This is more terrific and amazing — like a fun class. And I’m still reading and linking. But I wanted to ask what instruments were used for the measurements used in these calculations? What tool(s) facilitated the shift from measuring volume of gases to counting electrons?
6 Likes
Suuuuuuper basic here, probably mostly a lie for children with no stoichiometry or math. But here goes:
Tooth enamel is mostly made out of a hard substance called apatite. Primarily calcium and phosphorus. As you wear your teeth with use, phosphorus gets knocked, scraped and dissolved out of your teeth, leaving “potholes” in your enamel’s surface.
Unfortunately, teeth don’t grow like normal bone because they’re outside your body cavity and don’t have a blood supply on their surface.
But, fluoride is a game changer here. Fluoride is chemically similar enough to phosphorus, and reactive enough that it can fill in the potholes. In fact it’s in a way better than phosphorus because it binds to the matrix of your enamel more strongly than phosphorus.
So, by drinking fluoridated water, you are supplying a source of “spackle” that fills in the “holes” in your teeth, building them back up.
If there’s too much fluoride in the water, the effect is so strong that you can get buildups and deposits of fluoride big enough to be macroscopic. This isn’t a physiological problem. You’ll have practically bullet proof teeth. But cosmetically, they can end up with visible staining that gives teeth a speckled appearance.
Toothpaste:
Toothpaste and brushing have multiple mechanisms of action. Toothpaste is a mixture of fluoride (typically either sodium fluoride or stannous (tin) fluoride), a rough debriding material (typically very finely divided silica/sand), antimicrobial stuff, flavorant, and emulsifiers (propylene glycol usually) that foams up when mechanical energy is added.
Toothpaste works in conjunction with brushing by killing microbes directly, by mechanically scraping bacterial plaques and colonies off the surface of the tooth with the silica, and simultaneously dispersing and evenly applying fluoride to the surface of the tooth where it can then fill in the holes left behind by the bacteria and then binding to the surface of the tooth itself. Thereby repairing damage.
Yes I read Wikipedia for fun, and love having in depth conversation with my dental hygienist when she’s cleaning my teeth.
The Sheriff’s description of fluoride is pretty accurate:
“Basically Rusty, it’s like steroids for your teeth. Makes 'em strong, and cocky. Like an oiled up muscleboy, and your mouth’s the gogo cage.”
13 Likes
And when your teeth are badly damaged, you don’t want to eat any more because you’ve lost your apatite.
Hey, it’s a thread for 5 year olds. You should expect “Dad” jokes. 
15 Likes
The only tools really used for these measurements were balances of increasing accuracy, pressure gauges, and containers which measured volume. These tools haven’t changed, and are still how most stoichiometry (the measurments of quanitities) takes place. The leap to measuring the strength and presence of electrons were mostly conceptual, but the technological apparatus usually didn’t involve a major new invention.
The thing to understand is that the history of chemistry, much like history in general, is too often presented in terms of Great Men and Great Moments. But history (and I’m sure @anon61221983 will back me up on this) rarely actually works this way. History, like light, is a spectrum. These Great Men (the concept of Great Women is only now receiving attention, highlighting a big problem with the way we talk about the history of science) are often little more than mileposts. Newton was brilliant, but Leibniz was also discovering calculus at the same time. Einstein was a genius, but Lorentz and Poincaré were on the same track, and in some ways were more flexible than Einstein. More often than not, scientific progress is made more gradually and continuously than people think. Chemistry is no different, and the evolution of its tools and ideas was particularly gradual until the twentieth century. Often what you had were competing figures for atomic mass, until better and better apparatus were invented. Precision measurements had to wait for precision tools, and precision tools often had to wait for better methods of production, which often had to wait for better scientific and engineering methods, which often had to wait for… you guessed it: more precise measurements. This seeming catch-22 is why it’s such a jerky process, and why multiple people often had to rediscover and reinvent the wheel for themselves.
So with the discovery of the electron, for instance, the big leap was conceptual. J.J. Thompson was working with cathode rays. These are beams of electricity that people discovered by playing around with the stuff. Electricity was a curiosity that was poorly understood as early as the Ancient Greeks. Early experiments with electricity involved noticing static buildup when rubbing glass or amber with wool. Elektron, is in fact the Greek word for amber. Over time, the links between this quirk of clothing, lightning, and dancing frog’s legs were discovered. So when J.J. Thompson was experimenting with electricity, he was curious, and decided that if these beams of electricity could be deflected by magnetic fields, then they may have a measurable mass, and it was based on these deflections that he came up with a number for the ratio between the charge of the electron to its mass. But this involved the measurement of angles. While I don’t know how his setup looked, it was likely little more than a glorified protractor doing the measuring, because you can see where a cathode ray strikes glass.
You would have to wait until a man named Millikan to measure the charge of the electron. This was again done using tools that were almost stuff you’d find at a hardware store. He sprayed oil droplets and based on how they fell through space, found a way to give them charge. He sprayed these droplets in a mist, charged them and let them float between two electrically charged plates. By adjusting the charge on the plates, he could suspend them in midair, cancelling the effect of gravity. He knew the charge on the plates, the forces on the droplets from the air and from gravity, and so he could work out using math the force of the electrons. He invented the apparatus, but the apparatus itself didn’t need its own breakthrough beyond employing the conceptual breakthroughs.
But the oil drop and cathode ray experiments were just the first in series after series of experiments that repeated and then reconfirmed the results. No one apparatus is usually the only guide we have to how these things work. For example, the equations used to figure out the voltage a chemical battery will produce can be used to confirm what we know about how many electrons are transferred in a reaction.
This delves a little into complex topics in the philosophy of science, like Karl Popper’s idea of “the crucial experiment.” I encourage you to read this and Thomas Kuhn’s criticisms (which I find very persuasive). I tend to believe that there are no crucial experiments except in retrospect and based on agreement. That human beings are easily deceived by the illusion of coherent narrative.
Now, to tackle NMR. Oh, how I love NMR.
NMR stands for Nuclear Magnetic Resonance. This is a technique that relies heavily on the principles of quantum mechanics. This gets a little difficult to explain in simple terms, so bear with me. Remember how I said that the only thing that really makes an element a particular element is the number of protons? Remember how I said that an atom of the same element can, depending on how many electrons it has, form different ions? I told you to ignore the neutrons, but now they’re important. Two atoms of the same element with different number of neutrons can still be the same element, because only the protons matter, but they are different isotopes. Different isotopes have different masses, because they have different numbers of particles. (The reason we don’t care about the mass of the electron is that the mass is pitifully tiny, 1/1842 the mass of the proton.) Here, we’re less concerned about mass and more concerned with a quantum mechanical property that is called spin. We don’t know where it comes from. It’s a crisis. We do know that this spin means it behaves a particular way in a magnetic field. Here’s the problem, it only works when the isotope in question is one with an odd number of particles in the nucleus. So this technique only works for specific isotopes. Fortunately, hydrogen is one of them, and most hydrogen is the odd numbered isotope. In organic substances, most of the carbons are bound to at least one hydrogen.
So if you take a sample and place it in a very powerful, very strong magnetic field (I had to take off my mechanical watch every time I walked over to the machine to insert my sample), and gradually increase it, all of the hydrogen nuclei will start to align themselves. About half of them have the correct spin, and line up right away with the direction of the magnetic field. The other half have the opposite spin, and line up the other way, but because this magnetic field is so powerful, the spin is eventually forced to “flip” to align. When it does this, a radio wave is produced and this can be recorded. But electrons can shield a nucleus from this effect. So you’ll get different signals from different parts of the molecule. Since hydrogen actually shares its electrons with a system of other atoms in a molecule, you can use this information to figure out if that hydrogen is near a double bond, or near an oxygen or nitrogen. Also, because of a phenomenon called splitting, where the hydrogen nuclei influence each other’s magnetic field, you can tell how many hydrogen are attached to a neighboring atom. This becomes a game of deduction, where you look at the splitting, the location of the peaks on the graph, and start to count how many hydrogens there are, and where they are. Because it’s often safe to assume they are attached to carbons (and there are hints if the hydrogen is bound to something else) you can figure out how many carbons there are, and how they’re linked together based on how many hydrogen neighbors they have. This is where it becomes a skill and an art. It takes time to learn how to do this, but once you learn:
So here’s an example of something I had to do for an introductory class on these techniques. What I did as an undergraduate over the course of half a semester using modern techniques would have won me a Nobel Prize back in the bad old days of chemical testing and retesting.
I was given an unknown organic substance, and using MS, IR, and NMR, I had to figure out what it was. (I don’t have access to the media I have my work stored on, so I’ll add pictures later.)
For MS, I got a bunch of peaks on a graph that told me how much it weighed, and how much the fragments weighed. Based on the mass of the fragments, there were some signs I had at some oxygens. Since I knew it was mostly carbon and hydrogen, this means that I could come up with various combinations of the elements to add up to a total weight. I also knew based on the fragment weight that I probably had a ring structure, since molecules fall apart a certain way. This was all the information I had, and it wasn’t enough.
So I looked at the IR graph. What I saw was a distinct, super indicative peak that told me I had a carbonyl group. That’s a carbon double bonded to an oxygen. I assumed based on where it was that it was experiencing some effects from a nearby ring or nitrogen. I knew it couldn’t be a nitrogen because of the MS data, or it was at least unlikely. I also saw telltale signs of an aromatic ring structure. I still didn’t have enough information.
So I looked at the 1H hydrogen NMR and a second 13C carbon NMR. Carbon 12 won’t show up in NMR, which is unfortunate, because it’s the most common form of carbon there is. only 1% of carbon is carbon 13. So what you get from carbon 13 is a bit like looking through a glass, darkly. It didn’t give me much, but confirmed how many carbons I was working with. These NMRs gave me a ton of information. Suddenly I knew where the carbonyl was located off of the ring, and I knew there was a carbon that wasn’t attached to any other carbons. This didn’t make any sense until after a few hours of work, I came up with:
This is how I reconciled a carbon without neighboring carbons but only two hydrogens. It was out in the cold attached in an ether (oxygen singly bound to two carbons) ring. It’s piperonal, a substance found in pepper, and more interestingly a precursor that can be used to make MDA/MDMA in high yield. Quoth the lab instructor (a grad student who assisted the professor and didn’t design the course) when I went to him asking if I was on the right track, “Wait… you realize this is a controlled substance?” This gave me pause, but I got 100% on the report in the end, so I’m pretty sure I was right.
So you can see from this example that no one technique is ever good enough. It’s all about bringing together different techniques and weighing the evidence for your conclusions.
For really crazy molecules, unlike the simple example that I had to work on, these techniques alone may still contain ambiguity, and you may have to prove the exact structure by rebuilding it from scratch, a technique called total synthesis. This is a time consuming way to prove a point, but it can be incredibly useful in teaching us about chemical reactions.
ETA: I forgot to mention something about NMR.
It stands for Nuclear Magnetic Resonance. Wait… that sounds a lot like something else… in fact it sounds a lot like Magnetic Resonance Imaging (MRI).
They’re the same technique applied different ways. The medical people were wise to drop the word “nuclear” because people immediately think of dangerous ionizing radiation. The machine only generates harmless radio waves and strong magnetic fields. It uses the fact that your body is full of hydrogen, mostly in water, but also in carbon compounds in tissue. It uses the radio waves generated by the magnetic field flipping the spin of the hydrogen atoms to form a picture of your insides. The technology is the same, the output is different.
9 Likes
(back to lurking)
15 Likes
Chiming in to second the recommendation of Popper’s Conjectures and Refutations (interesting and important, but not very historically accurate) and Kuhn’s The Structure of Scientific Revolutions (much better on the history, but tends to overemphasise the revolutions and undervalue the slow grind).
But I’d also add Helen Longino’s Science as Social Knowledge. That one is an essential antidote to the “epistemological relativism” trolling of Feyerabend’s Against Method.
Read those four and you’ve got the basics of Philosophy of Science covered.
2 Likes
See?
#SEE?
Chemistry is sorcery! It’s the science of all matter. Or at least all matter that matters here on Earth.
8 Likes
Richard Rorty’s Philosophy and the Mirror of Nature is a one-stop shop.
Rorty surveys analytic philosophy and its precursors — the U.S./U.K. line which includes Kuhn. PMN restored a more scientific disposition toward concepts like representation and the risk of too-transparently accounting for the complexity of physical and social reality. I think it should be required reading for those engineers and scientists unaccustomed to critiques of foundationalism.
Alan Chalmer’s What Is This Thing Called Science? would also work as an easily readable single-book intro.
1 Like
Okay, my first question here:
#How do propane refrigerators work?
The non-electric kind. I’ve tried looking it up but the explanations don’t seem to make sense, probably because it requires a lot of prior reading. Is there any kind of simple explanation?
2 Likes
I’m pulling this out of my non-engineer arse, so I may be wrong, but:
Burn propane. Use the heat energy to drive a compressor. Use the compressor to compress a gas. Use a radiator to bleed off the heat of the compression.
Then, rapidly drop the pressure (by releasing the gas into a larger volume container). When you rapidly decompress a gas, it sharply reduces the temperature (ever noticed how the outside of a gas canister gets frosty when you dump the contents?). Use the chilled gas to cool the refrigerator.
IOW, it works the same as a normal refrigerator, but with the compressor powered by heat instead of electricity.
5 Likes
See this actually makes sense.
Most places seem to make it sound like a very different thing is happening. But the thermodynamic diagrams don’t make sense if anything other than what you’re saying is happening.
1 Like
I’ll third popper and Kuhn. They really get into the ideas of the process of Science
2 Likes
American yankee doodle here calling on help from across the pond:
ASBOs.
Go.
3 Likes
Oh, I had to look that one up. Anti-Social Behaviour Orders. ETA: What I read said that they’re generally used for misfit adolescents. But in this case, the dude definitely should know better.
3 Likes
Well, I know what it stands for, but I don’t know how any of it works.
3 Likes
Sorry, I edited it after I thought about how you probably wanted more. I guess the Brits expect more from their youths? I’m not sure our legions of lawyers and parents of special snowflakes would allow such a law. However, a seriously anti-social minor could theoretically be held for observation. I’m sure a Nazi salute would be protected under free speech here in the U.S.
1 Like
But do you need evidence? Is there a proceeding? What kind of misfits?
To me it sounds like British police hand them out to anyone who ding-dong ditches their neighbor. Is this far from the truth?
1 Like
I imagine there’s a proceeding (I thought that’s what was going on in the story, and he’s had a previous ASBO). I do know that right-wing hate speech isn’t protected in many European countries like it is here.
Paging @daneel or @GilbertWham:
Are ASBOs handed out willy-nilly?
ETA: Also, @anon73430903 just posted a little bit about this in the OT.
2 Likes
I don’t have much to add, really.
They’re supposed to be punishments for non-criminal antisocial behaviour. It was part of Tony Blair’s “Respect” policy. Sort of curfew type things; keep graffiting stuff, you’ll be banned from buying paint, keep vandalizing one shop, you’ll get banned from that street (or that side of the street), etc.
But they were (inevitably) largely ignored and seen almost as a badge of honour.
I thought they were going away/had gone away, to be honest.
8 Likes
