil', i.e. paraffin - this is not a very technical term and obviously means that the alternative product contains some kind of petroleum distillate)
Both regular and water-based formulations contain the same active ingredients: 1.14% prallethrin, 0.29% cyfluthrin.
I presume that there are particular properties of prallethrin that make it better for vaporization with an electric plug-in device (which I guess is not close to boiling - this should be evaporation) than certain other pyrethroids, but the question is:

if one were to refill the plug-in device with the 'spray'-type 1.14% prallethrin, 0.29% cyfluthrin petroleum-based product, would it be reasonable to expect it to have similar dispersal effectiveness to the 1.32% prallethrin petroleum-based 'vaporizer' product
given the existence of 'water-based' and 'petroleum-based' products with the same insecticides, is the 'water-based' less effective at dispersal, and does it have any advantage beyond presumably some sort of antipathy towards hydrocarbon vapours for health reasons.

In my opinion the only reason for using petroleum based products is that once deposited on plants, they won't be washed away by he next rain. The dispersal effectiveness is a secondary factor.

these products are anti-mosquito for use inside a home; they are not products for use on crops, so there wouldn't be getting any rain on them.

In short:  the petroleum most likely is used as a solvent.
Small detour:  tablets of asperin contain acetylsalicylic acid as the intended active compound.  But the tablets contain other chemical compounds as well, for example to retain the active ingredient in «good shape» (e.g., stabilizers, antioxidants, etc.) and not at last, fillers.  Because, to moderate the wanted action of aspirin in the body, you possibly need a smaller mass of the pure compound, than your balance at home allows you to measure. Thus, the active ingredient is diluted; you equally could say, it is dissolved in the solid matrix of the tablet.
Now to the pyrethroids in question.  Similar to perfume, only a relatively small number of the molecules per cubic foot / cubic metre of air suffices to trigger the action; here, against the insects without triggering nausea / headache etc. to you.  (It is a question of balancing the effects, and the cost producing the materials vs. return of investment for the company, too.)  Thus, the pyrethroids are distributed as diluted.  Given the structures of two of the compounds you mention (they differ)

(prallethrin, source)

(cyfluthrin source
there are no, e.g. hydroxy-groups which could render the substances well soluble in water (hydrophilic).  Instead, they are more likely to dissolve in apolar solvents, such as petroleum is, then called «carrier».  In the other recipe, the PEG and propylene glycol butyl ether may function as the slowly evaporating solvent (mixture).
(Possibly, the list of ingredients was rearranged for this question; often, the containers list them in decreasing relative amount to the total content.)  The list of ingredients from the Philippines still may indicate lots of water in the bottle because simultaneously there are other compounds which may mediate between the polar water and the less polar pyrethroids to yield a solution; especially isopropanol (an organic solvent miscible with water), and the detergent, sodium lauryl sulfate may increase the solubility here.
It may be, that overall the recipe using both prallethrin and a little of cyfluthrin is more efficient or / and is less costly to produce than using the other recipe of prallethrin  alone.  It may be local legislation setting an upper threshold for one, or the other compound differently in one country, to the other; which may be to protect you directly as a consumer, or the environment in general (they don't decompose well in normal wastewater treatments and thus may enter the food chain).  Because the evaporation of an aqueous solution may well differ from one based on petroleum, it is not evident that one may be used to refill the other in the electric nebulizer.

Hi, to clarify I don't think the ingredients are any particular order - the pyrethroids tend to be about 1-2% of the volume of any such product, but I'm sure that the 'water-based' products from both Indonesia and the Philippines are mostly water. I'm not sure if the electric vaporiser product also contains water, it's possible they wouldn't indicate it in the list of ingredients (which are 'what's inside', not a 'full list of ingredients'). Presumably it would ignite easily if the solvent is a petroleum fraction?

While it does seem reasonable that the water-based product might not work well in a device designed to create vapour from a petroleum-based solution, I don't think you mentioned whether it is reasonable to suppose there might be significant differences between a product designed to be 'sprayed', containing pyrethroids dissolved in petroleum, and a product designed to be evaporated, also containing pyrethroids dissolved in petroleum? In both cases presumably it's desirable for the spray to disperse effectively in air?

Just because «A» contains much of water (plus the detergent and alcohol) doesn't make it better or worse than petroleum-based formulation «B». While ignition of the petroleum of course depends on the amount of energy delivered, there is an interest to use a fraction of petroleum which does not evaporate not too quickly (not as quick as, say acetone); otherwise, the consumers are occupied with refilling the devices too often to see a benefit in the installation of the devices. And yes, the finer air-dispersed, the more effective (actually both against insects and you [lungs, skin, eyes], too).


text:
Why is aqueous solution of lauyl glucoside alkaline?
What is the reaction of dissociation of lauryl glucoside? It's aqueous solution pH is much higher than 7, it's definately alkaline. Why is that? It's nonionic surfactant and as far as I know it only creates hydrogen bonds with water (the hydrophilic part of the molecule, oxygen atom). Lauryl glucoside is the combination of glucose and dodecanol. Glucose itself when diluted in water is neither acidic nor alkaline. Why is glucoside then alkaline?


text:
How concentrated does sulfuric acid need to be for nitric acid synthesis (H2SO4 + nitrate salt distillation)?
So I have a home lab setup and recently found myself in need of some nitric acid. I could just buy some, but I thought it would be a useful exercise to make my own. I have plenty of nitrate salts around, as well as plenty of unused ~30% sulfuric acid (battery acid refills, fresh out of the box so there's no lead etc in it).
I know distillation of a mixture of a nitrate salt (ex. NaNO3) in sulfuric acid should yield nitric acid, but all the protocols I've found call for concentrated H2SO4. Would ~30% work? I know "concentrated" sulfuric acid usually means >90% if not fuming, but as long as I use stoichiometric ratios of the nitrate and the sulfuric acid, the reaction should still proceed fine, correct? Obviously the nitric acid will be more dilute, but I don't need very concentrated nitric acid anyway.
I realize I can distill the sulfuric aid to concentrate it, but I haven't done that before and would like to minimize the number of "dangerous" distillations I do. My vacuum pump is pretty shoddy and distillation of H2SO4 at ambient pressure requires so much heat,  it just seems like an unnecessary risk if 30% would work.

$1$ kg $30$% sulfuric acid contains $300$ g $\ce{H2SO4}$ (= about $3$ moles). It may react with $\ce{NaNO3}$ according to : $$\ce{H2SO4 + NaNO3 -> NaHSO4 + HNO3}$$ and may produce $3$ moles or $3·63 = 189$ g $\ce{HNO3}$, plus $700$ g water. The solution contains also $3$ moles $\ce{NaHSO4}$ ($360$ g). It is hard to describe the composition of the vapor that will distill out of such a mixture. It should be a mixture $\ce{HNO3 + H2O}$. In the beginning it should be nearly pure water. But slowly the vapor will contain more nitric acid. But at what temperature and how much ?

Related


text:
Ramachandran Plot for Phi and Psi angles of peptides
Why do we consider the normal and extreme limits for Ramachandran plots to be less than the Van Der Waal distances between two atoms?

The calculations are done on the basis of steric hindrance faced by the side chains when the angles are changed. For instance almost at all times (0,0) for (phi,psi) are not allowed because there is clash in between the side chains. This steric clash is calculated based on the normal limit and extreme limit the maximum and minimum distance allowed between the two atoms for the conformation to be deemed stable. Both these distances though are smaller than Van Der Waal distances.

Do you have a source for this algorithm? Which is the minimum and maximum distance? I am also confused by these definitions. It would help to have a source for the table. Where did you get these numbers?

I got the table itself as a part of a question paper, for reference material I suppose you could read this: http://skuld.bmsc.washington.edu/~merritt/bc530/local_copies/Ramachandran_article.pdf

A remarkable aspect of the Ramachandran plots is that the implementation of a hard-sphere potential with flexibility limited to the two backbone dihedrals proved sufficient to largely explain the conformational range sampled by real polypeptides. Take for instance the following map from a seminal article by Ramachandran (from Ref. 1; dashed line encloses the partially allowed region):

The map was generated with the following set of restraints:

In the article Ramachandran et al. explain how they selected the range of vdW distances used to generate their maps:

The next step towards working out the allowed conformations is to choose a set of permitted minimum contact distances between the different types of atoms. Only those conformations (4,4') which do not have any of the contact distances less than these minimum values now become allowed. Two such sets are given in Table II, termed "normally allowed" and "outer limit" contact distances. These are essentially the same as those given by Ramachandran, Ramakrishnan, and Sasisekha-ran (1963) arrived at from a study of the contact distances observed in structures of various organic compounds. Contact distances inbetween the two limits have also been observed in actual crystals, but not as frequently as the normally allowed values.

van der Waals radii are meant to define distances of closest approach. They are useful to identify conformational regions forbidden due to steric clashes. However, tabulated vdW radii are representative values. In real molecules the internuclear potential can vary somewhat depending on atomic types and the local electronic environment.
In addition, molecules are fairly flexible and can to a degree circumvent potential clashes by changing internuclear distances and angles, but a rigid model does not capture that. That a simple rigid model suggests a steric clash, because one or more vdW distance restraints are violated, does not guarantee that a conformation (phi/psi pair) is forbidden. Therefore, if you use a rigid model to generate a Ramachandran plot, that is, if only phi and psi are allowed to change, then you should allow "wiggle room", for instance by setting a lower threshold for the allowed values of the vdW distances. The use of "tolerances" in vdW radii should therefore not be surprising.
Close contacts can be energetically costly even with compensating rearrangements, so the associated configurational space - marked as "partially allowed" in a traditional diagram - is more sparse. In a modern approach a more descriptive potential for the protein backbone is employed, and ideally degrees of freedom other than the dihedral angles are relaxed, and energy isolines are displayed. The following, from Ref. 2, shows roughly how such a diagram might look:

References

G.N.Ramachandran, C.M. Venkatachalam, and S.Krimm. Stereochemical Criteria for Polypeptide and Protein Conformations II. Helical and Hydrogen-Bonded Polypeptide Chains. Biophysical Journal 6 (1966) 849-872.
ibid, 909-933.


text:
Why would the following reaction not proceed through SN2 reaction?
I made a guess that the OH- group cannot do a backside attack on the carbon because the C double bond with O and the 5 carbon ring may act as steric hindrance and prevent it from forming a bond but I'm not really sure.

Carbonyls undergo addition-elimination. SN2 reactions at sp2-hybridized carbons are extremely rare (I only know of one report in the literature for a contrived case), so much so that you should never propose them.

Maybe it does. Take a look at this earlier SE Chem question, Nucleophilic attack at a carbonyl - angle of attack.

when hydroxide ion attacks , there is a chance of negative electromeric effect at carbonyl group carbon atom, so backside attack is possible but backside to C double bond O

The substrate in this reaction is an Acyl Chloride, which is highly susceptible to nucleophilic acyl substitution, more so than other carboxylic acid derivatives, so SN2 attack (as mentioned in the comments) is highly improbable. The mechanism for this nucleophilic acyl substitution is as follows:

With equilibrium constant $K_c >> 1$ favouring the products on the right.


text:
Does the acid dissociation constant takes hydrogen ions from water into account?
For the dissociation of a weak acid:
$$\ce{HA(aq)<=>H+ + A-}$$
To calculate $\mathrm{pH}$ of a weak acid, we use the expression: $$K_\mathrm{a}=\ce{\frac{[H+] [A-]}{HA}}$$
The $\ce{[H+]}$ has two sources:

Water
Acid

Now, many problems that I've read on the calculation of $\mathrm{pH}$ of weak acids that doesn't take $\ce{H+}$ due to dissociation of water into account. I speculated that maybe in the expression of $K_\mathrm{a}$, the $\ce{[H+]}$ is the total hydrogen ion concentration (including hydrogen ions from water). Is that true? Or is that an approximation we have to make?
Which one of the following correctly represents acid dissociation in water?
$$\ce{H2O + HA <=> H3O+ + A-},\tag{R1}$$
$$\ce{HA <=> H+ + A-}?\tag{R2}$$
Does the first reaction imply that water also dissociated and the $\ce{H+}/\ce{H3O+}$ concentration takes into account the contribution from water?

More exact expressions do, simplified ones leading to pH=0.5*(pKa - log c) do not.

The only difference between those two reactions is the former explicitly mentions hydration of protons, as protons do not exist naked in water, being an extremely strong Lewis acid, reacting with the Lewis base H2O.

It may help to have a look at this answer https://chemistry.stackexchange.com/questions/60068/how-to-set-up-equation-for-buffer-reaction/95566. and the answer listed in the comments.

Your question and your formulas are really asking about two different things.
Let's look at your formulas first.
$$\ce{H2O + HA <=> H3O+ + A-},\tag{R1}$$
$$\ce{HA <=> H+ + A-}?\tag{R2}$$
For R1 the equilibrium equation should be:
$$\mathrm{K_a} = \dfrac{\ce{[H3O+][A-]}}{\ce{[H2O][HA]}}\tag{Eq-1}$$
but for R2 the equilibrium equation should be:
$$\mathrm{K_a} = \dfrac{\ce{[H+][A-]}}{\ce{[HA]}}\tag{Eq-2}$$
Thus the two equilibrium equations differ by a factor of $\ce{[H2O]}$ in the denominator. Since these equations are really only good for "dilute" aqueous solutions (say less than 0.1 molar ionic strength) the concentration of water can be taken as a constant. Thus equation Eq-2 is the accepted way to consider the equilibrium.
Not to leave the explanation hanging, let me point out that rather than concentrations the equilibrium expression should really be written using activities.
$$\mathrm{K_a} = \dfrac{a_\ce{H+}a_\ce{A-}}{a_\ce{HA}}\tag{Eq-3}$$
where the activity of a species does take into account the ionic strength of the solution.
Now let's consider water's contribution to the $\ce{H+}$ concentration.
Water disproportionates according to the chemical formula:
$$\ce{H2O <=> H+ + OH-}\tag{R3}$$
with the equilibrium expression
$$\mathrm{K_w} = \ce{[H+][OH-]}\tag{Eq-4}$$
Thus if an acid, HA, is added to water, then both equations Eq-2 and Eq-4 must be satisfied simultaneously for an exact solution. Thus for a monoprotic acid this leads to a cubic equation, for a diprotic acid a quartic equation, and so on. Fortunately in chemistry only 2-3 significant figures are usable, so we can usually make assumptions about the chemistry and simplify the mathematics to a quadratic equation for most situations.


text:
Why is chemistry unpredictable?
Disclaimer: I am not a chemist by any means, and I only have knowledge limited to what I learned in my university's Chemistry III course. Basic understanding of everything up to valence electron orbitals.
Why is there no set of rules to follow which can predict the product of chemical reactions? To me, it seems that every other STEM field has models to predict results (physics, thermodynamics, fluid mechanics, probability, etc) but chemistry is the outlier.
Refer to this previous question: How can I predict if a reaction will occur between any two (or more) substances? The answers given state that empirical tests are the best way we've gotten to predict reactions, because we can discern patterns or "families" of reactions to predict outcomes. Are we only limited to guessing at "family" reactions?
In other words, why am I limited to knowing my reactants and products, then figuring out the process? Can I know the reactants, hypothesize the process, and predict the product?
If the answer is "It's complicated", I would enjoy a push in the right direciton - like if valence orbitals actually do help us predict, or any laws of energy conservations etc, please give me something which I can go research.

Your best bet would be to look at the field of theoretical chemistry which has various branches of study as highlighted in the above link. Basically, if you are given a set of reactants and products, then you can have a huge number of combinations of atoms simply by looking at all possible arrangments of atoms. But, you can significantly narrow down your options by looking at lower potential paths (which are made from ab initio calculations), simulating movements of the nuclei (molecular dynamics), theoretical kinetics and so on.

You can make reasonably good predictions using such methods about the most probably pathways, and such predictions are increasingly getting better as better computational technologies are developed. In the meantime, even if you end up obtaining results deviating from your predictions, you can assume that you have probably missed out on some factor, and try to run calculations accounting for the same till you end up obtaining the expected result

BTW, just a digression: You said chemistry is the "outlier" in STEM. I have a gut feeling that maybe biology (which is also a part of STEM) may not be "predictive" enough for you as well, at least when you start out with the subject. Biology and chemistry together form an integral part of STEM, so it may be worth pondering over their seemingly "empirical" nature ;)

Chemistry is neither a set of easy rules, neither set of empirical knowledge to memorize. Or, it it both, intertwined in a wild mixture of both extremes. Most non chemists are too impatient to recognize many patterns, that are hybrids of rules and evidence.

https://doi.org/10.1016/j.drudis.2018.02.014

https://par.nsf.gov/servlets/purl/10065514#:~:text=Reactions%20are%20predicted%20at%20the,products%20and%20suggest%20possible%20structures.

Maybe these papers could point you in a helpful direction...

What would you say about biology? Going further, psychology, or sociology? The more complex is the object of study the harder are predictions. https://xkcd.com/435/

Who says there are no rules. Between comments and answers it should be clear that you start from a false premise.

Physics has much more serious problems than chemistry wrt predictability. Physics is filled with problems which might well be impossible to solve. So much for prediction! Start with the famous 3-body problem, continue with chaos, dark matter, dark energy, black holes (the ultimate in predictably unpredictable), QM, etc ...

Firstly, is physics so very different? As Schroedinger's equation has no derivation then perhaps chemistry does not look so bad. Quantum Mechanics exists only by observation confirming its axioms. Secondly, reactions between most molecules can be predicted with good confidence nowadays based on empirical evidence form many experiments.

continues ... and third, to calculate what happens is extraordinarily hard as the energy along a reaction pathway is only a tiny fraction of the total energy so any approximations made on the way, (e.g nature of potential energy  surface) has a huge effect on the outcome. Forth there is very little, i..e. no direct experimental evidence on potential energy surfaces for most reactions.

It's not unpredictable. It is just complex... sometimes to a point it is easier to just try (many times) and see instead of calculating something.

I think you're confusing "predictive" and "predictable". Chemistry is predictable - repeating the same experiment gives you the same results over and over. Exactly how predictive it is is a bit more complicated - and in the end, you need to think about economics. You're looking for the way that yields results the most efficiently - and quite often, that is "do 200 experiments every day for a year" rather than "do everything from first principles". Chemistry is ridiculously varied compared to modern physics (not necessarily harder or easier, mind).

Related: xkcd 435

Note that computational quantum chemistry is used to eliminate many molecules ( up to 99% ) as potential candidates for curing drugs, before they are tested or even synthetized, what is much cheaper and faster. But it is extremely demanding on computer power. E.g. relativistic quantum computations, probably together with some Monte Carlo methods, predicting with good accuracy mercury melting point ( some 140 deg C error without special relativity, some 1 deg C with it)., waited about 50 years for fast enough computers, put into running just few years ago.

Hello, I'm no chemist, but many branches in STEM deal with random processes.  Chemistry is one of them because the smaller the subject (molecules, atoms) the  more uncertainty you have.  For example, why is the Sun's surface white (many colors) if its temperature and composition are somewhat constant?  Because a temperature yields a distribution of colors, not a fixed, exact, deterministic color, to put it simple.  Random processes are not unpredictable, but their uncertainties can be accuratelly quantified and probability and statistics is STEM.

The answer is simple: a reaction has billions and billions of atoms interacting with one another, but we create analogies to the interactions using just a few symbols which we manipulate using rules; a symbolic analogy of countless atoms interacting, but this process implies a of information. The simpler the analogy, the higher the loss of information and the less accurate the analogy. The results of symbol manipulation will differ from the reality of the reaction. Quantum chemistry has much more complex models that are a better analogy to the reaction, thus it is a *better predictor.

Chemistry is increasingly becoming a broader and broader group project for us. The more we collect and share our information with each other, the better we seem to predict stuff.

First of all, I'd ask: what do you admit as "chemistry"? You mentioned thermodynamics as being a field where you have "models to predict results". But thermodynamics is extremely important in chemistry; it wouldn't be right if we classified it as being solely physics. There is a large amount of chemistry that can be predicted very well from first principles, especially using quantum mechanics. As of the time of writing, I work in spectroscopy, which is a field that is pretty well described by QM. Although there is a certain degree of overlap with physics, we again can't dismiss these as not being chemistry.
But, I guess, you are probably asking about chemical reactivity.
There are several different answers to this depending on what angle you want to approach it from. All of these rely on the fact that the fundamental theory that underlies the behaviour of atoms and molecules is quantum mechanics, i.e. the Schrödinger equation.*
Addendum: please also look at the other answers, as each of them bring up different excellent points and perspectives.
(1) It's too difficult to do QM predictions on a large scale
Now, the Schrödinger equation cannot be solved on real-life scales.† Recall that Avogadro's number, which relates molecular scales to real-life scales, is ~$10^{23}$. If you have a beaker full of molecules, it's quite literally impossible to quantum mechanically simulate all of them, as well as all the possible things that they could do. "Large"-ish systems (still nowhere near real-life scales, mind you — let's say ~$10^3$ to $10^5$) can be simulated using approximate laws, such as classical mechanics. But then you lose out on the quantum mechanical behaviour.
So, fundamentally, it is not possible to predict chemistry from first principles simply because of the scale that would be needed.
(2) Small-scale QM predictions are not accurate enough to be trusted on their own
That is not entirely true: we are getting better and better at simulating things, and so often there's a reasonable chance that if you simulate a tiny bunch of molecules, their behaviour accurately matches real-life molecules.
However, we are not at the stage where people would take this for granted. Therefore, the ultimate test of whether a prediction is correct or wrong is to do the experiment in the lab. If the computation matches experiment, great: if not, then the computation is wrong. (Obviously, in this hypothetical and idealised discussion, we exclude unimportant considerations such as "the experimentalist messed up the reaction").
In a way, that means that you "can't predict chemistry": even if you could, it "doesn't count", because you'd have to then verify it by doing it in the lab.
(3) Whatever predictions we can make are too specific
There's another problem that is a bit more philosophical, but perhaps the most important. Let's say that we design a superquantum computer which allowed you to QM-simulate a gigantic bunch of molecules to predict how they would react. This simulation would give you an equally gigantic bunch of numbers: positions, velocities, orbital energies, etc. How would you distil all of this into a "principle" that is intuitive to a human reader, but at the same time doesn't compromise on any of the theoretical purity?
In fact, this is already pretty tough or even impossible for the things that we can simulate. There are plenty of papers out there that do QM calculations on very specific reactions, and they can tell you that so-and-so reacts with so-and-so because of this transition state and that orbital. But these are highly specialised analyses: they don't necessarily work for any of the billions of different molecules that may exist.
Now, the best you can do is to find a bunch of trends that work for a bunch of related molecules. For example, you could study a bunch of ketones and a bunch of Grignards, and you might realise a pattern in that they are pretty likely to form alcohols. You could even come up with an explanation in terms of the frontier orbitals: the C=O π* and the Grignard C–Mg σ.
But what we gain in simplicity, we lose in generality. That means that your heuristic cannot cover all of chemistry. What are we left with? A bunch of assorted rules for different use cases. And that's exactly what chemistry is. It just so happens that many of these things were discovered empirically before we could simulate them. As we find new theoretical tools, and as we expand our use of the tools we have, we continually find better and more solid explanations for these empirical observations.
Conclusion
Let me be clear: it is not true that chemistry is solely based on empirical data. There are plenty of well-founded theories (usually rooted in QM) that are capable of explaining a wide range of chemical reactivity: the Woodward–Hoffmann rules, for example. In fact, pretty much everything that you would learn in a chemistry degree can already be explained by some sort of theory, and indeed you would be taught these in a degree.
But, there is no (human-understandable) master principle in the same way that Newton's laws exist for classical mechanics, or Maxwell's equations for electromagnetism. The master principle is the Schrödinger equation, and in theory, all chemical reactivity stems from it. But due to the various issues discussed above, it cannot be used in any realistic sense to "predict" all of chemistry.

* Technically, this should be its relativistic cousins, such as the Dirac equation. But, let's keep it simple for now.
† In theory it cannot be solved for anything harder than a hydrogen atom, but in the last few decades or so we have made a lot of progress in finding approximate solutions to it, and that is what "solving" it refers to in this text.

Let me contribute two more reasons which make chemistry hard to analyse from a purely theoretical standpoint.
The first one is that, viewed very abstractly, chemistry essentially relies on the study of geometry in very high-dimensional spaces, and even from a purely mathematical point this can be extremely difficult. An important part of chemistry is bond breaking and bond formation, which is behind most reactions. This turns out to require knowledge of the vibrational modes of a molecule. For a general molecule with $N$ atoms, there are $3N - 6$ vibrational modes. Each of these vibrational modes are a "spatial dimension" in what is called phase space. In principle, if we knew the potential energy in every point of the phase space for a molecule, we would know virtually everything there is to know about how it might react. For an idea of what this looks see, see the figure below:

Source: https://www.chemicalreactions.io/fundamental_models/fundamental_models-jekyll.html
Unfortunately, there is simply too much space to explore in very high-dimensional objects, so it's very difficult to get a picture of it as a whole. Also disappointingly, almost all of this space is "tucked away in corners", so it is also very difficult to get a reliable picture of the whole space by looking at small bits of it at a time. This has been called "the curse of dimensionality". Something as simple as benzene ($\ce{C6H6}$) has a $\mathrm{3 \times 12-6 = 30}$-dimensional vibrational phase space (though this particular phase space is highly symmetric, as benzene itself has a high symmetry). Now consider a general reaction which requires two reagents, and forms one product:
$$\ce{A + B -> C}$$
Each of the three molecules has its own phase space, and combining them all together means adding all the number of dimensions of each. In this view, a chemical reaction is nothing but a particular set of trajectories of points (for each atom) in the combined phase space of all molecules, such that the potential energy of the system is locally minimised throughout the trajectory. As such, one would easily find themselves trying to describe trajectories in objects with over 100 dimensions. Few people talk about chemistry at this level of abstraction because it is so complex, but it is a conceptual hurdle in describing chemistry "exactly". Thankfully, there is research into it, such as the CHAMPS collaboration.

The second complication is that, while many important reactions are direct reactions like the one shown above, in the general case, what really exists is a network of reactions, potentially forming a complicated, highly interconnected graph with dozens or even hundreds of intermediates and possible products (graph vertices) and as many reaction arrows connecting them (graph edges). The field of chemical reaction network theory uses graph theory to study these networks. It appears that some of the problems they grapple with are NP-hard.

Source: https://www.mis.mpg.de/stadler/research/chemical-reaction-networks.html
Of course, this second issue compounds on the first!

So given these two dizzyingly complex problems, even from a purely mathematical standpoint, how can we do chemistry at all? Well, with enough experimental parametrization (e.g. equilibrium constants, rate constants, enthapies and entropies of formation, etc.) and approximations, you can drastically simplify the description of a system. Fortunately, even after throwing away so much detailed information, we can still make decent predictions with what is left. You really should count ourselves lucky!

This is a really insightful and absolutely fantastic answer and extremely helpful to those outside of or not immersed in chemistry. I see stable manifolds in satellite trajectories and in papers about them in Chaos but never stopped to think of how general the concept was. Instead of doing what I was supposed to today I will go hide somewhere and (try to) read all of your linked sources :-)

@uhoh There are a number of articles studying chemical oscillators (which can display mathematical chaos) from a reaction network theory perspective. That alone is probably quite a rabbit hole to go down. You have my blessing!

And all of that only to describe reactions that occur on 1 electronic state. Photo-induced reactions can involve multiple electronic states which also requires accurate description of the coupling of all states.

Parts of chemistry have predictability but the combinatorial complexity of what is possible leaves a large amount of space for things that don't follow the rules
Some of the ways chemistry differ from physics in unpredictability are an illusion. Take gravity, for example. There is a strong rule–sometimes described as a law–that all objects near the surface of the earth fall with the same acceleration. That is a cast iron rule isn't it? Apparently not. Flat pieces of paper and feathers don't fall as fast as cannon balls and the exact way they fall is very unpredictable. "But we know why that is, don't we?" Yes, a bit, it is air resistance. But that doesn't enhance the predictability at all as any useful prediction would have to solve the equations for fluid flow and there is a $1m prize for even proving that those basic equations even have a solution all the time.
Arguably, physics is only predictable in school where only idealised versions of real problems are considered.
And it is unfair that chemistry is completely unpredictable. A good deal of physical chemistry is quite like physics in its laws and predictions.
I suspect that you are talking about general organic and inorganic chemistry where there are many predictable properties of compounds but a dictionary full of exceptions to even simple rules.
Or synthetic chemistry where reactions sometimes work but often don't.  But, there are plenty of chemical reactions that work fairly reliably (Grignard reactions make C-C bonds fairly reliably with many compounds; Diels Alder reactions create two at once with predictable stereochemistry.)
But this predictability is limited by a fundamental problem: the unfathomably large variety of possible compounds that could be made. Take a ridiculously small subset of possible compounds: all those that can be made just from carbon and hydrogen using only single bonds and disallowing any rings. For simple compounds where the 3D nature of the compounds does not interfere by constraining their existence in real space (atoms have finite volumes in 3D space and can't overlap in real structures) these are mathematically equivalent to simple trees (or the carbon skeleton is: we assume the hydrogens fill out the remaining bonds so each carbon ends up with 4). At the point where 3D space becomes a constraint on which can exist (about 16 carbons), there are already about 25k distinct possible compounds and by the time you get to 25 carbons there are more possibilities than all the chemicals that have ever been characterised in the history of chemistry.
And this is for very constrained rules for making the compounds that use only two elements and deny a huge variety of interesting structures.
The real issue making chemistry apparently complex is that unfathomably large combinatorial variety of possible chemicals that might exist. In such a large space there is very little possibility that simple rules will always work. And this complexity is just about the possible structures. There are a very large number of reactions that get you from one structure to another and those add another midbuggeringly large layer of complexity.
And this, I think, is the reason why many find chemistry so hard to generalise about. There are simply too many possible things that can exist and even more possible ways to make them for any simple set of rules to always work. And I thought physicists had a problem not being able to fully solve the Navier Stokes equations.

Predictabilty is essentially determined by the level of detail you need to accurately model the phenomenon you're trying to predict.  Models that require little detail to capture the phenomenon of interest typically can give reliable predictions, while those requiring enormous detail typically cannot.
The level of detail needed is determined by the complexity of the system, and the fineness at which you wish to model it; the amount of detail needed to model a complex system can be reduced if you only wish to make coarse-grained predictions.
This is true for all the sciences—biology, chemistry, physics, and geology. Thus, in this fundamental way, they all have the same predictability. I.e., there is no fundamental difference in the nature of prediction among these fields. Allow me to illustrate:
Physics:

Bending of light from a distant star by the sun's gravitational field.  Predictable.  Requres very little detail to model the phenomenon accurately: Just the mass of the sun, and the assumption that the distant star is a point particle at a distance much greater than the earth-sun distance.

The temperature of the sun's corona.  Not yet predictable. This problem requires far more detail to model correctly.  The system is so complex that we don't have a model to predict the temperature of the sun's corona, and thus can't explain why the corona is far hotter than the surface of the sun.

Chemistry:

Osmotic presure of a highly dilute solution.   Predictable.  Requires very little detail to model the phenomenon accurately: Just the concentration of the solute.

Folding of long (1000's of nucleotides) RNAs.  Coarse-grained predictions (the relative radius of gyration) can be made; fine predictions (the ensemble-average probability of each individual base pair) cannot be made (at least not reliably).

Biology:

Possible blood types (O, A, B, AB) of offspring, and their odds.  Predictable. Requires only the blood type of each parent.

Size at which cells divide. Not yet predictable.  Cells are enormously complex, and thus a model capable of predicting this would require enormous detail.

Granted, there is a practical difference among the fields, in that physics has more phenomena that can be predicted with simple models than chemistry, and chemistry more than biology, because as one goes from physics → chemistry → biology, one is typically studying successively higher levels of organization of matter.  But I regard that as practical difference rather than a fundamental one.

Reminds me of Rumsfeld's famous "known unknowns and unknown unknowns". Another classic example: the weather. In some cases it is not detail that matters (nonlinear dynamics), in fact accurate simple models can fail to provide useful predictions even when all parameters are known exactly.

"it seems that every other STEM field has models to predict results (physics, thermodynamics, fluid mechanics, probability, etc) but chemistry is the outlier"
This is only partially true, but there are areas of all of those fields where predictive power is difficult in practice due to the complexity of the system and convolution of features. In simplified cases, yes, we can do quite well, but once the systems grow in size and complexity, we do less well.
Physics is a good example of this. The laws of mechanics are quite well-understood. But how well can you handle a chaotic 3-body system? There may be features that are predictable, but not probably not the entire system.
With thermodynamics, how well do we handle mesoscopic systems? Computationally, they can be quite difficult. In thermodynamics, we're able to deal with this complexity by discarding features that we don't care about to focus on bulk properties that rapidly converge in ever-larger systems, but we can't handle the entire system.
Fluid mechanics. OK. We have Navier-Stokes. Have you tried solving Navier-Stokes? Entire volumes have been written about how to deal with Navier-Stokes, and we still don't have great understanding of all of its features.
Probability. This is trickier to talk about, but I think the difficulty and complexity is building an underlying probabilitistic model. When you build your machine learning model, there are generally hyper-parameters to set. What makes a good hyper-parameter and how do you pick one? Just the one that works?
The thing with chemistry is that real-life examples are already incredibly complex. Pick any reaction you want. Liquids or solids? You're already dealing with bulk properties, phase interfaces, and boundary effects. Or solutions and solution effects. Gases? Once you have non-trivial reactions, how many atoms are there? How many electrons? Now, consider the fact that your typical organic reaction involves compounds with tens or hundreds of atoms in solution. There may be multiple models of reactivity, some productive, some not. And in the laboratory, reactions can be quite sensitive to any number of reaction conditions, which a generalized reactivity model does not begin to account for.
But in chemistry, as with the other disciplines, we aim to find simplifications that allow us to deal with complexity. We've been able to find patterns of reactivity, which are somewhat general but don't capture the full complexity of the system.

There are some great answers to this question already, but I'd like to provide a more practical boots-on-the-ground answer from my own perspective as an organic chemistry PhD who did computational chemistry on the side.
Most fields at their cutting edge are unpredictable
It has been my observation that when you come up against the frontier of what is possible, progress comes by and large only through a long grinding process of Trial and Error. When a breakthrough in understanding is made and the process all of a sudden becomes easy, rapid progress is made until things become hard again. This is true of all sorts of complex projects. The theory helps you along so far, but at some point you have to go off the beaten track and make your own way.
Chemistry gets mathematically hard, fast
Someone who's completing their undergraduate in a STEM field is likely to have a bit of a skewed impression of the first fact, because they will have already reached that point of unpredictability with chemistry but not with physics. It takes a long time to learn the differential equations associated with things like mechanics, stress-strain, heat transfer, fluid dynamics, electromagnetism and quantum fields. These topics often have solutions for idealised situations that are amenable to being written on paper in closed form.
The (comparative) simplicity of these solutions along with the difficulty of learning the necessary maths along the way may give the undergrad physicist the mistaken idea that this is what all physics is like. Hard-but-tractable differential equations that yield elegant solutions. In reality once you get past the idealised conditions physics becomes much more about computer simulation and experimentation.
In contrast, the equations that describe what happens in the flask (kinetics and thermodynamics) go from trivial to mind-bendingly difficult with only a little bit of added complexity. Other answerers have gone into this part in more detail, so I won't talk but about it here. Suffice to say I spent many fruitless hours of my education trying to find a generalised maths approach to the problems I was facing.
In practice, at least for organic chemistry, the main objective is to synthesise compounds from other compounds, typically complex ones from simple ones. The theory sort of devolves into a broad, massive decision tree.
Want to make intermediate A? try reaction B, if that doesn't work try reaction C. C normally works for this kind of thing, so if it's not working, check that your reagents are pure. You could try D but that's likely to deprotect the other side of A.
Systematic studies of certain reaction patterns exist, and they can certainly be helpful. Take the substitution patterns of aromatic rings, for instance. Using a bit of orbital theory, you can predict the outcome of reactions on aromatic rings based on what's already on the ring and in what position. But again, these studies were done on simple substrates and may not necessarily apply to whatever synthetic behemoth you're working on.
Lab work is hard
Finally on to the practical aspect. Chemical reactions may fail for any number of reasons that aren't theoretical. There's basic stuff like the cleanliness of your equipment and the purity of your reagents. You can lose heaps of your material by choosing the wrong solvents to work up (extract the product from the reaction mixture) with. Most of the material has gone into the aqueous layer without you knowing it, and you've either discarded it (rookie mistake) or it's degraded or turned into something else before you realised.
Then there's the more subtle stuff. The reaction might only work with one particular stir bar because it was impregnated by a palladium catalyst at some point. Reactions often need rigorous exclusion of oxygen and water to work, but occasionally you actually need some oxygen present to make it go, and the only way you'd ever find this out is by noticing the sloppily set up reactions always seem to perform better than the rigorous ones. You have one bottle of reagent from the sixties from a company that no longer exists, and once that's used up, the new bottle of the same reagent just doesn't work (this one happened to me). The surface of your glassware is slightly too acidic for your reaction, and you need to silanise it to get it to work (also happened to me). Some reactions don't work because your country is just too damn humid. The procedure you're following was written by a student desperate to impress/placate their adviser, and the yields are inflated. Your current lot of acetonitrile solvent is lower quality because China shut down their polluting acrylonitrile plants in order to improve the air quality in preparation for the Olympics. And so on and so forth.
Chemistry as a subject is very bitsy and messy. The best chemists I've known often had excellent memories. But all subjects tend to be messy and bitsy once you get past the basic theory and into the fine details.

To me, it seems that every other STEM field has models to predict results (physics, thermodynamics, fluid mechanics, probability, etc) but chemistry is the outlier.

What about structural engineering? Within that field, it's quite easy to predict the strength of a beam of known material and dimension, like a steel I-beam or dimensional lumber. But what about some new material, like a composite of toothpicks embedded in Elmer's glue?
Whether the material is steel or toothpick-glue composite, couldn't one "just" predict the strength from more basic physical properties?
Well yes, but that would be very complex. But I think more importantly, that wouldn't be structural engineering anymore. It would be some more basic field of physics.
You argue chemistry is "unpredictable" because reactions are described by rules and patterns rather than being derived from first principles. I posit these rules and patterns are chemistry. Without them, you no longer have chemistry. So chemistry is "unpredictable" (in your sense) by definition.
This isn't unique to chemistry, really. Most fields of study are based on the application of more pure fields, adding their own rules and patterns to enable higher-level reasoning about more complex systems:

The answer is dimensionality reduction: a reaction has billions and billions of atoms interacting with one another, but we create analogies to the interactions using just a few symbols which we manipulate using rules; a symbolic analogy of countless atoms interacting, but this process implies loss of information about reality. The simpler the analogy, the higher the loss of information and the less accurate the analogy. The results of symbol manipulation will differ from the reality of the reaction. The average of a set of numbers is a good example: you reduce a set of n dimensions onto a single dimension. There is a loss in information.
Another example: Newtonian physics did not predict what scientist saw with the famous Double Slit experiment. The moment that happens, the rules and symbols you use to make predictions(like the yield of a chemical reaction) become useless. So, it's not that Chemistry is unpredictable, the symbols we use to make predictions about chemistry are not good enough. The only way to make 100% accurate predictions is to simulate every single atom and subatomic particle and be certain the rules we use to define the interactions are 100% analogous to what happens in reality. We know this to be impossible due to the uncertainty principle.
Quantum chemistry has much more complex models that are a better analogy to a reaction, thus it is a *better predictor, but never 100% accurate.

Any basic text in Organic Chem has a table of contents.So for a given transformation such as reduction it will list reagents( the chemicals or conditions,eg heat ,light,that appear above the arrow connecting reactants with products) For a simple reactions such as the sodium borohydride reduction of acetone to isopropanol I have absolute faith that if I carry this out in the laboratory it will work.If it didn't work I would check the labels on the reagent bottles and confirm the identity/purity of the chemicals used.If these checked out and the reaction still failed ,it would be in the category of dropping an apple and watching it ascend up toward the sky.It is not a matter of a failed opinion.Now if I  change the substrate to a large polyfunctional molecule,the analogous reduction  may  not occur at all or yield a highly rearranged product.Retroanalysis may provide a rationale but for the bench chemist doing the reduction,it is an opinion as to whether it is worth trying the reaction in the first place.

Simply put, it is because we don't have complete or near complete understanding of the forces that drive chemical reactions, every few atoms added to the structure of the compounds will add new forces and layers of complexity that we haven't accounted for in our simple 300 years of chemistry knowledge. You can sense this when you learn the theories show their limitation at some point where complexity surfaces (for instance Lewis, Huckel ... etc).


text:
What makes concentrated aqueous solutions of aluminium nitrate and aluminium chloride suitable to dissolve noble metals, specifically gold?
Aluminium nitrate is a strong oxidising agent and aluminium chloride acts as a Lewis acid. Also these solutions are claimed to have the abilities of aqua regia and therefore can dissolve noble metals.
References

Dissolution of noble metals in highly concentrated acidic salt solutions
Federica Forte, Sofia Riaño and Koen Binnemans, Chem. Commun., 2020, 56, 8230-8232, DOI: 10.1039/D0CC02298E
Dissolving Noble Metals without Aqua Regia,  David Bradley, Chemical Communications/Royal Society of Chemistry, 04 August 2020 DOI: 10.1002/chemv.202000071

It is very unusual when a question body  is not really a question.

Sorry I am new to this. Is there any more information you need to come up with the explanation?

Do you want us to read the articles for you? It seems you have all the information you need?


text:
How are class II force field cross-terms derived?
In some class II force fields, there are cross-terms that characterize bond-bond, bond-angle interactions and others. Example of equations below:
$$\sum_{bond-bond}^{}k_{bb}(b-b_{0})(b^{'}-b^{'}_{0})$$
$$\sum_{bond-angle}^{}k_{ba}(b-b_{0})(\theta^{'}-\theta^{'}_{0})$$
My question is: how do I get the $k_{bb}$ and $k_{ba}$ values?
I thought of two possibilities:

$k_{bb}$ must be obtained by some combination rule (e.g., Lorentz-Berthelot) that involves two $k_{b}$ values (of the bonds of interest). A similar procedure could be done for $k_{ba}$.

Or $k_{bb}$ and $k_{ba}$ are adjusted as a total energy of the molecule of interest (e.g., I take the force field and adjust the "global" equation for various geometric configurations, where the energy values are obtained by quantum chemistry).

If "1." approach exists, could someone kindly provide me with an article that reports the existing combination rules for that purpose (i.e., if you face it)?

The images for your equations aren't working. You can write mathematical expression here using Mathjax

It was working properly. I will correct, thanks.

Check out:  https://pubs.acs.org/doi/pdf/10.1021/ja00085a036
In short, you'll need to do QM calculations and frequency analysis to get the force constants. You find more information and references here: https://gaussian.com/freq/


text:
Halogen vs H-Bond Donor/Acceptor: How do they differ?
I know that in rational design sometimes the researcher might use Cl or F instead of  a H-Bond donor/acceptor. What kind of interaction does a halogen provide compared to the H-bond donor/acceptor? When might it be better to use one compared to another for best binding affinity?
Below is a picture of a small molecule in an active site. I am considering if should use a H-bond donor or a Halogen to complement the carboxylic acid (skinny line structure with red oxygen in upper left). I want the most attraction to occur between the small molecule and the active site.

https://en.wikipedia.org/wiki/Halogen_bond


text:
Is it OK to dispose of potassium hydrogen phthalate into the sewer system?
I have been using potassium hydrogen phthalate (KHP) for creating primary standards. I have looked into the Flinn catalog, and the disposal consideration is classified as "organic acids, must be neutralized prior to drain disposal." That seems easy enough, but with recent publications of health concerns of phthalate exposure I just want to make sure. I have looked everywhere, and I can't find any additional information as to how this stuff can be disposed.
I know some of you use this in your labs. How do you dispose of it? I could reduce it down in solution or hire a company to take it away, but if it's safe to neutralize and empty into the sewers I'd rather save the time and money.
Thanks in advance for any help!

No chemicals of any kind go down our drains. Everything goes out as chemical waste through appropriate disposal paths. Check your appropriate local-state-federal laws for your area.

Check out the MSDS of KHP.

The MSDS is helpful. "Do not let this chemical enter the environment" Thank you!

Generally, mixing with amphoteric NaHCO3 thereby creating more likely insoluble/low soluble compounds is a good start. Then, dispose of as regulations further dictate. This is wise in that one should not assume, for certain, that correct disposal procedures are correctly actually always implemented to the certain detriment of the environment.


text:
Can you make n-Hexane a supercritical fluid?
When making aerogel using supercritical fluids the liquid in the gel is replaced by a gas. Can this be achieved using n-Hexane as the supercritical fluid? What would the resulting gas be? Would it be safe to do or even possible?

A supercritical fluid is just a state of matter. like solid, liquid or gas. Hexane is no more or less toxic if it has become supercritical and then brought back to standard temperature and pressure, as would water, if it changed from liquid to ice or to supercritical "steam" and back to water.
Further, supercritical $\ce{CO2}$ is used for dry-cleaning clothes, removing contaminants from printed circuit boards, and even to produce decaffeinated coffee.
So the question is not whether it's safe to use supercritical hexane, but whether there is a process that can use hexane to make an aerogel. A quick search on the web found articles on ceramic aerogels, polypropylene aerogels, and magnetic carbon-fiber aerogels using hexane in the synthesis, and you should be able to find others.


text:
What did I do wrong in my orbital notation for this Os (Osmium) question?
I was given a question where I had to write the orbital notation of Osmium (Os) and I got it wrong.
The question:
6s2 --> ↑↓  4f --> ↑↓↑↓↑↓↑↓↑↓↑↓↑↓  5d6 --> ↑↓↑↓↑↑
The arrows above are my answer. What did I do wrong? I think it might be my answer to 5d6 but I am not sure. How many unpaired electrons are in Osmium?

According to Hund's rule of Maximum Multiplicity, a higher spin multiplicity is more stable. In simple terms, this means that a given atom would try to have the maximum number of unpaired electrons. For Osmium, which has a $\mathrm{5d^6}$ configuration, there are 5 d-orbitals and 6 electrons to fill, so the maximum number of unpaired electrons in this arrangement would be 4 (two electrons pair up and occupy one orbital, and the remaining 4 occupy a single orbital each). Here's the orbital diagram (taken from this Chemistry Libretexts article, which you should read)


text:
liquid crystal and light
The liquid crystal can be control using polarizes light. This means we can control the arrangement of the liquid crystal molecules, then when the light passes through it can be manipulated.
But why exactly is this possible? What is the relationship between liquid crystal material and light pass through it? How do they interact and affect each other?

Please check your English grammar. It very unclear who controls what.

And you don't want to use the word "control" anyway. Look it up in a dictionary, it's a total misapplication, resp. bad labslang.

Cross posted on Physics SE (where it probably belongs anyway).  Suggest closing here.

This text is not English. A liquid crystal cannot be "controlled". Furthermore, nobody can "control" an arrangement, with or without polarized light. And this arrangement cannot be "manipulated".

Liquid crystals are controlled by electric fields and their structure manipulates the polarisation of light
Liquid Crystals are usually made from long rod-shaped molecules which behave in some ways like liquids (they flow) but in other ways like crystals (they form structures with a lot of regularity). The name is appropriate.
The key features that matter are that the structures they form are easy to manipulate with electrical fields and that the rod-like nature of the structure interacts with light in roughly the same way as polaroid: it selectively alters the polarisation of the light passing through.
In LCD devices the liquid crystals are placed between two polaroid filters and in front of a backlight so the the default orientation of the filters plus the liquid crystal layer lets most light through. When a small electric field is applied across the liquid crystal it can be arranged so that it changes the orientation of the molecules in the layer and twists the light so it no longer passes through the polaroid layers. This can be arranged to happen one pixel at a time with suitable electronics. So an electrical field can drive a display.
That's the basic principle. Real displays have evolved to have some very complex arrangements of these effects to improve speed and the twistiness of the LC layer to make faster responding and higher contrast pixels.


text:
How does the Benedicts Test work?
What exactly is the mechanism by which Benedict’s test works for reducing sugars? As I understand it the free hemiacetal groups on reducing sugars opens up and an aldehyde is formed, this is then oxidised by Cu2+ which then results in a red copper(I) oxide.
RCHO + 2 Cu2++ 5 OH- → RCOO- + Cu2O + 3 H2O.
The equation above isn’t really clear to me, what is exactly going on?
Additionally why are alkaline conditions required for the test? And if a red precipitate is formed how come there is such a wealth of colours observed (green, yellow, orange) for varying concentrations of reducing sugars?
Perhaps these are a couple of questions too many :D

In addition to the post linked above you can look at https://chemistry.stackexchange.com/questions/62352/why-is-the-reduction-by-sugars-more-efficient-in-basic-solutions-than-in-acidic

See also: https://chemistry.stackexchange.com/questions/40127/why-does-fructose-reduce-tollens-reagent-and-fehlings-solution

And: https://chemistry.stackexchange.com/questions/82677/why-are-different-colors-according-to-sugar-concentration-observed-in-benedict


text:
Can H+ ion exist independently?
Can H+ ion exist independently since it does need to form any bonds.Why do we say that it needs to be in H30+ ion form?

Please explain. What is the meaning of "independently" ? Independently from what ? Or do you want to know whether H+ exists alone, without any water arounds it ? In the vacuum of a mas spectrometer, the ion $\ce{H^+}$ does exist. Not in water.

Yes I want to know whether H+ ion can exist alone.

As long as there is something (partially) negative around, a proton will be drawn to it. In aqueous solution it forms (in first approximation) $\ce{H3O+}$.

Be aware that $\ce{H+}$ is nothing else then a proton $^1_1\mathrm{p}$. Protons have an extreme charge density, typically $10^{15}$ higher than  any other ion ( but bare electrons or atomic nuclei).
Protons can exist free in vacuum or plasma, but even in gases they quickly react with many gaseous molecules, forming molecular ions, like dioxidanylium $\ce{O2 + p -> HO2+}$.
Protons act in aquaeous environment as extremely strong Lewis acid with an electron pair of $\ce{H2O}$(Lewis base), forming a ion called hydronium, a particular type of oxonium ions.:
$$\ce{p + H2O -> H3O+(aq)}$$
For convenience, we often use notation $\ce{H+}$ or $\ce{H+(aq)}$, supposing it is at least $\ce{H3O+(aq)}$.
Experiments mentioned in the above Wikipedia link confirm hydronium is usually coordinated with 6 water molecules as $\ce{[(H3O)(H2O)6]+}$, but multiple various hydration structures have been observed or assumed, like
Zundel ion $\ce{H5O2+}$,
$\ce{H9O4+}$($\ce{H3O+ . 3H2O}$)
or a cluster $\ce{H3O+ . 20 H2O}$
Note that the hydronium ion forms solid salts with acids with dissociation constant $K_\mathrm{a} \gt 10^{9}$.
E.g. a crystallic monohydrate of perchloric acid is hydronium perchlorate $\ce{[H3O+][ClO4-]}$

If we try to answer this question at a rather simple level, we could say that yes the ions can exist in a particle-free environment like vacuum. But if ions are formed in an aqueous medium, like the H+ ions in acidified water then the H+ readily combines with H2O to form hydronium ion. Which means that the H+ is not stable but since it can't combine with anything in vacuum, it exists. But as soon as there's something it could combine with, it'll readily do so. That is the reason they say we must mention it to be in hydronium ion form when it is formed in an aqueous medium.