# Why is copper(II) oxide green?

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?

• These two questions seem kinda separate. I think the community will be able to cover them both, provided that you ask them separately. – M.A.R. ಠ_ಠ Jun 27 '15 at 7:33
• Just hazarding a guess, it might be due to formation of $\ce{CuCO3}$ (by reaction with $\ce{CO2}$ in the air). – orthocresol Jun 27 '15 at 7:57
• As I said in an earlier comment, you need to separate the two questions. – user15489 Jun 27 '15 at 8:18
• 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. – Ivan Neretin Feb 7 '16 at 21:25
• @orthocresol that's a good guess. CuCO3.Cu(OH)2 is green – Aditya Dev Mar 8 '16 at 12:01