Copper sulfate solution is used as an invisible ink.
name: NH3 replaces water molecules from the Cu coordination shell with formation of a blue well defined complex.
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Cooper sulfate dissociates in the water solution and gives hydrated Cu+2 ions (Cu(H2O)2+) and SO42- ions:
CuSO4 +6H2O → Cu(H2O)62+ +SO42- (light blue)
When it comes into contact with ammonia vapor, NH3 replaces water molecules from the Cu coordination shell. The formation of a bright blue well defined complex is observed.
(Cu(H2O)6)2+ + 4NH3 → (Cu(H2O)2(NH3)4)2+ + 4H2O (blue)