MadSci Network: Chemistry

Re: Is the tarnish on a penny considered a precipitate or color change.

Date: Thu Mar 2 07:21:20 2000
Posted By: Peter Blau, Staff, Metals and Ceramics, Oak Ridge National Lab
Area of science: Chemistry
ID: 951524723.Ch

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 

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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