Skip to main content

Sunday chemistry blogging: why do hot things glow?

I got the idea for this one looking at an electric burner on a stove, but let's start with a slightly different example.  When you're looking at campfire flames, what are those flames actually made of?

Via Wikimedia

The answer is superheated glowing gas, mostly carbon dioxide from the burning of carbon.  But that just raises another, more interesting question: why does the hot gas glow?  Classical physics had an answer for this; basically they said that it's from the vibration of the molecules, each one emitting electromagnetic radiation (light) corresponding to the frequency of vibration.  However, if one looked closely at the physics, it turned out there were a few problems with this theory.

It had been known for a long time that a black body (meaning a perfectly absorptive mass—like frictionless planes, these don't actually exist but are good for learning stuff) at thermal equilibrium would emit radiation in roughly this pattern:

The problem with classical physics was that the usual model there predicted that any blackbody at thermal equilibrium would emit infinite radiation at small wavelengths.  This was later awesomely called the "ultraviolet catastrophe."  Max Planck, working on the problem from a completely different direction, discovered that energy is quantized (meaning it only comes in discrete little squirts—this is where "quantum mechanics" comes from), and later came up with the correct answer to the catastrophe, though the derivation was rather implausible and no one took it seriously for awhile.  Einstein applied this same basic idea to light, postulating that light also only comes in little squirts he called photons.

Setting aside a lot of math, this gives us the modern picture of how atoms and their electrons behave.  Electrons swirl around atoms in mathematical corrals called "orbitals," and those suckers are also quantized according to strict laws.  When an electron absorbs some energy, it shoots up to a higher energy orbital for a time, then relaxes back down to the original level, emitting a photon of equal energy to the difference between the orbital levels.  The higher energy the situation (i.e., hotter), the larger the energy difference and the higher energy the photon.  If we're talking about visible light, different energy photons means a different color, so blue flames are hotter than yellow are hotter than red.

That's it for today.  Corrections, comments, and especially topic suggestions are always welcome.


  1. Ryan, I've got a slightly more in-depth comment for you. So I went to a talk by George Whitesides (famous chemist at Harvard) who mentioned that his lab has been using electric fields to control flames. It turns out that with a high enough AC voltage (electric field of sufficient intensity) you can extinguish a flame completely. This is largely because a flame can also be thought of as an ionized gas (plasma). It turns out that the combustion process generates a considerable number of electrons and ions, which you can move around using an oscillatory E field. Thus, applied potentials can be used to shift, shape and extinguish flames. Although it may not be super-applicable, I thought this was pretty frickin' cool.

    While his data is mostly unpublished, I tracked down an abstract from a talk that sums this up rather well:
    "Here, we described one such approach based on oscillating electric fields applied to the flame. The approach is based on the perspective that flames can be considered not only as the hot, gaseous products of an exothermic oxidation process but also as weakly ionized, nonequilibrium plasmas – which include electrons, molecular ions, and various charged carbonaceous particles (i.e., soot) created as chemical byproducts of the combustion process. The movement of these charged species under the action of an appropriate electric field can couple to the hydrodynamic flow field surrounding the flame boundary, thereby enabling one to shape, deflect, and even extinguish the flame. We demonstrate how such field-induced flows can be used to stabilize lab-scale flames and discuss the mechanism controlling flame stability."
    That was taken from here:

    Anyway, I thought you (and any of your other technically-minded readers) would appreciate that little bit of knowledge. Basically, fires are awesome.


Post a Comment

Popular posts from this blog

Why Did Reality Winner Leak to the Intercept?

So Reality Winner, former NSA contractor, is in federal prison for leaking classified information — for five years and three months, the longest sentence of any whistleblower in history. She gave documents on how Russia had attempted to hack vendors of election machinery and software to The Intercept , which completely bungled basic security procedures (according to a recent New York Times piece from Ben Smith, the main fault lay with Matthew Cole and Richard Esposito ), leading to her capture within hours. Winner recently contracted COVID-19 in prison, and is reportedly suffering some lingering aftereffects. Glenn Greenwald has been furiously denying that he had anything at all to do with the Winner clusterfuck, and I recently got in an argument with him about it on Twitter. I read a New York story about Winner, which clearly implies that she was listening to the Intercepted podcast of March 22, 2017 , where Greenwald and Jeremy Scahill expressed skepticism about Russia actually b

Varanus albigularis albigularis

That is the Latin name for the white-throated monitor lizard , a large reptile native to southern Africa that can grow up to two meters long (see pictures of one at the Oakland Zoo here ). In Setswana, it's called a "gopane." I saw one of these in my village yesterday on the way back from my run. Some kids from school found it in the riverbed and tortured it to death, stabbing out its eyes, cutting off its tail, and gutting it which finally killed it. It seemed to be a female as there were a bunch of round white things I can only imagine were eggs amongst the guts. I only arrived after it was already dead, but they described what had happened with much hilarity and re-enactment. When I asked why they killed it, they said it was because it would eat their chickens and eggs, which is probably true, and because it sucks blood from people, which is completely ridiculous. It might bite a person, but not unless threatened. It seems roughly the same as killing wolves tha

The Conversational Downsides of Twitter's Structure

Over the past couple years, as I've had a steady writing job and ascended from "utter nobody" to "D-list pundit," I find it harder and harder to have discussions online. Twitter is the only social network I like and where I talk to people the most, but as your number of followers increases, the user experience becomes steadily more hostile to conversation. Here's my theory as to why this happens. First is Twitter's powerful tendency to create cliques and groupthink. Back in forum and blog comment section days, people would more often hang out in places where a certain interest or baseline understanding could be assumed. (Now, there were often epic fights, cliques, and gratuitous cruelty on forums too, particularly the joke or insult variety, but in my experience it was also much easier to just have a reasonable conversation.) On Twitter, people rather naturally form those same communities of like interest, but are trapped in the same space with differe