I was able to find a couple of articles that describe various aspects
of the chemical reaction that you are describing. Ultimately the blue
color has to do with the properties of the Copper II (Cu2+) ion in
solution. Here?s what I found.
- - - - - - - - - - - - - - - - - - - -
This first article discusses the Copper-Amine complex that forms
during a reaction of copper and ammonia.
?When ammonia is dissolved in water, the interaction of the ammonia
molecules with the water molecules results in the production of
ammonium ions and hydroxide ions. One possible reaction for the copper
(II) ions is the combination with the hydroxide ions to form a
slightly soluble copper (II) hydroxide, which can be identified by the
formation of a cloudy, pale blue precipitate.
Another possible product of a reaction of the copper (II) ions,
perhaps less easily recognized from the reactants in this system, is
the formation of a copper (II) ion ammonia complex, usually identified
by a characteristic deep blue color.?
?Copper(II) hydroxide (chemical formula Cu(OH)2) is the hydroxide salt
of the metal copper, with atomic mass approximately 97.561 atomic mass
units. The typical color of copper hydroxide is blue.?
- - - - - - - - - - - - - - - - - - - - -
I hope that you find this information useful. If you have any need of
further clarification, please let me know how I can help.
penny ammonia blue color
copper II hydroxide color