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I'm a high school student doing a project where we need to identify chemicals and one of the substances had a very unique change when heated.

Before heating it was an extremely fine, flour-like white powder. I placed it on a hotplate at 350 degrees Celsius in its solid form and it "boiled off", I presume a hydrate evaporating. Before the hydrate appeared to evaporate, the powder had small pink dots appear all over it. This also happened when exposed to $\ce{NaClO}$ and $\ce{NaOH}$.

Once the temperature was raised to 400 degrees Celsius, the remaining solid turned yellow and then spontaneously caught fire, turning it all black before the fire ran out.

I had ran other tests on this substance, being insoluble in water, ethanol, and hexane. It also produced no apparent flame color.

My initial thought was that this compound was starch, however now that I think about it, it may be an insoluble magnesium compound, due to the lack of flame color? Just curious as to if anyone knows what substances display these properties when heated.

Here is a list of potential substances based off of a predetermined list.

  • Aluminum nitrate
  • Ammonium chloride
  • Ammonium nitrate
  • Ammonium sulfate
  • Barium acetate
  • Barium nitrate
  • Barium oxide
  • Barium sulfate
  • Calcium carbonate
  • Calcium chloride
  • Calcium oxide
  • Calcium nitrate
  • Calcium sulfate
  • Iron(II) sulfate
  • Iron(III) sulfate
  • Lauric acid
  • Lithium carbonate
  • Lithium nitrate
  • Magnesium carbonate
  • Magnesium chloride
  • Magnesium hydroxide
  • Magnesium oxide
  • Magnesium sulfate
  • Potassium bromate
  • Potassium carbonate
  • Potassium chloride
  • Potassium iodide
  • Potassium metabisulfite
  • Potassium nitrate
  • Potassium persulfate
  • Potassium phosphate
  • Potassium sodium tartrate
  • Potassium thiocyanate
  • Sodium acetate
  • Sodium bisulfate
  • Sodium borate
  • Sodium carbonate
  • Sodium citrate
  • Sodium chloride
  • Sodium fluoride
  • Sodium nitrate
  • Sodium nitrite
  • Sodium oxalate
  • Sodium sulfate
  • Sodium sulfite
  • Sodium thiosulfate
  • Starch
  • Strontium carbonate
  • Strontium hydroxide
  • Strontium nitrate
  • Sucrose
  • Tin(II) chloride
  • Urea
  • Zinc nitrate
  • Zinc sulfate

Of these, as stated earlier, an Mg compound (due to insolubility) or starch is most likely.

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The fact that it turned black when heating tells you that:

  1. It was an organic substance and did not burn completely leaving some carbon behind, or
  2. It was a transition metal that had a lower oxidation state and was oxidised during burning.

Let's remove the obvious ones:

  1. Not a nitrate. These will release toxic brown gas, which is not something you give to high school students.
  2. Not an alkali (Na, K, etc) or alkali earth (Ca, Mg, etc) element because they're not pink nor they turn black.
  3. I doubt it's a sulfur compound of any kind. You should have detected a sulfur smell when burning.

Maybe this can help you a bit.

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Let's look at magnesium carbonate. It's white. When heated it remains white, and upon decomposition it gives off carbon dioxide leaving behind magnesium oxide -- which is white. So can your color changing substance really be magnesium carbonate?

Only one of your remaining candidates really matches up with the color criteria you present.

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