Re: Why does zinc metal turn black when exposed to hydrochloric acid?

Boy what a question!!

As far as I am able to find, nearly all Zinc compounds are colorless or
white.  The only ones that are dark are the chromate, ferrate, and 
telluride
salts.
I suppose that lesser technical grades of hydrochloric acid might have
traces of chromium and iron but telluride is pretty uncommon.  I would also
be very surprised if much iron or chromium existed in commercial HCl.

Advanced Inorganic Chemistry by Cotton & Wilkinson, has no mention of
complexes formed by zinc and chloride that are black.

To take a guess at this phenomenon, I might speculate that a surface 
etching
mechanism might be responsible.  Extremely finely ground metals appear
black.  The formation of hydrogen gas may form little pockets in the zinc 
as
it is dissolving.  It may be that these fine pockets "trap" incoming light
and reflect very little making the surface appear dark.
There does not seem to be any simple, easy, explanation.  Perhaps an open
appeal to all of the other Mad Scientists out there to give this question 
some thought might stir up some reasonable explanation.

I apologize for the lack of a good answer.

Dr. Michael M. Gallagher

Dr. Dan Berger adds:

I think Dr. Gallagher's response is right on target. I've seen the results of
experiments in which zinc metal is dipped in a solution of copper sulfate;
metallic copper deposits on the surface of the zinc -- but it's not copper-
colored, it's black! The reason is that it does not deposit smoothly, and
the many pits and so forth absorb light.

I'd bet the reaction of zinc and HCl makes just such an irregular, "black"
surface as Dr. Gallagher suggests.