According to this paper, the green colour from the reaction of copper and nitric acid could be explained:

The color is greenish, probably as a result of a combination of the pale-blue color of the traces of dissolved salt and the pale yellow color of the acid. Another reason for the color difference might the absence of water in the system; hence, no aqua complexes are formed.

The deep green colour arises when copper contact with nitric acid solution, I think the colour might be due to copper complexes. After consulting my teacher, he said that when nitric acid solution is concentrated, the main component would be [Cu(H2O)4(NO3)2] which is responsible for the colour. I wonder if this is true?

  • $\begingroup$ The shift from blue to green could be also caused by dissolved NO2. $\endgroup$
    – Poutnik
    Feb 26 at 7:44
  • $\begingroup$ My friend once conducted electrolysis of CuSO4 experiment. The cathode turned green also. The device was new and CuSO4 solution was prepared from CuSO4.5H2O. Couldn't explain it $\endgroup$
    – Shira
    Feb 26 at 8:54
  • $\begingroup$ @Shira your green cathode is probably not the same as the copper nitrate in nitric acid. Copper plating at the cathode could be accompanied by reduction of water to hydroxide ion thus precipitating copper hydroxide. Industrial copper platers add acid when plating from a sulfate bath. $\endgroup$ Feb 26 at 13:20
  • $\begingroup$ Well but copper hydroxide is not green, isn't it? Might have been some faults while experimenting $\endgroup$
    – Shira
    Feb 26 at 13:48
  • $\begingroup$ Blue to green from what I read. $\endgroup$ Feb 26 at 18:58


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