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Why does displacing copper using zinc produce a black precipitate?

Why does displacing copper using zinc produce a black precipitate?


1 Answers

Sunil Kumar FP
askIITians Faculty 183 Points
6 years ago
The reaction occurs as
CuSO4(aq) + Zn(s) ---> ZnSO4 + Cu
Here are two hypotheses.
Hypothesis 1: tiny copper crystals look black because they scatter light in all directions.Perhaps copper precipitated very quickly from the highly concentrated solution, forming microscopic particles. Most of the light that falls on these tiny copper particles is reflected by the many microscopic facets, and scattered in all directions. Very little of the incident light is reflected back to the eye, so the copper looks black, not shiny.

Hypothesis 2: zinc contains an insoluble black impurity.Commercial zinc is tainted with impurities such as lead, cadmium, iron, cobalt, copper, and nickel. The black precipitate left behind as the zinc dissolves may be oxides of these impurities.

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