Why is Potassium gas Green?

So my interest was really spiked here when I found that when reacting with water, potassium gives off a green gas!

While the green of potassium gas has been known about for over a century, being found in Encyclopedia Britianica articles as far back as 1911, the origin of that green color is proving a little more elusive.

If you know what you are doing it’s relatively easy to observe.  250 mg of potassium is ‘the right’ scale to work with.  Much smaller and it’s all over before you get a chance to do anything, much bigger and you run into problems with the metal exploding, or the hazards of hydrogen build-up.  All you need is a heavy glass vessel of about the dimensions of a wine glass.  Insert a burning acetone taper through a small hole in the top of the vessel and to burn out all the oxygen, and then drop in your potassium.  The green color is easily visible.

I’m tempted to propose it’s due to the solvated electron, which is a hot research topic at the moment as it’s part of the principal mechanism by which radiation damage happens to DNA.  It’s known to be stable and blue in liquid ammonia, but survives only picosecond in water.

A bead of Sodium Potassium alloy in anhydrous liquid ammonia. The blue colouration is due to the presence of solvated electrons

It’s an interesting suggestion, but I think the observation that you get the green color when there is no water around pretty much kills this idea.

So how do you get a green gas.  Well the only other metallic gaseous vapor I’ve seen was mercury which I once boiled to test the calibration on a thermocouple.  That has no color at all that I can remember.

Well as you will recall virtually all the gases are colorless, e.g. all the Noble gases (helium, argon etc) and all the first period gases (nitrogen, oxygen, and mostly fluorine).  However the heavier halogens have colored vapors that get more heavily colored as you go down the group.  Chlorine, light green, bromine, brown etc.

Chlorine (left) and bromine (right).

The reason these gases have these colors is the same reason the sky is blue, Rayleigh scattering.  That’s related to the polarizability of the molecule.  This is a very different mechanism from the electronic transitions that give the classical flame colors of the alkali metals!

It’s well known that all the alkali metals have stable bound states as diatomic molecules, similar in electronic configuration to hydrogen.  IF K2 had a similar polarizability to Cl2, it may well be green like chlorine!  Regrettably finding the polarizability K2 is not as easy as it sounds.  A significant difference between H2 and K2 is the bond energy.  H2 has a bond energy of about 400kJ/mol, while K2 has a bond energy of ~50kJ/mol.  For reference, the hydrogen bond, the thing that holds water together as a liquid, has a bond energy of ~20kJ/mol.  If you heat any bound state and eventually the species will gain enough energy to separate and become individual species.  With water this happens at about 100 oC (boiling, ~400 K).  For hydrogen it’s about 3000 K.  You can actually do the real calculations, it’s just I’m too lazy at the moment, and so I’m just going to do a linear extrapolation between these two.  That gives K2 breaking up at about 2-300 oC.

Well that would fit nicely with green gas being evolved at lower temperature, but as the temperature rises, the diatomic species break up, and the relevant polarizability of the molecule, and hence the Rayleigh scattering, and hence the color is lost.

Great, so if this is a working hypothesis, then the diatomic metals should match their corresponding halogen right?  The bummer is sodium.  Sodium gives off a blue vapor when it boils.  Fluorine is almost colorless.  ARSE!

The game is not over yet!

I’ve decided I need to see sodium vapor for myself, but how to do it with only the junk I have to hand!  What I really need is a nice small sealed silica tube that can take temperatures over a thousand degrees C.  Hmmmm, thinking, thinking…..


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21 Responses to “Why is Potassium gas Green?”

  1. dogsoldier Says:

    Dear friends,

    I am from Slovakia, it lies in the middle of Europe. First off, sorry for my English, but I’m poor in it I think.
    So my problem is this:
    I will go to YMCA Camp for little childer as an animator. It will be in Slovakia. And there have every year lectures for children
    and they washing those little brains. I have an idea, because nobody knows that, I am an atheist 😉 I want to prepare myself
    for that lectures with good arguments situated exactly to this lectures. There will be talks, etc. on it, so I want to know
    about weaknesses or weakest points in that topic. So if you all be so kind and try to give me some good tips how to argue in
    that topic. Above this text I paste some translated topics so if you have time and look in it please. Camp starts in July, so
    we have time.

    Thank you for your time, I appreciate it.



    LECTURES (sorry for my translation):

    1. Joshua will be Gods leader.
    Verse: Joshua 1:9
    Have I not commanded you? Be strong and courageous. Do not be afraid; do not be discouraged, for the LORD your God will be with you wherever you go.

    It’s about journey to Kanaan with Moses and ten commandements …

    Idea in this topic is: Who leads your life?

    2. Red string in window
    Verse: Romans 8:1
    Therefore, there is now no condemnation for those who are in Christ Jesus.

    I do not know this story very much. It is about Raachab, she was a whore? 😀

    3. Fall of Jericho
    Verse: Jeremiah 33:3
    Call to me and I will answer you and tell you great and unsearchable things you do not know.

    It is about war in Jericho. Joshua waited for sign from Thor (maybe) or any other god and they then
    circled around Jericho seven or three or any magic number times.

    4. Sin of Achan
    Verse: Proverbs 28:13
    Whoever conceals their sins does not prosper, but the one who confesses and renounces them finds mercy.

    Israelits have won (from “to win”) with Jericho. It was they first win in Kanaan. Joshua prayed a lot. It is about
    gods punishments I think. Bolts from Thor 😀

    5. Story about ?Gibeonians?Gibeons? (in slovak: Gibeónci)
    Verse: Proverbs 3:5
    Trust in the LORD with all your heart and lean not on your own understanding

    God wants that we were adicted on him. Gibeons have had some kind of pact with Joshua. King hide in cave…

    6. Joshua gives Goodbye to his people.
    Verse: Joshua 24:15
    But if serving the LORD seems undesirable to you, then choose for yourselves this day whom you will serve, whether the gods your ancestors served beyond the Euphrates, or the gods of the Amorites, in whose land you are living. But as for me and my household, we will serve the LORD.

    It summary of all topics. Something about Kaleb, Moses and pact with his people which is in law (bible maybe).

    Okay, so thas is all folks. Thank you very much again.
    Please give me some feedback that you read this mail or something.

  2. dogsoldier Says:

    please send me rather email if it is possible and delete my posts from discussion. Thank you.

  3. Grenator Says:

    I remember being quite impressed by that Brainiac video, when I’d first seen it. A fake? I’m intrigued. And subscribed!

  4. Grand Lunar Says:

    To think that we consume potassium!
    I’m surprised you didn’t mention that, BTW. Perhaps next time? Just to give something else for us to think about?

    This “adventure” in science really is a learning experience for us all.

    • Anonymous Says:

      We do consume potassium, and it is essential for all life on earth, as it is a fundamental component of DNA, RNA, and the phospholipid layer which surround every animal cell, and also vital for propogation of every nerve signal. The exploding stuff is potassium in it’s elemental, metallic form. We consume it by eating plants, animals or salt (KCL), which is better for you than table salt (usually NaCl)

  5. Heather Gauld Says:

    Caesium exploding like that in water is fake!? what!? they showed us videos of that in high school… Or do I misunderstand what you’re saying?

  6. commonsoldier Says:

    thank you for editing and pointing out that brainiac is a fucking fake ass science show… It has less to do with chemistry than a methamphetamine addict..

    Please dont use any of there crap in the future, cause it harms you more than methamphetamine 🙂

    Btw, i love you kinda 🙂

  7. Anonymous Says:

    ” they showed us videos of that in high school…”

    Caesium reacts vigorously with water but that video is fake.

  8. polescholesch Says:

    You almost gave the full recipe for making methamphetamine, be careful!

  9. poleschs Says:

    After thinking about this green gas for a while, my best suggestion is quantum electronical effects in the Rayleigh scattering. This is due to the electrons in the outer layers of the potassium atom.

  10. Joe Says:

    I am honestly excited to see what you discover TF, I’ll be watching patiently.

  11. John Says:

    I don’t think Rayleigh scattering could possibly explain this green color. According to the wiki, the intensity of the scattered light by the atmosphere at 1 atm is 10^-5 of initial intensity per meter of the atmosphere. Considering that the potassium gas layer thickness is on the order of millimeters, and the amount of ambient light portion necessary to see the color, as intense as seen in the video, is about 10% or more (from my limited experience working with spectrophotometers), then the polarizability of K2 molecules would have to be 10^3-10^4 fold that of the atmosphere molecules.

    This sounds a bit stretching. I’d rather say that the intensity of the color suggest a genuine electronic transitions rather than scattering.

    Then again, I don’t have any experience with diatomic alkali molecules.

    • magnus271 Says:

      Agreed. It’s molecular rotational-vibrational band absorption in K2 molecules.

      The Sodium and Potassium Absorption Bands

      Phys. Rev. 30, 429–437 (1927)

      I DO WANT to still want to see the blue color of Na2 though, that would be wonderful. This is a very obscure phenomenon that not even most chemists are aware of, I imagine.

  12. John Says:

    I suspect that the different colors of potassium vapor are complimentary colors of absorption and emission.

  13. Anonymous Says:

    Boiling a tiny piece of sodium in organic lab for elemental analysis produced a black vapor (I was surprised.) NO2 (red) turns to colorless N2O4 as it cools- I guess the color is from electronic transitions of the odd electron. Ozone is dark blue- i would guess from the highly polarized electronic structure that there are low energy electronic transitions. I would guess K’s color to be from the K radicals not K2 molecules. Basically a lot of guesses and 1 undergrad observation.

  14. poleschs Says:

    I just found out that barium and barium salts give off a green vapour.

  15. Bill Says:

    Thanks for the science 🙂 It was really interesting reading about solvated electrons, especially the metal/nonmetal transition in amonia/potassium solutions. It got me thinking about liquid superconductors and it seems that amonia/metal solutions are being investigated for that.

    It would be interesting to see the kind of magnetohydrodynamic forces at work in thousands of amps flowing through a liquid superconductr.

  16. Patricia Bianconi Says:

    Cs’s melting point is very close to room temperature (can’t remember it exactly). It’s close to liquid already, and the heat of the evolution of H2 from the surface is enought to melt all of it. When it touches water the liquid phase reacts much more quickly than a solid does. So all the hydrogen is evolved almost simultaneously, the explosion is a big one.

    The mp of Li is 181 degrees C. The heat of the H2 evolution from the surface of the solid is not enough to melt it, and allow a rapid liquid-phase reaction. K’s mp is 65 degrees; at some point it becomes a liquid, and reacts more instantaneously. And can vaporize in some form.

    I don’t know what the red color is!

    What we’re seeing is not very different thermodynamics, i.e. so much difference in the amount of energy released that the explosion is more vigorous. It’s a difference in kinetics: which metals melt more quickly to the liquid phase, which phase reacts much more instantaneously.

    The colors of ammonia solutions of all the alkali metals are the same blue. That’s one reason it was deduced that the same species (solvated electrons) were forming, and that the blue color was not some alkali-metal species

  17. Cam Says:

    I was completely unaware that alkali metals formed diatomic bonds, can anyone provide a link where I may be able to read more about this topic? I’ve tried several combinations of google searches with no luck.

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