Kagutsuchi
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Identifying a substance
From a source of mine, I recently got a small metal barrel of a black powder that is very-very homogeneous, supposedly metallic, but couldn't quite
figure out what it is.
Upon heating it first glows red and upon cooling down it produces a yellow homogeneous product. Does anyone have any idea what this can be?
Also it does not react with water, nor with dilute sulphuric acid.
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gdflp
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That sounds like tungsten powder. Try seeing if a small amount will dissolve in 30% H2O2, it does then it is likely tungsten.
That yellow powder sounds like WO<sub>3</sub> produced from oxidation of the tungsten at high temperatures.
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Kagutsuchi
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Thanks for the tip, I'm currently out of h2o2 but I will get some and see if it does the job. I was hoping it's not my other tip (arsenic). It can be
tungsten because it is realllllllly heavy.
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gdflp
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Since it's really heavy, that sounds more and more like tungsten. It would be hard to confuse the densities of arsenic and tungsten, there's a
threefold difference! And I don't know what the yellow compound would be if it was arsenic, since arsenic forms two oxides and both are white.
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Kagutsuchi
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It could form yellow arsenic, an allotrope of arsenic, but that would be turned back to gray arsenic by light but it does not happen fortunately 
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MrHomeScientist
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It might be bismuth. Rather than melting on heating, Bi powder oxidizes to form bismuth trioxide.
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gdflp
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Isn't bismuth powder soluble in sulfuric acid though? I know bulk bismuth isn't, but I presumed the higher surface area of the powder would speed the
dissolution rate. It is a possibility though.
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Magpie
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Consider molybdenum. When I was playing with some molybdenum ore I remember making an oxide that was yellow. But it turned white on cooling.
Probably MoO3.
[Edited on 26-10-2015 by Magpie]
[Edited on 26-10-2015 by Magpie]
The single most important condition for a successful synthesis is good mixing - Nicodem
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woelen
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It might also be vanadium. Vanadium is oxidized at high temperature and forms yellow/ochre V2O5.
You can test for this as follows:
Add some of the yellow solid to a solution of NaOH in water. It quickly dissolves, giving a colorless solution. Then slowly add excess dilute acid
(any acid like 10% HCl or 10% H2SO4 will do). The color will change from colorless to orange and on adding more acid the color will fade again to pale
yellow.
If the yellow material does not react as described above, then the black solid is no vanadium.
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diggafromdover
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How about a flame test mate? The color may be dispositive.
Enjoying second childhood with REAL chemistry set.
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Kagutsuchi
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The bad thing is that I have never found a chart with the metals and the colors of their oxides. Maybe if no-one has done it earlier, I'll invest the
energy and do it myself, for the good of the science community.
[Edited on 26-10-2015 by Kagutsuchi]
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maleic
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I can't judge its constituents, but I hope it could be gold
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Kagutsuchi
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I have no clue what it is, by reactons I determined it's none of the above, but when boiling it with ammonia, it turns back to the original material
if it could help.
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The Volatile Chemist
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Did you do a flame test, see what color it was? Most colors are documented.
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gdflp
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That's bizarre. Ammonia is a terrible reducing agent, I can't believe that the yellow compound is an oxide, and the ammonia reduced it back to the
metal. It doesn't sound like vanadium though, as I believe V<sub>2</sub>O<sub>5</sub> is reasonably soluble in ammonia. If
you could figure out what it is soluble in, that would be quite helpful. Perhaps try a concentrated solution of NaOH and a concentrated solution of
an acid like HCl or HNO<sub>3</sub>. Testing the same two solutions on the yellow material, a suspected oxide, would also be helpful.
Heat may be needed to assist the dissolutions.
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woelen
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V2O5 is soluble in ammonia, giving a nearly colorless solution. So, if the yellow material reverts back to a black compound on adding ammonia, then
the black material is not vanadium.
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Metacelsus
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Maybe the yellow compound was only a thin layer of oxide over the metal, and the ammonia dissolved it away.
Can you weigh the compound before heating, after heating in air, and after adding ammonia?
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Kagutsuchi
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Okay, I'll try weighing it. It is just red hot when being oxidised, the yellow thing is insoluable in water, it does not color flames, I'll do tests
with the yellow compound today.
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Amos
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Quote: Originally posted by Kagutsuchi  | | Okay, I'll try weighing it. It is just red hot when being oxidised, the yellow thing is insoluable in water, it does not color flames, I'll do tests
with the yellow compound today. |
While you're at it, pictures of it before and after heating might be helpful to us.
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AJKOER
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Per Wikipedia https://en.m.wikipedia.org/wiki/Iron_oxide:
"Iron oxides are widely used as inexpensive, durable pigments in paints, coatings and colored concretes. Colors commonly available are in the "earthy"
end of the yellow/orange/red/brown/black range."
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The Volatile Chemist
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I doubt it's iron or black iron oxide...
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