Metallic Mixtures / Alloys Alchemist’s “Gold” Metallic Mixtures / Alloys

Prior to the times of Lavoisier, 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 Lavoisier 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.

The concept behind this investigation is METALLIC BONDING & ALLOYS OUR DEMONSTRATION We will be using copper as our base metal, and turning it to “gold”. The concept behind this investigation is METALLIC BONDING & ALLOYS

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 before) 6 M NaOH. (approx. 100 mL) Zinc (powder or granular, 5 g)  400 mL ceramic evaporating dish 250 mL beaker (2) glass stirring rod metal tongs hot plate Bunsen burner Paper towels iron wool dilute HCl solution

SAFETY PRECAUTIONS!!!! ***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 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.) 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. 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 paper towel. BE SURE TO LAY PAPER TOWEL OUT FLAT TO DRY OVERNIGHT – DO NOT CRUMPLE UP AND PLACE IN TRASH CAN!!! THIS CAN BE A FIRE HAZARD!!!! 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. Do not leave in heat too long!

What to think about…. What is an alloy? How are they formed? What alloy makes for brass? What have you produced? And HOW was it produced? Were these changes chemical or physical in nature? Why is this really considered to be a metal mixture as opposed to a new metal compound? Why do you think heat is needed to cause the zinc coating to diffuse into the copper?