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Copper(II) oxide is black, but when we find the percentage of oxygen in air by volume using ammonia, copper, and ammonium chloride, the color of $\ce{CuO}$ turns green. What is the reason for this?

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    $\begingroup$ These two questions seem kinda separate. I think the community will be able to cover them both, provided that you ask them separately. $\endgroup$ – M.A.R. ಠ_ಠ Jun 27 '15 at 7:33
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    $\begingroup$ Just hazarding a guess, it might be due to formation of $\ce{CuCO3}$ (by reaction with $\ce{CO2}$ in the air). $\endgroup$ – orthocresol Jun 27 '15 at 7:57
  • $\begingroup$ As I said in an earlier comment, you need to separate the two questions. $\endgroup$ – user15489 Jun 27 '15 at 8:18
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    $\begingroup$ Cupric oxide is not green, nor does it turn green. What you see is another compound, probably $\ce{CuCl2}$. BTW, the question would benefit from a concise description of the process you are using, so that we wouldn't have to resort to guesswork. $\endgroup$ – Ivan Neretin Feb 7 '16 at 21:25
  • $\begingroup$ @orthocresol that's a good guess. CuCO3.Cu(OH)2 is green $\endgroup$ – Aditya Dev Mar 8 '16 at 12:01

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