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Iron compound

  1. Jul 27, 2007 #1
    We have this compound in our water that is insoluble in water, partially soluble in HCl (turns it yellow), it is orange when it comes out of the water as a precipitate but whenyou heat it up it turns slightly magnetic with a color change to black then brown but then degrades (or somthing like that) over time it becomes completely non-magnetic with no visible color change. I don't have the right materials to do a reduction with H2. Doesanyone have a clue what this might be? I was always told it was rust. We have methane dissolved in our water too. I have reaserch and i cannot find what it is. Salicylic acid turns it purpleish blue. it does undergo a thermite reaction when mixed with aluminum.
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  3. Jul 27, 2007 #2


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    Its iron. The yellow color in HCl, the salicylic acid test and the ferromagnetic properties are conclusive enough results. The thermite reaction is not specific.
  4. Jul 27, 2007 #3
    I think that you have a mixture of iron hydroxides and iron oxides (rust). HCl would give iron chloride which explains the yellow color. Salicylic acid does form a purple iron(III) complex. When the mixture is heated, the hydroxides decompose to the oxide and thus you are able to initiate a thermite reaction. The magnetic behavior is due to magnetite (Fe3O4).
  5. Jul 27, 2007 #4
    would a mixture of Fe2O3 and Fe3O4 make the compound brown?
  6. Jul 28, 2007 #5
    Fe2O3 is reddish-brown; Fe3O4 is black.
  7. Jul 28, 2007 #6


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    Yes. Rust is rarely pure Fe2O3 - there are various oxides (stoichiometries) - and hydrated oxides, and even oxyhydroxides. There are most likely other impurities.
  8. Jul 29, 2007 #7
    You can't really say the powder that makes up rust is either pure Fe2O3 or pure FeO or pure Fe3O4. It is a crystal matrix with Fe(II) and Fe(III) in it and hydroxide impurities at the crystal-air interface. The formula which indicates the crystals composition is usualyl that which best describes it.

    In this case I would say you have iron oxide, when it's just precipitated it will be best described as Fe3O4 which explains why it's magnetic and when you leave it in air it will convert into orange Fe2O3 as the powder gets further oxidized.
  9. Aug 1, 2007 #8
    Are the hydroxides of iron magnetic?
  10. Aug 1, 2007 #9
    Not to my knowledge but when the hydroxide ions make up only a tiny portion of the total amount of anions, the crytal in it's entirety might still be mostly describable as Fe3O4 and thus magnetic.
  11. Aug 1, 2007 #10


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    Yes, some are.
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