Which white powder first "boils off" and then catches fire at high temperatures? 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.
The fact that it turned black when heating tells you that:


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


Let's remove the obvious ones:


Not a nitrate. These will release toxic brown gas, which is not something you give to high school students.
Not an alkali (Na, K, etc) or alkali earth (Ca, Mg, etc) element because they're not pink nor they turn black.
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.