Saerynide - 12-1-2005 at 05:06
Does anyone know why iron oxide turns black when heated but turns back to red when it cools down?
This also happens to brown iron oxide.
mick - 12-1-2005 at 12:38
If you are doing it on a small scale you might be going from Fe(II) to Fe(III) to Fe3O4 and back again. On a larger scale, if you quench the iron
covered in Fe(II)O, after heating, when it turns black, in oil then it might stay black. The oil keeps the O2 out and a bit of carbon from the oil
seems to stabilise the oxide layer.
mick
cyclonite4 - 12-1-2005 at 17:11
Does the color darken in proportion to the heat applied? If so it would be kind of a innacurate temperature indicator. 
Mephisto - 12-1-2005 at 17:47
Because there are countless non-stoichiometric variations of iron oxides + oxid-hydroxids, oxid-hydrates, oxid-hydroxid-hydrates,... it's not
possible to answer this question decently. At least in an ideal case, red-brown Fe2O3 with a mass-proportion of ≈28% Fe will turn into black
Fe3O4 above 1100 °C and become again red-brown under this temperature.
Such a change between two modifications would be described by a diagram of state with a changing point and no real continuously colour-changing in
proportion of the heat applied.
Saerynide - 19-1-2005 at 05:53
Not really about rust changing colors, but still about rust (I didnt want to start a new topic for 1 question).
People always say that bleach and vinegar (used to make rust from steel wool) makes Cl2, but I cant think of an equation nor find one that supports
that.
I thought:
NaClO + HAc --> NaAc + HClO
2HClO --> 2HCl + O2
Is this correct? And if so, are "regular" people just confusing the HCl with Cl2?
[edit]: But once, when I added vinegar to bleach, the whole solution turned light green 
I've tried many times again, using the same bottles of bleach and vinegar, but nothing happened. How do we explain that?
[Edited on 21-1-2005 by Saerynide]
frogfot - 19-1-2005 at 08:24
From the red-ox reactions table I found thisone:
HClO + H+ + 2e- --> Cl- + H2O
electrons being form the iron.
Could the color change on heating be because of change in crystal shape? IIRC there are like several modifications of Fe2O3 (alfa through
gamma-Fe2O3), or maby that's totally different thing..
JohnWW - 19-1-2005 at 12:16
Hematite, in its pure powdered form, pure Fe2O3, is bright red in color.
There are also hydrous oxides, particularly the mineral limonite, FeO(OH), which is yellow-orange, and the main constituent of the "ironstone
pan" in leached acid soils containing iron.
Iron oxides containing Fe(II), viz. FeO and Fe3O4, having more unutilized 3d electrons, are electrically conducting, and so strongly opaque and
reflective like a metal, and also ferromagnetic, due to some of these unutilized electrons lying in the conduction band.
mick - 19-1-2005 at 12:57
People always say that bleach and vinegar makes Cl2, but I cant think of an equation nor find one that supports that.
Don't know what it has to do with rusting
but here is my bit
The bleach (I think I did this with bleaching powder which is calcium hypochlorite not sodium) should be dry otherwise there is no bleach there, it
would have hydrolysed slowly. Vinegar has got water in it (I used pickling grade vinegar). The reaction seems to work because the product of the
reaction is anhydrous calcium acetate which is hydrosopic so it takes the water out and the heat kicks out the chlorine as a gas.
mick
You drip the vinegar onto the bleaching powder.
mick
[Edited on 19-1-2005 by mick]
neutrino - 19-1-2005 at 16:09
Iron oxide with plenty of water in it is yellow.
The vinegar + bleach reaction does work, my nose can confirm that.
HAc + NaCl (byproduct of bleach production and decomposition) -> NaAc + HCl
HAc + NaOCl -> HOCl + NaAc
HCl + HOCl -> Cl<sub>2</sub> + H<sub>2</sub>O
This reaction proceeds slowly because acetic acid is a weak acid. Mixing vinegar and household bleach will give you detectable chlorine gas, but it
will not bubble out of solution like the reaction with sodium bisulfate will.