Why does displacing copper using zinc produce a black precipitate?

I tried to displace copper (II) sulfate from solution using zinc. I got a black precipitate, reddish-brown in parts, instead of the copper metal I expected. Why?
Andrew 06/06/98

Here are two hypotheses. Can you devise an experiment to help you decide which is correct?

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.

Platinum electrode plates look black for this reason. Platinum is a very shiny metal, but tiny particles of metal deposited on the plates effectively scatter visible light. And people who are sloppy when handling silver nitrate solutions learn the hard way just how black microscopic metal crystals can be. Silver nitrate spilled on the skin photoreduces to silver metal, making a black stain that won't wash off.

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.

References

Zinc and Health (National Library of Medicine)
An extensive bibliography from 1990-1998, containing over 3500 citations.
http://www.nlm.nih.gov/pubs/cbm/zinc.html (01/22/00)

Zinc Uses (American Zinc Association)
Zinc is primarily used to galvanize steel- but other uses include zinc-air batteries, water purification, high-tech tape, and toy manufacture.
http://www.zinc.org/zincuses.html (01/22/00)

Zinc (Minerals Council of Australia)
History, occurrence, mining, and uses of zinc.
http://www.minerals.org.au/pages/page3_47.asp (11/22/99)

Author: Fred Senese senese@antoine.frostburg.edu



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