|MadSci Network: Chemistry|
From a metallurgist's standpoint, I do not believe that the change you see should be considered to be a precipitate because that refers to the insoluble product of a chemical reaction occurring within a solution. Rather, the term 'tarnish' was created specifically to describe the a thin film on a metal that forms as a result of exposure of the metal to a reactive chemical environment. Tarnishes are typically thin oxides or sulfide films (like the brownish black tarnish on silver). The optical properties of tarnish films can cause light to change color as it passes through them and is reflected back out. The colors you see are related to the thickness of the film at different places on the surface. When the reaction of the surface is more intense and thicker reaction products are formed (like rust on iron), then the term 'scale' is used rather than 'tarnish'. In your case, the penny (which is about 90% Cu and 10% Zn) has apparently reacted with the acidic solution to produce a corrosion product on its surface. At zinc contents over about 15% Zn (by weight) in Cu, the problem of 'dezincification' can occur. In that case, Zn will come out of the alloy and form a surface film, weakening the Cu underneath. At lower zinc contents, like the penny, this is not as important a problem, but some reaction of the Zn with the acetic acid in vinegar is possible, causing a discolored surface. If I recall my chemistry correctly, the term 'color change' mainly refers to reactions that occur within liquids (like pH indicators and the like). Therefore, I don't believe that the term 'color change' strictly applies to tarnishing reactions or other types of corrosive surface attack.
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