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Heating Magnesium to make Mg0

  1. Aug 5, 2010 #1
    Hi

    If you were making MgO by heating Mg in a crucible with a lid on, why might the MgO escape when you lift the lid. Isn't MgO a solid?

    thanks
     
  2. jcsd
  3. Aug 5, 2010 #2

    Borek

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    It is solid, but nothing stops solids from becoming airborne if they are powdered. Try to blow into pinch of starch.

    --
    methods
     
  4. Aug 5, 2010 #3
    thanks
     
  5. Aug 5, 2010 #4
    Furthermore, consider that magnesium boils at ~ 900°C (it's for this reason that you can ignite it with a normal flame; with aluminum you can't, it boils at ~ 2500°C). The combustion of Mg with air is extremely exothermic (temperatures reached are much more than 900°C) and Mg boils off quickly.
     
  6. Aug 5, 2010 #5

    alxm

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    That's just plain wrong. The boiling point of a substance, which is related to the energy of its intermolecular bonds, has nothing at all to do with its combustibility, which is related to the energy required to initiate a reaction with oxygen.

    Heat a bit of aluminum foil in a crucible to a relatively modest 5-600 C and you'll see it rapidly oxidize and much of it will vaporize.
     
  7. Aug 5, 2010 #6
    Probably "ignite" it's not the proper term, don't know. What I mean is that you can make an Mg tape (for ex.) start burning and making a flame, simply using a bunsen/lighter flame. When you "ignite" (tell me if it's the proper term) a piece of wood, it starts burning and making a flame because of the gaseous chemicals generated, which mix with air and burn. The same with Mg: it melts, then a small percent vaporizes and that gaseous part mixes with air and burns. You can do this with a bunsen/lighter flame because Mg metal boils at 900°C only. You can't do it with Al metal.
    Hope it's more clear now.
     
  8. Aug 6, 2010 #7

    alxm

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    No, you're still entirely wrong. Solids and liquids burn just fine, things do not have to be mixed with air in the gas phase to burn. Burning magnesium gives off a lot of heat, so some of it will melt, and you'll have droplets of molten, burning magnesium, but little or none of the metallic Mg will boil off or vaporize. Your description is simply not what's happening.

    A bunsen burner does not get anywhere near 900 C, unless you've got a huge burner and it happens to be inside a refractory furnace that you're pumping air into.
     
  9. Aug 6, 2010 #8
    Sorry, it's you the one who is wrong.

    Why you didn't read what I wrote? Now I underline it:
    <<you can make an Mg tape (for ex.) start burning and making a flame, simply using a bunsen/lighter flame>>

    You have seen bunsen burners in pictures only, isnt'it? I easily (= a few seconds) melt copper threads with my own bunsen burner, at home. Which is the melting point of copper? 1083°C?
     
  10. Aug 6, 2010 #9

    Borek

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    I think a lot depends on what you mean by "burn". As far as I can tell from my observations (and I am a pyromaniac :devil:), flame requires gaseous fuel mixed with air. For example when you burn wood, initially when heated it emits flamable gases which burn and form the flame around the piece of wood itself. Once only charcoal is left, there are no longer flames present, just glowing embers.

    Could be thats kind of a language problem - for example in Polish first phase of the process has different name (wood "płonie" - with flames) than the second one (charcoal "żarzy się" - it glows, emits heat, but there are no flames present).
     
  11. Aug 6, 2010 #10

    alxm

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    Well, I define 'burn' as 'thermal oxidation' (I'm not sure if I would narrow it down to 'with air' or not). I don't think it's so far fetched, people consider charcoal to be 'burning' even when there's no flame. It's not uncommon to say "burns without a flame" in English if you want to specify the latter.

    In either case, a magnesium strip does not burn with a flame in the same way that a piece of wood does; there are no (or few) vaporized partially-oxidized species flying around.
     
  12. Aug 7, 2010 #11
    No, I remembered wrongly, it's zinc whic boils at ~ 900°C. Magnesium boils at ~ 1090°C . Anyway a bunsen burner reach easily that temperature.
     
  13. Aug 7, 2010 #12
    It cannot burn exactly in the same way that a piece of wood does, since a burning piece of wood doesn't melt, it releases gaseous chemicals (mostly methanol and acetone) by thermal decomposition; magnesium instead melts when it burns; I used the example of the wood just because of the flame; our controversy is if it vaporizes significantly or not during the process. When a thin piece of magnesium is heated with a flame, it starts burning *suddenly* and with a *very bright combustion*. Why? And why the same doesn't happen with aluminium, even if its melting point is very similar to that of Mg (Mg = 650°C, Al = 660°C)?
     
  14. Aug 7, 2010 #13

    alxm

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    No, the 'controversy' is that you equated boiling points with ignition temperatures when they have nothing to do with eachother. There's nothing to debate about this, it's factually false.

    Why is explained in chemistry textbooks. Boiling and melting points have nothing to do with it.

    Aluminum burns brightly and gives off quite a lot of energy as well, just look at a thermite reaction. It just has a higher ignition temperature, and will generally burn slower due to how the oxidation occurs at the surface interface.

    Which again has nothing to do with boiling or melting points.
     
  15. Aug 7, 2010 #14
    ??? Not at all!

    Of course it's false, but I also have *never* stated that...

    Really? I was waiting for you to tell me this :smile:

    So you don't want to read what I wrote. Please, try to ignite with a bunsen burner a piece of aluminium tape/ribbon in the same way as you do with magnesium:
    http://www.amazingrust.com/experiments/how_to/Mg.html [Broken]

    If that was still not clear:
    *take a piece of Al ribbon and try to ignite it with a bunsen burner*

    And why it has a (very) higher ignition temperature if it has (almost) the same melting point? And why it should burn slower due to how the oxidation occurs at the surface interface, since this oxidation at the surface interface (obviously) happens with Mg too?
     
    Last edited by a moderator: May 4, 2017
  16. Aug 8, 2010 #15

    Borek

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    Properties of oxides are different, one sticks to the surface isolating metal from the oxygen, other falls off, exposing more metal ready to react. For me that's a sufficient explanation.
     
  17. Aug 8, 2010 #16
    It is, in the case of a solid phase; but here we are talking of a liquid phase, many little droplets of liquid metal burning inside a very hot surrounding (we can have an idea of the temperature from the white colour of the flame); in these conditions, even Al2O3 is continuously removed/broken from the liquid surphace. You say it could be rimoved with greater difficulty compared to MgO, ok, it's possible; I say however that a small but significant percent of Mg also vaporizes and this contributes to the burning of the Mg ribbon.
     
  18. Aug 10, 2010 #17

    alxm

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    Yes you did. you wrote: "Furthermore, consider that magnesium boils at ~ 900°C (it's for this reason that you can ignite it with a normal flame; with aluminum you can't, it boils at ~ 2500°C)"

    Why would I bother? I know that I know the chemistry here. I've got a M.S in phys chem after all.

    Yes, why don't you? It will oxidize into Al2O3. It will just not burn as violently as magnesium does.
    I have done it, in effect, by leaving an empty aluminum pot on a gas stove. Within 10 minutes it'd burned a hole straight through it.

    You just repeated your idiotic equating of ignition temperature with material properties. They have next to nothing to do with eachother.
    And aluminum does not have a much higher ignition temperature. Aluminum powder will ignite at around 650 C, not much higher than for magnesium. The reason why block aluminum metal does not readily ignite is due to surface interface effects.

    Because the rate of combustion is limited by that. Maybe you should look up how dust explosions occur.

    They don't have the same crystal or oxide structures.
     
    Last edited: Aug 10, 2010
  19. Aug 11, 2010 #18
    You are right, don't know what I was thinking about here :smile:
    "As violently"? I've never been able to ignite an aluminium ribbon with a bunsen burner *at all*. Ah, yes, it oxidizes, of course, but it doesn't make a flame with sudden heat and light generation, I mean this with "igniting".
    Aluminum powder is not an aluminum ribbon.
    Maybe. But maybe it's not the only reason.
    I can make mistakes and I'm ready to accept it, even because I don't have a master in chemistry (but it's not the only reason). Anyway you have made at least two mistakes :

    1. you wrote a bunsen burner cannot reach 900°C:
    2. you wrote magnesium cannot boil in the flame:
     
    Last edited: Aug 11, 2010
  20. Aug 11, 2010 #19

    Integral

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    With out a doubt. I personally have burned Al in a camp fire. Wish it had been foil but it was a relatively new pancake griddle. I was a teenager, what more need I say.
     
  21. Aug 11, 2010 #20

    alxm

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    So read your own damn posts before you accuse someone else of being wrong and misquoting you.

    So try aluminum powder instead. Or a thermite reaction. The fact that you can't ignite an aluminum ribbon with a bunsen burner does not mean that aluminum cannot ignite at that temperature, much less that aluminum doesn't burn.

    How is that a mistake? An ordinary bunsen burner flame barely reaches 900 C, which means that if you heat a solid object with it, in an open environment, it will be much lower than 900 C. Again you're taking an overly simplistic view. A burning substance does not have a specific temperature, it's all due to circumstances. Charcoal burns slowly on a fire, but charcoal dust in air will explode. Charcoal in a blast furnace will melt steel, charcoal in a barbecue will not.

    No, I didn't. I wrote that 'little or none' of the magnesium will boil off. Because it burns faster than it can boil off. Magnesium, like all metals, has a huge heat of vaporization. It does not instantly turn into gas just because you're at or above the boiling temperature.
     
    Last edited: Aug 11, 2010
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