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Physics, Astronomy & Cosmology / Re: Why don't atmospheric gases separate according to specific gravities?

« on: 29/03/2019 18:58:12 »

There is a very small amount of separation, but this is largely due to the fact that most of the gases that make up the atmosphere have very similar molecular masses (N₂ 28, O₂ 32, Ar 40).

The difference in gravitational potential energy that would occur by stratifying would be less than the energy equivalent of the decrease in entropy (due to unmixing) at the temperature that the atmosphere is. Same reason that vodka doesn't spontaneously separate into water and ethanol (which is about 20% less dense than water).

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Chemistry / Re: Why would you burn coal, then turn the carbon dioxide back into "coal"?

« on: 07/03/2019 21:12:53 »

As far as I can tell evan_au's objection stands.

It doesn't really matter what is done with the "coal." I have worked on a few carbon sequestration projects myself--it really only works out if (a) the energy being used to capture and convert the CO₂ is 100% carbon free, and (b) if the product has value. There may be use in products from electrocatalytically reduced CO₂, but if you think of the carbon footprint required to generate the raw materials to build a CO₂-reducing plant on the scale needed to have any effect on the atmospheric composition, it is unlikely to pay for itself (in terms of net CO₂) before it breaks!

Photosynthetic organisms already capture and convert CO₂ far faster than we could hope to do it (in the next few decades, at least). Just look at the slope of the jags on the Keeling curve compared to the slope of the overall trend:

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We just need to focus on not burning the fossil fuels in the first place, and allowing the photoautotrophs to do the work (ie preventing deforestation, promoting reforestation, and promoting healthy oceans). As long as we can allow the downward parts of the jags to extend a little bit, and the upward parts of the hags to be a little less intense, we can return to a reasonable equilibrium pretty quickly.

There might be a tiny little nudge we can do technologically to speed the process along, but focusing money and attention on these sorts of "band-aid fixes" merely diverts it from the "severed jugular" that the actual problem is.

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General Science / Re: Is there any science behind divining water or oil?

« on: 04/03/2019 14:59:11 »

Indeed--there is no reasonable physical mechanism to facilitate divining. There are no properly controlled studies to indicate that it works better than chance. Major corporations each spend (many) millions of dollars on devices that they use to find water and oil with--why would they opt to do that if a dude with a stick could do the same?

Divining/dowsing is fakery.

https://xkcd.com/808/

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Technology / Re: Why are Some Fridges Hard To Open Again Once Just Closed?

« on: 04/03/2019 14:52:23 »

I have noticed this before as well. I suspect it has to do with pressure:

Fridges don't have a perfect airtight seal, but they are pretty good.

Let's say that the fridge interior is kept at 8 °C (281 K), and the air in your kitchen is 24 °C (297 K), so there is a fairly significant difference between the two (otherwise you wouldn't need a fridge!). When you open the door, the air mixes--for the sake of argument let's say that the air inside reaches 22 °C (295 K) by the time you are ready to close it. Then when the door is closed, the mixing no longer happens, and the air inside begins to col rapidly (the walls of the fridge and all the stuff in it is still at 8 °C, and has a larger heat capacity than the newly replaced air. If the air inside then cools down to 10 °C (283 K) from 22 °C (295 K), then the pressure would drops to 96% of atmospheric pressure (283/295). Standard atmospheric pressure is 101.3 kPa, so the difference in pressure between the inside and outside would be about 4 kPa, or 4 Newtons per square meter. Thus if your fridge door has an area of 2.5 m², you would need to supply 10 Newtons of additional force to open the door (about the same as lifting a 1 kg weight). This isn't much, but it is significantly more than would be required to open the door in the first place. (There may be more at play, as this number does seem low--even increasing the temperature difference within reason only gains a factor of 2. Perhaps condensation of humidity in the air compounds the effect--the vapor pressure of water falls from about 3 kPa at 24 °C to about 1 kPa at 8 °C, so this would add another 50% to the 24-8 difference...)

If , however, instead of trying to open the door again right away, you give it a few minutes, there is enough time for the pressure to fall, and then more air is able to leak in, re-equalizing the pressures.

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Chemistry / Re: Is there a way to separate the HCl from a mixture of salt and vinegar?

« on: 24/02/2019 01:29:08 »

Sodium chloride and vinegar will not be a useful combination for distilling HCl.

The pK_a of acetic acid (the acid in vinegar) is more than 12 units above the pK_a of hydrochloric acid (4.8 vs about –9), so the concentration of HCl could never be more than one trillionth of the concentration of acetic acid (and vinegar is about 5% acetic acid). Also, acetic acid and water are both fairly volatile, so you would probably just end up distilling water and then acetic acid, leaving sodium chloride behind.

It is possible to convert sodium chloride into hydrochloric acid by adding acid and distilling, if one uses much stronger acid with a much higher boiling point (and little to no water). I believe that sulfuric acid and phosphoric acid both would do the trick.

However, I should note that pure, hot, gaseous HCl is *extremely dangerous.* If you don't have the right equipment to condense it, or aren't working with enough ventilation, or something goes wrong, you could easily *permanently* damage your eyes and/or lungs. HCl can be purchased (in multiple concentrations), and this is a much safer way to get it (cold dilute aqueous solutions of HCl can still blind you, but at least you are unlikely to kill yourself with them...)

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Chemistry / Re: If i dry-distil sodium acetate, will i get acetone?

« on: 23/02/2019 12:07:03 »

Chris has correctly identified that the oxidation state of a carboxylic acid (like acetic acid) is higher than that of a ketone (like acetone), but there is another problem with this approach--molecules of acetic acid have two carbons in them, and molecules of acetone have three carbons in them.

There are ways of converting acetate to acetone, but they are much more complicated and less efficient than envisioned by the OP.

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Chemistry / Re: Can an electron have two orbital directions?

« on: 21/02/2019 20:46:55 »

m_l is the closest thing to an "orbit direction"

ie if l = 0 then there is no need to describe a direction
if l = 1 then m_l can be +1 or –1 or 0 (representing each of the axes that the spin could be about)
if l = 2 then m_l can be +2 or +1 or –1 or –2 or 0 (representing each of the pairs axes that the spin could be about) (this one is more of a stretch)

there is no reasonable interpretation of an m_n

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Question of the Week / Re: QotW - 19.02.18 - Why does alcohol make you drunk?

« on: 18/02/2019 18:50:35 »

As far as I know, the exact mechanisms of alcohol intoxication are not known. However, it appears that the primary effects are related to potentiation of GABA pathways in the brain (so many of the effects are similar to those of benzodiazepines, GHB/GBL, gabapentin, and barbiturates, each of which potentiates or activates this pathway in different ways.)

Unlike these other drugs, especially benzos, which are quite potent and selective (a 2 mg dose of valium wouldn't be unusual), alcohol is very weak and unselective--requiring doses typically measured in ounces or deciliters!

As I understand it, activating GABA pathways dampens other signals in the brain--with the effects manifesting more strongly for processes that require more neurons (especially if thee neurons are in different parts of the brain). This means that just a little bit of alcohol can potentiate the GABA pathways a little bit, causing the most change in higher cognitive functions like reasoning and self-control. As the dosage increases, the effects there increase, and it also starts manifesting in fine motor control and response time. When really large doses are reached, even the ability to breath can be compromised.

Acute toxicity of alcohol (for a single large dose) can largely be attributed to loss of the ability to regulate breathing, or aspiration of vomit. But most injuries and deaths due to acute alcohol intoxication are due to loss of gross motor skills and response time, especially when it comes to things like automobiles and stairs...

However, much of the harm from alcohol is not from acute, but rather chronic abuse. Because the dosage is so high, alcohol is very hard on the liver and kidneys. Also, the neural pathways that alcohol acts on become resistant, then accustomed to and then dependent on the alcohol. Those who drink excessively for a long time can suffer from the potentially fatal DTs (delerium tremens), which can cause hallucinations and seizures (both of which can be treated with some of the GABA potentiators mentioned earlier).

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New Theories / Re: How do we measure the energy of a photon?

« on: 07/02/2019 04:46:54 »

Space is curved by massive objects. Light goes "straight" within that space. This is very different from the force of gravity on other massive objects.

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Chemistry / Re: How do I identify bridging Mo-O and terminal Mo=O in FTIR spectrum?

« on: 04/02/2019 22:10:24 »

The terminal bonds will have higher stretching frequencies than the bridging ones. I don't know off the top of my head where these will all lie, but based on Co-O and Co-O-Co vibrations, which I am more familiar with, I would expect bridging Mo-O-Mo vibrations to be around 300 cm^–1, terminal Mo-O-H to be more like 600^–1, and Mo=O might be closer to 1000^–1...

I am certain this info can be looked up. You can also model it using DFT.

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Chemistry / Re: Can water be avoided in the Limestone cycle

« on: 29/01/2019 19:04:57 »

I don't know what the optimal amount of water would be (and it might be zero).

I think that sounds like the starting point of a great science fair project. You could either do tests with controlled atmospheres (in sealed air-tight containers) that have essentially no water in them, and possibly higher concentrations of CO₂ than air has, assuring that there is no reaction with water, and only with CO₂. Or you could test equal amounts of CaO in setups that are identical other than how much water is added first.

It might be difficult to measure how much CO₂ has been absorbed, but not impossible.

Good luck!

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Chemistry / Re: Mercury, can you really safely dip your hand into?

« on: 29/01/2019 15:04:15 »

Yes. You can do it once or twice without any ill effect.

However, mercury is quite toxic. So how can this be?

Elemental mercury (ie the liquid metal) cannot easily pass through skin, so every time you touch it the dosage is incredibly small--each touch will deliver far less than a toxic dose. However, mercury is also very slow to leave the body, and can accumulate. This means that repeated exposure to small doses eventually adds up to a large dose.

Those who work with mercury on a regular basis without proper protection will be poisoned.

It is especially dangerous to heat mercury up, as it has a fairly high vapor pressure (for a metal--it boils at 360 °C). The vapors can condense in your lungs, and then will stay there until it has been absorbed (even though it is absorbed slowly, there isn't anywhere else for it to go, so you will get the whole dose).

Also, some compounds of mercury are MUCH more easily absorbed. Dimethylmercury is a notorious example. A single drop can pass through a rubber glove and through the skin within seconds, delivering a fatal dose (this has happened).

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Cells, Microbes & Viruses / Re: What molecules can pass freely from cell membranes?

« on: 28/01/2019 15:59:21 »

There is certainly NOT a complete list of molecules that can pass through membranes freely (there are almost certainly  more than 10⁴⁰ such molecules!!)

There are some rules of thumb. Lipisnki's rule of 5 is not exactly what you're looking for, but I think it is probably a good place to start https://en.wikipedia.org/wiki/Lipinski%27s_rule_of_five

It is also important to note that there are also compounds (DMSO comes to mind) that can make membranes more permeable--allowing though molecules that wouldn't normally cross easily, or increasing the crossover rate for molecules that already could cross.

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Chemistry / Re: Can water be avoided in the Limestone cycle

« on: 28/01/2019 15:41:57 »

Yes, it is possible for CaO to react directly with CO₂ to produce CaCO₃. However, in practice, in the environment, there is far more available water than CO₂. The CaO could come into contact with liquid water, resulting in very fast conversion to Ca(OH)₂, or undergo a more sluggish conversion by reaction with gaseous water in the air. Even in very dry conditions, almost 1% of the air is water, while CO₂ only makes up about 400 ppm (0.04%). Thus a CaO surface is much more likely to interact with water molecules before CO₂ molecules. (I also think that the reaction with water will be much faster, even if the concentrations were equal, because it is effectively an acid-base reaction, or proton transfer, which is among the fastest types of reactions known).

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New Theories / Re: How do we measure the energy of a photon?

« on: 28/01/2019 15:00:41 »

To clarify my point about direct vs indirect:

I think we can agree that when I talk to someone on the phone, that is a form of indirect communication: the sound waves of my voice are captured by a microphone, converted into electronic signals, which are then converted into electromagnetic radiation, before being received, turned back into electronic signals, which the speaker reconstitutes into my voice (this is a very simple version).

But what I urge people to consider is that talking face-to-face is also indirect. My vocal chords vibrate, bumping into air molecules, which bump into air molecules, which bump into air molecules, which bump into air molecules, which bump into air molecules, etc. etc. until those air molecules reach the ear of my listener. But wait, we're not done yet! The ear is really complex. Those moving molecules bump into their ear drum, causing vibrations that then emanate from the eardrum, and goes into a funny spiral-shaped organ, that is able to physically do a Fourier transform of the vibrational frequencies (it sorts them out by frequency due to how easily the sound waves are able to go around a bend), ultimately detecting those vibrations with little hair-like structures, that then encode the vibrations into neural signals, to be processed by the listeners ear.

My point is: when considering information transfer on a molecular scale or smaller, one must accept that there is not really any such thing as a direct measurement, other than the actual interaction between two particles (and there is no way for that kind of interaction to directly transfer the information from a single particle to any sort of macroscopic device.)

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New Theories / Re: How do we measure the energy of a photon?

« on: 24/01/2019 03:41:38 »

1. Yes, it is possible to generate photons without the involvement of electrons, but moving electrons (in atoms, or other sources of electric/magnetic fields) is one of the easiest ways.

Photons can be generated by accelerating charged particles (including electrons, or protons, or muons, or alpha particles etc.)

Photons (high energy) can be produced by the annihilation of particles with their antiparticles.

Photons (also typically high energy) can also be produced from nuclear reactions.

Electrons don't necessarily have to change energy levels--simply changing vibrational states in molecules or crystals can lead to emission (or absorbtion) of photons in the infrared region of the spectrum.

2. Electrons can "leap" to higher energy levels for many reasons, including colliding with other electrons or particles. Or high energy vibrations can be converted into excited electronic states with very little vibration. A good example of both of these phenomena is the blackbody radiation of hot objects. Thermal energy is essentially kinetic energy of vibrations and random motions. If there is enough thermal energy (if the sample is hot enough) then some of this kinetic energy is converted to light--hence hot objects glow, and the hotter they are, the more shorter wavelengths can be emitted (blue is hotter than yellow is hotter than red). Another example of collisions creating light is "triboluminescence"

There are also chemical reactions that produce light directly (like in fireflies and glowsticks).

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Geology, Palaeontology & Archaeology / Re: How can you tell silicon from germanium non-destructively?

« on: 23/01/2019 05:06:02 »

Silicon and germanium do have a very similar luster, but germanium is more than twice as dense (5.3 g/mL vs 2.3 g/mL) it would be very easy to tell the difference between the two just by picking them up (for anything larger than a lima bean), and trivial to distinguish smaller pieces using an analytical balance and a small graduated cylinder.

One could even put a sample in a vial containing diiodomethane (density of 3.3 g/mL). If it floats, it's silicon, if it sinks, it's germanium (assuming you know it is one of the two).

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New Theories / Re: How do we measure the energy of a photon?

« on: 23/01/2019 02:46:14 »

it seems to me that none of these arguments involve directly measuring the energy E in joules of a single photon as per the E=hf equation. 

That is correct. There is no good way to directly measure the energy of a single photon.

But, if you go back and look at the answers that were given to you, you will see that we can transfer the energy from photons to electrons, and then measure the energy of the electron. We can do tests to make sure that each electron is only absorbing the energy from a single photon, and we can do tests to make sure that all of the energy from the photon goes to the electron. And we can do tests to measure the wavelength of the photons. Therefore, we can indirectly get very accurate measurements of the energy that a photon has as a function of wavelength.

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Physics, Astronomy & Cosmology / Re: How fundamental is time?

« on: 11/01/2019 16:50:14 »

Change and time are not equivalent. Time is a dimension, and change refers to differences within one dimension.

This is most well defined within calculus (sometimes called the math of change), where the symbol ∂ (sometimes d) is used to denote microscopic change, often relating the change of one thing with respect to the change of another.

In this way, we can define instantaneous (microscopic) change of position along a spatial dimension (x) with respect to instantaneous (microscopic) change in time (t) as ∂x/∂t which is equivalent to the velocity in the x direction.

Although time is a very common dimension (variable) to compare changes against, it is not the only one. For instance we can look at the change in altitude with respect to change in forward distance traveled ∂y/∂dx (slope) or the change in cost of making chocolate with respect to the change in the cost of labor ∂$_chocolate/∂$_labor etc. etc.

What I am trying to learn in this thread is whether our concept of velocity (or any other rate of change with respect to apparent time) is best described as a ∂x/∂dt, or if there is a more fundamental variable (I believe I called it α earlier in the thread) such that ∂t/∂α is well-defined, and ∂x/∂α holds up better to the boundary condition that the big bang appears to impose on ∂t.

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Physiology & Medicine / Re: What could cause a too high potassium count?

« on: 09/01/2019 01:51:59 »

Obviously cooking won't make extra potassium appear out of thin air.
I think that cooking just makes the potassium more easily available by breaking down the cell walls.
Or it could be that the reported numbers don't account for loss of water during cooking. I could imagine 100 g of veggies cooking down to 90 g due to evaporated water. Thus 100 g of cooked veggies would have about 11% more potassium than 100 g of raw...