Why does the displacement reaction of zinc in copper(II) sulfate solution result in dark metallic copper?
All metals look black or dark when presented as a sufficiently thin powder. The metals display their color only when the dimension of the grain is greater than the wavelength of the light, namely 400 to 700 nm. This is about 1000 times bigger than the atoms. When prepared from a precipitation reaction, the metal atoms are never arranged neatly in a regular fashion. So they do not reflect the light like a metallic plate.