Re: WHY DO PENNIES TURN SILVER WHEN PLACED IN NAOH,HCL AND ZINC AND HEATED?

Well, you have uncovered one of the secrets of pennies, which used to be 
made primarily of copper.  According to the US Mint's website,  
http://www.usmint.gov/facts/composition.cfm, the 
composition of pennies was changed in 1982 from an alloy of 95% copper, 
5% zinc to copper plated zinc. The change was made because zinc is cheaper 
than copper.

When you place a copper-plated zinc disk in HCl, the copper dissolves in 
the acid to form copper chloride which should color the solution blue.  If 
you are using a solution of all three chemicals, you are still probably 
dissolving the copper, depending upon the concentration of each, in order 
to end up with a "silver" penny.  The zinc will also probably dissolve in 
time, it is just that the copper layer is very thin.

Heating a metal in a flame will generally cause the surface of the metal to 
oxidize by reacting with oxygen in the air.  The oxide layer itself may be 
colored, or it may pick up some of the partially burned hydrocarbons 
within the flame to add color to an otherwise clear oxide layer.  The thin 
oxide layers may also be similar to the thickness of the wavelength of 
light to produce a range of colors.  This is particularly noticible with 
some metals such as pure copper or some of the steels.  A drill bit which 
is overheated will often show this type of coloration, caused by reacting 
with oxygen in the air.

And you're right in thinking that these are chemical changes.  Chemistry is 
all around us and the more we understand about it, the more we can 
appreciate and use it as a helpful tool.  So get a small piece of copper 
tubing and see what colors you can produce by heating it in a flame (just 
remember; copper conducts heat well so don't burn yourself by holding it in 
your hands).