Mabus - 25-7-2015 at 12:22
I recently melted some car stick-on wheel counterweights made of zinc. During the melting process, the resulting slag turned yellow. After I poured
the molten zinc into another can, I let the slag to cool, and as it cooled, it turned white.
Was the yellow color from zinc peroxide?
BromicAcid - 25-7-2015 at 12:44
As I recall zinc oxide is thermochromic and changes color due to heating alone. Usually peroxides are unstable at higher temperatures so it would
surprise me to find out that a transient peroxide was the cause of the color change.
Mabus - 25-7-2015 at 13:16
I believe you are right:
<iframe sandbox width="640" height="360" src="http://www.youtube.com/embed/cEIujFx2Mro" frameborder="0" allowfullscreen></iframe>
Mystery solved.
chemrox - 29-7-2015 at 13:41
This could also be due to Fe impurities in the weights. Maybe they're an alloy?
Pumukli - 30-7-2015 at 09:44
Bromic is right, ZnO changes its color depending on the temperature. I heated pharmacy grade ZnO in a small ceramic crucible and it became yellow when
was on fairly high heat. Upon cooling the original white color came back.
If you have some known pure ZnO you can see that even at room temperature it is not 100% WHITE, as snow white or TiO2 white (tooth paste white). It
has a bit yellowish tinge!
MrHomeScientist - 30-7-2015 at 09:48
Correct! I also saw the thermochromism when decomposing zinc hydroxide into the oxide. Since both are white powders, I used the yellow color change as
an indicator for when the conversion was complete. Cooling turned it back to nice and white.