Theory

Prior to the times of Lavoissier, in the 1700’s, chemists - who were known as alchemists - spent most of their time looking for a way to turn matter into gold.  Alchemists spent years developing techniques and trickery that made their audiences believe that they had the ability of turning lead (and other substances called "base metals") into gold.  One such trick involved filling a bored-out metal stirring rod with gold dust.  The tip of the rod was then sealed with wax in order to contain the dust and to keep it from being seen by others.  To prove himself as a master alchemist the charlatan would stir a heated, empty crucible with this rod and (as the wax melted) the vessel would slowly fill with molten gold.  Although alchemists were measured by the “success” of their trickery, their less-than-systematic research played an important contribution to laying the foundation for chemists like Lavoissier and others to build on.  It was the alchemists that discovered and developed the various techniques of chemical purification such as fractional distillation and sublimation.

When our penny is heated in a solution of sodium hydroxide in the presence of zinc powder, atoms of zinc which have gone into solution migrate towards the penny and deposit a coating of zinc onto the copper.  This process is known as wet-chemical plating, as opposed to electrochemical plating.  The coating of zinc gives the penny an appearance of silver.  If the “silver” penny is heated gently the zinc on the surface mixes with the outer layers of copper atoms of the penny, producing an alloy which looks like gold.  In making this alloy, the atoms must actually exchange places to mix - this is accomplished by adding heat to cause the natural vibrational motion of the copper and zinc atoms to turn in to fluid motion.  The atoms then flow into each others' "territory."

Materials:

Copper pennies (1981 or beforepreferred; Canadian is fine)
6 M NaOH. (approx. 100 mL) and zinc (powder or granular, 5 g)  (these may be pre-mixed)
400 mL ceramic evaporating dish
250 mL beaker (2)
glass stirring rod
metal tongs
hot plate
Bunsen burner
soft cloth
iron wool
dilute HCl solution

* Sodium hydroxide solutions are corrosive, and skin burns are possible.  Heat the NaOH carefully, to prevent spattering.
* Zinc solutions are irritating to the skin and cause itchy rashes.
* HCl solutions (hydrochloric acid) are corrosive and especially irritating to the eyes and mucous membranes

Procedure

1. Pour NaOH solution into a ceramic evaporating dish.  Add 5 g zinc powder to the solution and heat slowly.  (This solution may be pre-mixed.)
2. While the zinc solution is heating, dip a penny in dilute HCl, rinse in tap water, and scrub shiny with iron wool.  Repeat until you have a shiny clean penny with a consistent surface.
3. Place the penny into the zinc/sodium hydroxide mixture.  Stir the contents of the dish while continuing to heat, until the penny develops a silver appearance.  Remove the penny with metallic tongs and dip it in cool water a few times to wash off any adhering solution.  The penny may be dried and polished with a soft cloth.
4. Use tongs to pass one of the silver pennies back and forth through the flame of a Bunsen burner.  As the penny is heated it will develop the appearance of gold.

Disposal

• Zinc dust is a fire risk so follow your instructors directions for disposal.
• Keep or spend the penny  J.

 1. What is an alloy?
 
 
 
 

 2. What two metals compose the alloy brass?
 
 
 
 

 3. Why do you think heat is needed to cause the zinc coating to diffuse into the copper?
 
 
 
 

[Brass Pennies score sheet][MHS Chem page]