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In the reaction where $\ce{Zn}$ reacts with $\ce{HCl}$ to give off Hydrogen gas & Zinc Chloride solution, does the zinc turn a dull black color after a while? If so, why?

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    $\begingroup$ Could you provide a little context? Are you asking because you tried this or saw it done and the zinc turned black? Are you trying to find a way to make zinc turn black? $\endgroup$ – Ben Norris Sep 16 '13 at 11:14
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    $\begingroup$ Under which conditions does it happen? Only when you dip a zinc sheet into hydrochloric acid or also when you hold the sheet above to only react it with the hydrogen chloride in the vapour phase? $\endgroup$ – Klaus-Dieter Warzecha Jan 21 '14 at 22:03
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Look at it under a microscope. If the surface is microporous, light goes into the reflecting cavities, bounces around, and is absorbed before it can escape. Wilkinson Sword sold bolted stacks of double-edged razor blades as low power laser beam dumps, Each sharp edge was mirror bright with a slight taper toward the edge. Light bounced in and could not escape. The sharp sides of the stack were astoundingly black. 375 blades x .004" = 1.5" square target.

Or, you have impure zinc and crud is accumulating at the dissolving surface. Alloy 8111 aluminum foil contains about 2% or so of strengthening dispersed intermetallics. Dissolving aluminum foil in acid or base leaves black residues.

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The $\ce{Zn}$ cation polarises the $\ce{Cl}$ anions to a good extent and reduces the energy gap between certain energy levels. Therefore the electrons in $\ce{ZnCl2}$ are able to jump this gap by absorbing certain radiation, when this radiation is absorbed all the radiations except these are reflected and can be seen as complimentary color which in this case happens to be black.

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It doesn't become pitch black, but it does become dull gray/dark gray. The reason is that even if metal surface is mirror bright, it is usually composed of multiple crystals with slightly different composition and orientations. Because of this differences, different faces of the microcrystals are etched with different speed, forming a complicated surface shape.

For the same reason, fine metal powders usually have dark coloration.

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I've been doing this as art for a while and when I let the muriatic acid dry over night on the zinc it not only turns perfectly black but becomes amazingly hard to sand down or buff back for aesthetic purposes.enter image description hereThe image not only illustrates the deeper darker parts but near the center of the image is a dark area that is dark and not deep and I used a muriatic wash to try to get it off. It even resisted that. So my thought was that it was some type of zinc oxide that had occurred and was harder than the zinc.

Curious about everything .

Charlie W.

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